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Behavioral Ecology Vol. 11 No. 4: 378–386 Nest and mate choice in the red bishop (Euplectes orix): female settlement rules Thomas W. P. Friedl and Georg M. Klump Institut für Zoologie, Technische Universität München, Lichtenbergstr. 4, D-85747 Garching, Germany We investigated female settlement in a colony of red bishops (Euplectes orix), a territorial and highly polygynous weaverbird widely distributed over sub-Saharan Africa. An earlier study showed that male reproductive success is mainly determined by the number of nests a male builds in his territory, which appeared to be a good indicator of male quality. Because males provide no parental care or food resources within the territory, females sharing a territory do not compete for material resources and might therefore be expected to settle preferentially in territories of males that build many nests to gain the possible genetic benefit of high-quality offspring. An analysis of female settlement, however, revealed that females did not show a preference for territories of males with many nests and that the distribution of female breeding attempts with regard to the number of vacant nests within a territory could be explained best by random female settlement in 3 out of 4 years. Females settled more often than expected by chance (in 3 out of 4 years) in territories already containing occupied nests, indicating that resident females did not discourage settlement of additional females. However, sharing a territory with other females might impose costs in terms of an increased predation risk because nests in territories that contained other nests with young suffered from higher predation than nests in territories that did not contain other nests with young. Females therefore might trade the possible benefits of settling in territories of males with many nests against the costs of sharing a territory with other females. This might result in the mating pattern found with random female settlement and male reproductive success being directly proportional to the number of nests built. We discuss possible implications of this mating pattern for sexual selection on males. Key words: Euplectes orix, female choice, male quality, predation, random female settlement, sexual selection, weaverbirds. [Behav Ecol 11:378–386 (2000)] I n polygynous species with resource defense polygyny, territorial males provide resources that are important for reproduction such as food or nest sites within the territory to attract more than one female. Females often have to make a choice between a male without mate and an already mated male. If there is variation in the amount or quality of the resources provided by the males, it might pay for the female to choose an already mated male with, for example, a better territory, instead of an unmated male, because the advantage of higher territory quality outweighs the disadvantage of sharing the territory and the available food within the territory with another female. If the quality differences among territories are large, this will lead to high variation in male mating success (polygyny threshold model; Verner and Willson, 1966; Orians, 1969), and female mating preferences can be explained by direct benefits (e.g., better offspring survival resulting from higher food resources within the territory). In contrast, in polygynous species, where males provide females with little material resources, such direct benefits of female mate choice are less likely to fully explain female mating preferences. Instead, indirect mechanisms as proposed by ‘‘good genes’’ models of sexual selection may play a more important role. We studied sexual selection in the red bishop (Euplectes orix), a sexually dimorphic and polygynous weaverbird from Southern Africa. Males in breeding plumage establish and defend territories in reedbeds or bullrush stands around water, where they construct several nests during a breeding season to which they try to attract females. Most territories contain one or more empty nests at any time during the breeding Address correspondence to T. W. P. Friedl. E-mail: Thomas. [email protected]. Received 7 April 1999; revised 24 August 1999; accepted 30 October 1999. 2000 International Society for Behavioral Ecology season. Besides the nests, the males provide no other material resources such as parental care or food within the territories, and territory quality appears to be similar between males (at least in the colony that we studied). Therefore, possible direct benefits of female mate choice seem to be low, and quality differences between males should become more important in forming the females’ mating decisions. More details of red bishop breeding behavior are given in Craig (1974, 1980) and Friedl (1998). The only factor found to be related to male mating success in the red bishop is the number of nests a male builds on his territory. Males that built many nests had more nests accepted by females and sired more offspring than males that built fewer nests (Friedl and Klump, 1999). The number of nests built by the males is a good indicator of male quality because males that survived and established a territory in the following season (and thereby demonstrated high quality) built more nests than males that did not establish a territory in the following season (Friedl and Klump, 1999). Females do not compete with each other for nests, paternal care, or food resources within the territory, and, consequently, the costs for females of sharing a territory with other females seem to be limited. By mating preferentially with males that build many nests, female red bishops could gain indirect genetic benefits in terms of high-quality offspring without incurring high costs from sharing the territory (given that there is some heritable component of male quality). However, the linear relationship found between the number of nests built by red bishop males and the number of nests accepted by females (Friedl and Klump, 1999) indicates that males that build many nests do not have a disproportionally high mating success (i.e., that females