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AMER. ZOOL., 26:71-79 (1986) Niche Expansion and Contraction in a Variable Environment1 H. RONALD PULLIAM Department of Zoology, University of Georgia, Athens, Georgia 30602 SYNOPSIS. When food is abundant, sparrows are found in great variety of habitats and appear to specialize on particularly profitable types of seeds. However, during periods of food scarcity, each sparrow species occupies a very narrow range of habitats but consumes a great variety of seeds within each occupied habitat. As a result, average dietary overlap may underestimate the potential for competition for food within habitats during periods of food shortage whereas average habitat overlap overestimates the potential for competition since each species occupies a different habitat during periods of food shortage. occur in approximately the same relative abundance when food is plentiful and when it is scarce. Quite to the contrary, Schoener (1974, 1982) argues that when food is abundant, certain food types are "superabundant" and these food types are particularly attractive to all consumer species by virtue of their abundance. According to Schoener's view, when food becomes scarce diets diverge because each species is forced to specialize on the particular prey types that it consumes most efficiently. In this paper, I examine various aspects of niche expansion and contraction for several species of sparrows in a variable environment, the arid grasslands of southeastern Arizona. During the non-breeding season these sparrows eat seeds and seed production can vary by orders of magnitude between years. First, I discuss habitat utilization of sparrows during years of high and low seed production. I then discuss the relationship between changes in habitat utilization and shifts in geographic range. Finally, I examine the diet of one species, Chipping Sparrow, in detail and discuss the potential for seed size partitioning within habitats during years of seed scarcity. INTRODUCTION Many ecologists have argued that high overlap in resource utilization invariably leads to competition and competitive exclusion of inferior species. Resource levels, however, fluctuate greatly, and many species may be resource limited only during infrequent periods of resource shortage (Wiens, 1977). If two species overlap greatly when resources are abundant but diverge in resource utilization during infrequent periods when resources are scarce, any measure of average resource overlap will greatly overestimate the potential for competition. On the other hand, if resource utilization converges between species during periods of shortage, average overlap may underestimate the potential for competition. A minor controversy has arisen concerning whether dietary overlap between closely related species increases or decreases during periods of food shortage. Most of the theory and empirical results of optimal foraging studies suggest that animals are most specialized when food is most abundant (Pyke et al., 1977; Krebs et al, 1983). According to this view, when food becomes scarce animals are less selective and this results in greater dietary overlap between species. In other words, food scarcity leads to dietary convergence because all species become generalists. An implicit assumption of this argument is that food types HABITAT UTILIZATION The abundance of wintering sparrows in grassland and woodland habitats at The Research Ranch in southeastern Arizona is strongly influenced by seed production (Pulliam and Parker, 1979). The seeds consumed by sparrows are mostly produced in the fall and the magnitude of fall seed pro1 From the Symposium on Mechanistic Approaches to duction varies enormously depending on the Study ofNatural Communities presented at the Annual Meeting of the American Society of Zoologists, 2 7 - summer rainfall. Table 1 shows the fall seed production of all grasses and those forbs 30 December 1983, at Philadelphia, Pennsylvania. 71 72 H. RONALD PULLIAM TABLE 1. Seed production and sparrow densities (mean ±sumes roughly 1 kg of seeds each winter SE) in woodland habitats at The Research Ranch.' (mid-September through March). In years of low seed production (< 10 to 20 kg ha"1), sparrows consume most of the seeds proSparrow diensity Grasses Forbs' (ind ha duced; however, in many years seed production is far greater and sparrows have 1972-73 little impact on overall seed density. Based Oak woodland 10.0 0.0 7.0 + 1.4 Riparian woodland 88.9 on the relationship between rainfall and 6.1 13.0 + 3.5 seed production, Pulliam and Parker (1979) 1973-74 estimated that sparrows are locally food Oak woodland 1.2 0.9 + 0.4 tr* limited at the ranch, in the sense that the Riparian woodland 2.0 1.6 + 1.1 1.6 wintering sparrow population is large 1974-75 enough to consume over 75% of the seeds 5.4 + 1.4 Oak woodland 23.6 0.3 produced, roughly one in every five years. Riparian woodland 12.2 18.8 + 5.5 82.4 In years of low seed production, fewer 1975-76 sparrow species were found in each habitat Oak woodland 3.1 tr 2.2 + 0.7 at The Research Ranch than in years of 1981-82 high seed production (Table 2). Two vegtr Oak woodland 43.8 11.8 + 5.0 etatively distinct grassland habitats and two a Seed production was not measured in the riparian distinct woodland habitats were censused woodland in 1975-76 and no censuses were con- each year from mid-September through ducted in riparian woodland in 1981-82. b Includes Amarantkus spp., Mollugo verticillata, and mid-April for four years. A sparrow species was considered resident in a particular habPotulaca spp. c Production < 0.05 kg/ha. itat only if one or more individuals were seen in that habitat on more than half of the censuses conducted. As shown in Table known to be eaten by sparrows (see Pul- 2 there were four winter resident species liam, 1980). Based on the calculations of in the riparian woodland during the winter Pulliam and Brand (1975) and Pulliam and of 1974-75 when seed production was 82 kg ha~' but none in 1973-74 when seed Parker (1979), a typical 20 g sparrow conSeed production (kg ha-) TABLE 2. The resident sparrow species in four habitats and grass seed production (kg ha ')for those years." Grassland #1 1972-73 Seed production Resident sparrows 23.0 Ammodramus Grassland #2 Oak woodland Riparian woodland 7.2 10.0 88.9 none Spizella Spizella Pipilo r 1973-73 Seed production Resident sparrows 1973-74 Seed production Resident sparrows 0.2 — 1.2 Spizella 2.0 none none none 19.1 Ammodramus Pooecetes 6.9 23.6 82.4 Pooecetes Spizella Pipilo Spizella Pipilo Zonotrichia Pooecetes 1975-76 — 3.1 6.0 Seed production 7.6 Spizella Resident sparrows Ammodramus Ammodramus none * The sparrow species included are Grasshopper Sparrow (Ammodramus savannarum), Brown Towhee (Pipilo fuscus), Vesper Sparrow (Pooecetesgramineus), Chipping Sparrow (Spizella passerina) and White-crowned Sparrow (Zonotrichia leucophrys). 73 NICHE EXPANSION AND CONTRACTION production was only 2 kg ha~'. Comparing all 14 site-year combinations for which both seed production and sparrow densities were measured, the rank correlation between seed production and number of resident species per habitat is statistically significant (P < 0.05, Kendall's tau). Typically, each sparrow species was found to be resident in a lesser variety of habitats when seed production was lower. For example, Chipping Sparrows were resident in two habitats during each of the two years of high seed production