do not settle preferentially in territories of males with many nests). Instead, the linear relationship between the number of nests built and male mating success might be the result of either random female settlement or females settling according to an ideal free distribution (Fretwell and Lucas, 1970). Although the outcome in both cases would be the same Friedl and Klump • Female settlement in red bishops (i.e., a linear correlation between the number of nests and the number of mates), the mechanisms resulting in such an outcome would be fundamentally different. In the case of random female settlement, females exert no choice at all, while in the case of female settlement according to an ideal free distribution, females actively choose the males with the highest available number of empty nests at the time of choice. In this study we investigated the female perspective of nest and mate choice in the red bishop in more detail. We analyzed the temporal pattern of female choice of nesting sites in relation to the number of vacant nests within a territory to distinguish between random female settlement and female settlement according to an ideal free distribution. Because the apparent lack of a female preference for males with many nests might reflect a trade-off between potential costs and benefits of mating with males of high quality, we attempted to identify possible constraints on female choice of nesting sites. We investigated female choice of nesting sites in relation to the number of occupied nests within a territory to find out whether resident females affect further settlement. Furthermore, we analyzed clutch and brood losses in relation to the number of occupied nests within a territory to find out whether there were costs of sharing a territory with other females in terms of a higher predation rate. Such costs should influence the female’s decision of where to start a breeding attempt and might explain why female red bishops do not show a preference for males with many nests. MATERIALS AND METHODS Study colony We studied female breeding behavior and female choice of nesting sites in a colony of red bishops in the Addo Elephant National Park, Eastern Cape, South Africa (33⬚26⬘ S, 25⬚45⬘ E) during four consecutive breeding seasons (1993–94 to 1996–97), lasting from October to April. The breeding site was a small dam (approximately 250 m2) surrounded by bullrushes (Typha capensis) and common reeds (Phragmites australis). General field methods Throughout the study, we counted nests daily to search for new nests built by territorial males; these nests were then marked with numbered yellow plastic tags that were attached to reeds or bullrush stems close to the nest. We checked all tagged nests daily to record laying dates of the eggs, clutch size, hatching dates, number of hatchlings, and number of fledglings. Throughout the breeding seasons, detailed observations of territorial behavior and aggressive interactions between males were made to obtain information on the location of the territories and identities of the territory holders. Based on these data, we drew territory maps and updated them every other week. Social parents of nestlings were assigned through behavioral observations, identifying both the owner of the territory in which the nest was situated and the female incubating or feeding the brood. Field methods are described in more detail in Friedl and Klump (1999). Data analysis Female settlement in relation to the number of vacant nests within a territory We investigated the patterns of female settlement in relation to the number of vacant nests within a territory by comparing the actual choice of females with the alternatives available at the time of choice (Garson, 1980). We considered the laying 379 date of the first egg as the day the choice was made. All breeding attempts for which we did not know the exact laying date of the first egg were excluded from the analysis (for example, in the first season we could only commence data collection about 3 weeks after the breeding season had begun; therefore, we excluded all breeding attempts that were started in the first 3 weeks of this season). Because the status of all nests within the colony on every day during the breeding season was known (with the exception of the first 3 weeks of the breeding season 1993–94, see above), we could calculate how many vacant nests were available within each territory for every day a female made a choice. Nests that were empty on a day a female laid her first egg but had contained eggs before were regarded as vacant because in several cases females accepted such nests. In contrast, empty nests that had contained nestlings before were not regarded as vacant because females were never observed starting breeding attempts in nests that had previously contained young (Friedl, 1998). For every day a female laid her first egg, territories were assigned to categories according to the number of vacant nests (i.e., all territories with one vacant nest formed the first category, all territories with two vacant nests formed the second category, and so on). We calculated the expected probability of being chosen for every territory category, assuming that females were occupying nests at random and that the probability of a territory being chosen by a female was proportional to the number of vacant nests within this territory. A territory with, for example, two vacant nests had twice the probability of being chosen compared to a territory with only one vacant nest, and so on. Thus, the analysis was conducted at the level of the nests even if we formed and analyzed territory categories. The expected number of females choosing a territory of a particular category was calculated by dividing the number of vacant nests available within the territory category by the total number of nests available on that day and then multiplying the resulting value by the number of breeding attempts that were started on that day. Summing up the calculated probabilities for each territory category for every day within a breeding season on which breeding attempts were started resulted in the expected total number of nests accepted in the different territory categories for the entire breeding season. We then compared this expected distribution with the observed distribution of the number of nests accepted using chi-square statistics. Female settlement in relation to the number of occupied nests within a territory We analyzed female settlement in relation to the number of occupied nests within a territory to evaluate effects of resident females on further female settlement within the same territory. We adopted a similar approach as in the analysis of female settlement in relation to the number of vacant nests described above. All nests that contained eggs or nestlings were classified as occupied. For every day a female started a breeding attempt, we first combined all territories with the same number of occupied nests at that particular day to form one territory category (that is, all territories with no occupied nests formed the first category, all territories with one occupied nest formed the second category, all territories with two occupied nests formed the third category, and so on). Territories with no vacant nests on a particular day were excluded from the analysis for this day. We then calculated the probability for every territory category to be chosen under the assumption of random female settlement with respect to the number of occupied nests within a territory. That is, we assumed that the probability of a territory category to be chosen by a female was proportional to the number of available vacant nests within this territory category. The sum over all days 380 in a breeding season on which breeding attempts were started yielded the expected frequency of nests accepted for the different territory categories. This frequency distribution could then be compared with the observed distribution of accepted nests in the different territory categories. Nest predation in relation to the number of occupied nests within a territory To test whether there are costs or benefits of sharing a territory with other females in terms of higher or lower predation rates, we analyzed predation in relation to the number of occupied nests within a territory. We considered every nest in which eggs or nestlings were missing compared to the previous day as predated, even if there were no signs of nest damage or destruction. The predominant predator in the study colony was the cape cobra (Naja nivea), accounting for an estimated 80–90% of all losses. Cape cobras were often observed moving through the colony and robbing nests, and nests that were predated by cape cobras never showed any sign of nest damage (Friedl and Klump, personal observation). Sometimes predated nests were found with holes in the bottom part of the nest, which is typical for predation by house rats, Rattus rattus (Craig, 1982). Other potential nest predators common at the study site were boomslang (Dispholidus typus), grey heron (Ardea cinerea), fiscal shrike (Lanius collaris), small grey mongoose (Galerella pulverulenta) and yellow mongoose (Cynictis penicillata). We calculated a random predation distribution similar to the random distributions described above. For every day we found nests that were predated, we formed territory categories according to the number of occupied nests within a territory. Territories with no occupied nests on a particular day were excluded from the analysis for that day. We then calculated the probability for every territory category to be affected by predation under the assumption that predation occurred randomly—that is, that every occupied nest had the same probability of being predated irrespective of whether there were other occupied nests within the same territory. We summed up the probabilities for all days in a breeding season on which nest predation took place and compared the resulting random predation distribution with the observed frequency distribution of predation events. Statistics All data analyses were conducted using the software package SPSS/PC (SPSS Inc.). Descriptive statistics are given as means ⫾ standard deviation unless otherwise stated. All p values given are two tailed. RESULTS Breeding activity and multiple breeding attempts During the 4 study years, 1254 nests were recorded. Females started 670 breeding attempts and laid 1992 eggs; mean clutch size was 2.973 ⫾ 0.598 (range 1–5 eggs per clutch). A total of 1072 eggs hatched, and 518 nestlings fledged successfully. Out of 386 individually marked females, 117 were observed starting breeding attempts in the colony. Several females started more than one breeding attempt within a single breeding season. Because the attending female could not always be identified for breeding attempts that failed early in the breeding cycle, the observed numbers of females starting multiple breeding attempts must be regarded as minimum numbers. Twenty-three of the 117 females (19.7%) started two breeding attempts within one season, and in 7 cases both attempts were successful in producing fledglings. In six instances females started with a second clutch only about 1 week after chicks Behavioral Ecology Vol. 11 No. 4 from the first brood had left the nest. These females were still observed feeding the fledglings while at the same time incubating their second clutch. Seven females produced a replacement clutch soon after the eggs from their first clutches disappeared. This can be as soon as 10 days after the start of the first clutch (median 16 days, range 10–22 days). These females never used the same nest for the replacement clutch. In fact, in five out of the seven cases the replacement clutch was laid in a different territory. Two females (1.7%) started three breeding attempts within a season. Both of them first produced hatchlings that were predated, while in the second and third breeding attempt the eggs disappeared before the hatching date. The time spans between the start of the first and second breeding attempt were 23 and 46 days, and between the second and third attempt 20 and 19 days. One female (0.9%) even started four breeding attempts within a single season, with the first two producing fledglings; in the last two the eggs disappeared before hatching. Time spans between the four breeding attempts were 34 days, 44 days, and 11 days, respectively. Female settlement in relation to the number of vacant nests To be able to distinguish between random female settlement and female nest choice according to an ideal free distribution, we compared the observed distribution of breeding attempts among territories with a different number of vacant nests with the expected distribution of breeding attempts calculated under the assumption of random female settlement (i.e., under the assumption that every vacant nest had the same probability of being chosen; see Methods). Female nest choice according to an ideal free distribution predicts that females choose the territories with the highest number of available vacant nests at the time they make their choice. That is, for the territory categories with many vacant nests, the observed frequency of breeding attempts should be higher than the expected frequency calculated under the assumption of random female settlement. The results from the analysis of 521 breeding attempts in the four study seasons, presented in Table 1, clearly show that this is not the case. Territory categories with many vacant nests were mostly chosen less often than expected by chance, indicating that the pattern of female settlement cannot be explained by female choice of nesting sites according to an ideal free distribution. In three out of the four seasons investigated, the observed distribution of breeding attempts was not significantly different from the distribution expected by chance, suggesting that in these three seasons the observed pattern of breeding attempts can be explained by random female settlement. Only in the breeding season 1996– 97 was the observed distribution of breeding attempts significantly different from the distribution calculated under the assumption of equal probabilities of being chosen for all vacant nests within the colony (see Table 1). Female settlement in relation to the number of occupied nests To see whether the females’ mating decisions were affected by the number of other females breeding within the territories, we compared the observed distribution of breeding attempts with the distribution calculated under the assumption that every nest has the same probability of being chosen independently of the number of occupied nests within a territory at the time of choice (see Methods). If females started breeding attempts in territories with many occupied nests more often than expected by chance, this would indicate that females actively chose such territories. If, on the other hand, females were observed to breed less often in territories with Friedl and Klump • Female settlement in red bishops 381 Table 1 Observed distribution of breeding attempts compared with the distribution expected under the assumption of random female settlement with regard to the number of vacant nests within a territory Season Number of vacant nests within a territory 1 2 3 4 1993–94 Expected Observed 30.16 35 24.3 22 17.05 13 1994–95 Expected Observed 36.72 43 68.8 72 49.23 46 1995–96 Expected Observed 13.69 17 29.48 28 1996–97 Expected Observed 34.39 28 42.63 70 3.34 5 5 6 7 2 Total p 0.15 0 75 2.776 .428 23.0 17 5.25 5 183 3.013 .556 21.74 19 16.46 16 4.63 6 86 1.638 .802 39.32 45 32.06 24 177 35.189 .0005 17.4 8 occupied nests than expected by chance, this would present evidence for females actively avoiding territories with already breeding females. The analysis revealed that in the breeding season 1995–96 the observed distribution was almost equal to the distribution expected by chance, indicating that in this season females settled at random with respect to the number of occupied nests within a territory (Table 2). In the three breeding seasons 1993–94, 1994–95, and 1996–97, the observed distribution of breeding attempts was significantly different from the distribution expected by chance (Table 2). In all three seasons this difference seemed to be caused by territories with no occupied nests being chosen less often than expected by chance. To test this, we performed a second analysis with only two territory categories (i.e., territories with and without occupied nests). The results confirmed that in the breeding seasons 1993–94 and 1996–97 territories without occupied nests were chosen significantly less often than expected by chance (breeding season 1993/94: 2 ⫽ 21.98, p ⬍ .001; breeding season 1996–97: 2 ⫽ 5.21, p ⫽ .022). The same tendency was found in the season 1994–95; however, observed and expected distribution were not significantly different from each other in this season (2 ⫽ 1.125, p ⫽ .289). 10.37 1 0.83 1 Predation in relation to the number of occupied nests If the predation rate is influenced by the number of breeding females within a territory, females should either actively seek territories with already breeding females or avoid such territories, depending on whether the number of simultaneously breeding females decreases or increases the predation risk. We tested for effects of the number of simultaneously breeding females on predation rate by comparing the observed distribution of predation events with the frequency distribution calculated under the assumption that every occupied nest has the same probability of being predated (see Methods). There was no significant difference between the observed distribution and the calculated random distribution of predation for any of the four breeding seasons (Table 3), suggesting that predation risk was not related to the number of occupied nests within a territory. However, since predators might be more easily attracted by nestlings than by eggs, we performed a second analysis considering only nests that contained young. For this analysis territories were classified in only two categories (i.e., territories with and without nests containing nestlings). We then tested whether the observed probability of an Table 2 Observed distribution of breeding attempts compared with the distribution expected under the assumption of random female settlement with regard to the number of occupied nests within a territory Number of occupied nests within a territory Season 0 1 2 3 1993–94 Expected Observed 39.28 19 15.35 28 14.47 21 1994–95 Expected Observed 55.59 49 48.44 48 42.93 39 1995–96 Expected Observed 51.15 50 26.17 24 6.75 9 1.68 3 0.25 0 1996–97 Expected Observed 57.23 43 48.47 41 29.99 48 24.83 28 10.21 9 4.73 6 22.2 24 4 5 6 0.1 1 0.53 0 0.54 0 9.63 14 2.23 6 1.76 2 2.93 2 2.53 3 7 0.15 1 0.55 3 8 0.07 0 0.26 0 2 p 75 31.807 .0005 183 14.491 .043 86 1.986 .575 177 27.295 .0005 Total Behavioral Ecology Vol. 11 No. 4 382 Table 3 Observed distribution of nest-predation events compared with the distribution expected under the assumption of random nest predation with regard to the number of occupied nests within a territory Number of occupied nests within a territory Season 1 2 3 4 5 6 1993–94 Expected Observed 21.82 22 40.72 35 29.99 27 9.55 12 5.0 11 0.91 1 1994–95 Expected Observed 23.3 23 34.7 42 22.56 22 14.25 11 1995–96 Expected Observed 37.38 45 23.45 19 11.45 10 5.72 4 1996–97 Expected Observed 26.23 23 28.56 42 31.54 26 14.95 10 4.32 4 4.49 6 occupied nest (both nests with eggs and/or young) being predated was affected by the presence or absence of nests containing young within a territory. The results of this second analysis are shown in Table 4. In three out of the four seasons nests in territories with nestlings suffered higher predation rates than expected by chance. In the season 1993–94 the difference from a random distribution was statistically significant, whereas in the seasons 1994–95 and 1996–97, despite a similar trend, there was no significant difference from the calculated random distribution. Only in the season 1995–96 were observed predation rates similar to the predation rates expected by chance. These data suggest that at least in some years sharing a territory with other females might impose costs in terms of an increased predation risk. DISCUSSION Polygyny and female nest and mate choice in birds In polygynous mating systems with female choice of mates and nesting sites, female settlement rules should depend on the Table 4 Observed distribution of nest-predation events compared with the distribution expected under the assumption of random nest predation with regard to whether there were nests with young within a territory Season Territories without nestlings Territories with nestlings Total 2 p 1993–94 Expected Observed 61.13 35 46.87 73 108 25.737 .0005 1994–95 Expected Observed 46.23 38 57.77 66 104 2.638 .104 1995–96 Expected Observed 51.72 51 26.28 27 78 0.03 .863 1996–97 Expected Observed 42.16 34 68.84 77 111 2.547 .111 3.56 2 4.05 2 7 8 1.27 0 0.46 1 0.77 1 Total 2 p 108 8.94 .111 104 2.955 .707 78 3.099 .377 111 11.616 .114 amount of material or genetic benefits provided by the males. In resource-defense polygyny, males often provide paternal care and/or food resources within the territory. The degree of polygyny in such a species will then be determined by the relative importance of these unshareable material benefits for female reproductive success. If male parental care is essential for successfully rearing young, the degree of polygyny should be low because females settling in the same territory compete for paternal care. Such competition can lead to a lower reproductive success of secondary females compared to monogamous or primary females due to reduced male assistance in feeding nestlings or nest defense, as has been demonstrated for several species (e.g., Alatalo and Lundberg, 1990; Catchpole et al., 1985; Johnson et al., 1993; Kempenaers, 1994; Smith et al., 1994; Veiga, 1990). If, on the other hand, male assistance in feeding young is limited and not essential for nestling survival, the costs for females sharing a territory with other females are low, and a higher degree of polygyny is to be expected. This expectation was confirmed by Searcy and Yasukawa (1995), who presented data on male contribution to feeding of nestlings and the degree of polygyny for 10 populations of 8 polygynous passerines. They found a negative correlation between these two variables; that is, as male contribution to feeding the nestlings decreases, the degree of polygyny increases. The relative importance of unshareable material benefits provided by males for female reproductive success should not only affect the degree of polygyny but also female matesearching behavior. In species in which females suffer high costs of being secondary females of polygynous males, they should sample males to gather information on their mating status to avoid mating with an already mated male. In species in which females suffer low costs of sharing a male’s territory with other females, they should reduce or even cease matesampling behavior; that is, they should settle randomly with respect to the available nesting sites. Lightbody and Weatherhead (1987b, 1988) proposed a ‘‘neutral mate-choice’’ model to explain polygynous mating in species where females neither suffer nor gain from sharing territories with other females. This model assumes that females settle independently of each other and independently of variation in male or territory quality. In support of this hypothesis, Lightbody and Weatherhead (1987a,b) presented data from a population of highly polygynous, marsh-nesting yellow-headed blackbirds (Xanthocephalus xanthocephalus), suggesting that (1)paternal Friedl and Klump • Female settlement in red bishops care is limited and not essential for nestling survival, and (2) females settled independently from each other and did not affect each other’s reproductive success. Similarily, Hartley and Shepherd (1995) presented evidence for random female settlement in the corn bunting (Miliaria calandra), in which polygyny is apparently not costly to females (Hartley and Shepherd, 1994). Is there evidence for nonrandom nest choice? Since in the red bishop males provide no parental care or food resources within the territories, and since the territories appear to be of rather uniform quality, both a high degree of polygyny and restricted or even absent female mate-searching behavior is to be expected (see above). In a study on determinants of male reproductive success in the red bishop, we could indeed show that the degree of polygyny is extremely high in this species, with some males mating with up to 18 females within a single breeding season (Friedl and Klump, 1999). Furthermore, we found a linear relationship between the number of nests built by the males and their reproductive success, indicating that females might settle randomly with respect to available nests within the breeding colony (Friedl and Klump, 1999). The detailed analysis of female settlement in relation to the number of vacant nests within a territory confirmed that females in three out of four seasons settled randomly with respect to the number of available nests within territories (see Table 1). The alternative possibility, that the linear relationship between the number of nests built by the males and their reproductive success is the result of females choosing territories according to an ideal free distribution (see Introduction), was not supported by our data (Table 1). We have no explanation for the strong departure of female settlement from random with regard to nest availability in the season 1996–97; there was no obvious difference between this season and the other three seasons (for example, the breeding season 1994–95 was characterized by a similar high breeding activity in terms of the number of breeding attempts; see Table 1). Evans and Burn (1996) analyzed female settlement rules in the wren (Troglodytes troglodytes), a small monomorphic passerine with high levels of polygyny. The mating system of the wren is similar to that of the red bishop; male wrens defend small territories where they build several nests to which they try to attract females, and male mating success has been found to be correlated with the number of nests built by the males (Evans and Burn, 1996). Using a similar type of analysis as we did in this study, Evans and Burn (1996) found that territories with many vacant nests were chosen significantly more frequently than expected by chance, and they concluded that females were more likely to nest on territories with large numbers of vacant nests. Furthermore, they showed that the probability of a nest being occupied by a female was independent of the number of other vacant nests on the territory (i.e., every nest had the same probability of attracting a female). As pointed out by Evans and Burn (1996), these findings could be interpreted as the result of females using a random settlement rule, similar to what we found in the red bishop. Random female mating and sexual selection As we have shown, female red bishops mostly settle randomly with respect to the number of vacant nests within the territories. What are the effects of such settlement rules on the males? It is clear that under random female settlement, males that build many nests obtain a higher mating success simply by chance. Even without any female preferences, there will be a strong sexual selection on males to build as many nests as 383 possible. The number of nests built will then be limited by energetic and time constraints, which per se will act differently on males, depending on male quality and condition. The number of nests built by red bishop males has been shown to be a good indicator of male quality; males that survived and established a territory in the following season built more nests than males that did not establish a territory in the following season (Friedl and Klump, 1999). Janetos (1980) presented a theoretical analysis of different female matechoice strategies and found random mating to be the worst strategy, with the average fitness or quality of males chosen being equal only to the mean of the male population, while other mate-choice strategies like ‘‘fixed-threshold decisions’’ and ‘‘best-of-n-males’’ consistently resulted in an average quality of males chosen above the mean of the population. However, the model assumed a mating system in which random mating results in an equal probability of every male being chosen irrespectively of male quality. This is in contrast to the mating pattern found in the red bishop, where random mating results in high-quality males having a higher probability of being chosen because they offer more nests than low-quality males. Thus, in the red bishop the average expected fitness of males chosen by females adopting a random settlement rule is above the mean of the male population, and females therefore have a good chance of getting a high-quality mate without incurring any costs of mate-searching behavior. Even without female choice there can be strong sexual selection on males. In fact, such a pattern might be common and could explain highly biased male reproductive success in many species in which males provide little or no material resources and for which there is no evidence of active female mate choice (see also Sutherland, 1985). In many bird species it has been shown that male territory size is an important factor determining male reproductive success (for review, see Andersson, 1994). Random female settlement would result in males with larger territories getting more mates and, consequently, there would be strong sexual selection for males to establish and defend breeding territories as large as possible. Because males that are able to defend large territories are probably males of good condition and/or high quality, females would have a good chance of mating with high-quality males even if they settled randomly (Lightbody and Weatherhead, 1988). Random female mate acquisition might also best explain the mating system found in many lek-breeding birds, anurans, and insects, in which lek attendance has been shown to be the most important factor determining male mating success (for review, see Andersson, 1994; Höglund and Alatalo, 1995). In lekking species males provide no material benefits but only their genes to the next generation (Höglund and Alatalo, 1995), and intrasexual selection often takes the form of endurance rivalry (see Andersson, 1994). A random female mating pattern would result in a considerable skew in male mating success, with males with the highest lek attendance obtaining most matings, an outcome that is typical for many lek-breeding species (Höglund and Alatalo, 1995). Random female mating in leks would select for males that stay at the lek as long as possible, with males of better condition and higher quality being able to attend the lek for longer than others. Random females settlement is, however, not restricted to species in which males provide little or no material resources to the females they mate with. For example, Dale and Slagsvold (1990) presented evidence for random female settlement in the pied flycatcher (Ficedula hypoleuca), a polygynous passerine in which female reproductive success depends heavily on male help in nestling provisioning (e.g., Alatalo and Lundberg, 1990), and secondary females receive less male assistance than primary females (Alatalo et al., 1982). Thus, ran- 384 dom female settlement in the pied flycatcher cannot be explained by the neutral mate-choice model proposed by Lightbody and Weatherhead (1987b, 1988), which assumes no costs of mating with an already mated male (see above). Instead, Dale and Slagsvold (1990) suggested that random female settlement in this species might best be explained by high costs of searching for a better mate. Do resident females affect further settlement? Our results demonstrated that female red bishops mostly (in three out of four seasons) settle randomly with respect to the number of vacant nests within the territory, despite the fact that they could get high-quality males if they would actively choose males with many nests. So why don’t they show a clear mating preference for males with many nests? One possible explanation could be that female settlement is restricted by female–female aggression. In polygynous species in which female reproductive success relies on male parental effort, there are conflicting interests of primary and secondary females because they compete for male assistance in nestling provisioning. Resident females should try to discourage or at least delay further settlement to obtain as much male help as possible for their own brood. Delayed settlement of secondary females has been shown to be advantageous for primary females because males of several polygynous species direct more of their parental care toward the second brood if the hatching interval between first and second brood is short (Bruun et al., 1997; Kempenaers, 1995; Lifjeld and Slagsvold, 1990; Sandell et al., 1996; Smith et al., 1994; but see Leonard, 1990). Indeed, aggressive behavior of resident females toward female intruders has been reported for numerous species (see review by Slagsvold and Lifjeld, 1994). There are other possible benefits that might select for female–female aggressiveness even in polygynous species like the red bishop, in which there is no competition among females for male parental care or food resources within the territory. For example, by discouraging other females from breeding in the same territory, nests would be spaced out more, which might reduce predation risk. Furthermore, female–female aggression might prevent intraspecific nest parasitism. On the other hand, polygyny might be beneficial to females. One possible benefit could be increased safety from nest predation due to either joint nest defense or dilution of predatorial attempts. Joint nest defense of females sharing a breeding territory against conspecific female intruders might also decrease the risk of intraspecific nest parasitism. Our analysis of female settlement in relation to the number of occupied nests within a territory in the red bishop showed that in one out of four seasons female settlement was random with respect to the number of resident females, whereas in the other three seasons territories without occupied nests were chosen less often than expected by chance (Table 2). The finding that females were more likely to nest on territories already containing another nesting female could have been the result of females avoiding territories of inferior quality. However, because all the territories in the breeding colony were rather uniformly structured patches of reedbeds and bullrush stands, and therefore territory quality appeared to be similar between males, this explanation seems unlikely. In any case, our data demonstrate that there is no negative effect of resident females on further settlement because females settled not less often than expected by chance in territories with many resident females. Thus, the fact that female red bishops did not choose males that built many nests more often than expected by chance cannot be explained by a discouraging effect of resident females on further settlement. Behavioral Ecology Vol. 11 No. 4 Is the predation risk influenced by the number of breeding females within a territory? Another possible reason for the absence of a clear female mating preference for males with many nests could be that sharing a territory with other females, and thereby increasing nest density, might increase predation risk. Indeed, in our study colony the probability of a nest being predated was significantly higher for nests in territories that contained other nests with nestlings than for nests in territories that did not contain other nests with nestlings in one season, and in two more seasons there was a nonsignificant tendency in the same direction (Table 4). Only in the season 1995–96 did nest predation events appear to be random with respect to whether or not there were other nests with nestlings within the territory. The season 1995–96 was also the only season in which we found random female settlement with regard to the number of occupied nests within a territory (see Table 2). We think that this might be due to the fact that overall breeding activity was much lower in 1995–96 than in the other three seasons, with only 86 breeding attempts in 1995–96 compared to more than 150 breeding attempts in the other seasons. Our findings of an increased predation risk for nests in territories that contain other nests with nestlings are in contrast to those reported by Picman et al. (1988) on effects of nest clumping on predation risk in marsh-nesting red-winged blackbirds. They placed groups of artificial nests with eggs either near (experimental groups) or away (control groups) from an active redwing nest and found that the experimental groups of nests suffered less predation compared to the control groups. Similarily, Westneat (1992) found in another population of red-winged blackbirds that increased breeding synchrony lowered the probability of nest predation. We think that the different effects of nest density on predation risk found in our study on red bishops compared to the studies on red-winged blackbirds by Picman et al. (1988) and Westneat (1992) are probably due to different types of predators. The main predators in the red-winged blackbird colony studied by Picman et al. (1988) were marsh wrens (Cistothorus palustris), which could be displaced through aggressive behavior by redwings (Picman et al., 1988). Predation by marsh wrens in this population should decrease with increasing nest density simply because the probability of detecting the marsh wrens is higher if more females are nesting within the same territory. At the breeding site of the redwing population studied by Westneat (1992), both mammalian predators (raccoons, Procyon lotor; short-tailed weasels, Mustela erminea; mink, Mustela vison) and avian predators (American crows, Corvus brachyrhynchos; blue jays, Cyanocitta cristata; common grackles, Quiscalus quiscula) were common. Here the decrease of predation risk with increasing breeding synchrony might be due both to a dilution effect on mammalian predators and to mutual nest defense against avian predators. At our study colony of red bishops the main predators were cape cobras. We never observed physical attacks of red bishops against cape cobras, and even intense mobbing by colony members did not prevent the cobras from robbing nests. Furthermore, cape cobras are opportunistic feeders and exploit food sources such as a red bishop breeding colony more with higher food availability (Friedl and Klump, personal observations). Therefore, predation risk in our study colony is unlikely to be reduced by mutual nest defense or dilution effects, which might explain why we found—contrary to the studies on red-winged blackbirds—an increased predation risk of nests in territories that contained other nests with young. Conclusions We have shown that in the red bishop, a colonial polygynous passerine, female settlement appears to be random (in three Friedl and Klump • Female settlement in red bishops out of four seasons) with respect to nest availability, with every vacant nest in the colony having the same probability of being chosen. Females showed no clear preference for territories of males that built many nests, although they might benefit from such a preference because the number of nests built is a good indicator of male quality (Friedl and Klump, 1999). The lack of a clear preference for territories of high-quality males could not be explained by a discouraging effect of resident females on further settlement because in three out of four seasons females settled more often than expected by chance in territories that contained already nesting females. However, it might be explained by costs of sharing a territory with other females in terms of an increased predation risk for nests if there are other nests containing nestlings within the same territory. Females therefore face a trade-off between possible benefits of breeding in territories of high-quality males in terms of high-quality offspring (if there are heritable components of male quality) and costs of sharing these territories with other females in terms of an increased predation risk. This could lead to the observed mating pattern with apparent random female settlement and male mating success being directly proportional to the number of nests built within the territory. However, female choice of a copulation partner is not necessarily restricted to the social mate (i.e., to the owner of the territory where the females started their breeding attempts). In many bird species extrapair copulations are common, and they often result in high proportions of young sired by males other than the social mate (Birkhead and Møller, 1992). Females of several species have been reported to control or even actively seek extrapair copulations (e.g., Gray, 1996; Houtman, 1992; Lifjeld and Robertson, 1992; Neudorf et al., 1997; Sheldon, 1994; Smith, 1988; Venier et al., 1993). Furthermore, females have been shown to selectively solicit extrapair copulations from males that are of higher quality than their social mates, consistent with the hypothesis that females solicit extrapair copulations to enhance the genetic quality of their offspring (Graves et al., 1993; Hasselquist et al., 1996; Houtman, 1992; Kempenaers et al., 1992; Otter et al., 1998; Weatherhead and Boag, 1995). The proportion of extrapair young in the red bishop colony we studied in Addo was 17.6%, and 30.5% of all broods contained at least one extrapair young, as determined by multilocus DNA fingerprinting (Friedl, 1998; Friedl and Klump, 1999). Since in red bishops females control copulations and male mate guarding is absent, the relatively high proportion of extrapair young highlights the potential for females to modify their mate choice by selectively seeking copulations with males other than their social mates. Thereby they could at least in part escape the limitations set by the choice of a nesting site and hence a social mate. We thank Mike Cherry and Horst Klump for support in different phases of this field project. Adrian Craig and Albert Schultz helped with mist netting in the field. We are grateful to the National Parks Board of South Africa for permission to conduct this study in the Addo Elephant National Park, and the whole park staff for continuous support. The comments of Matthew R. Evans, Ken Norris, Ulrike Langemann, and an anonymous referee on a previous version of the manuscript are gratefully acknowledged. 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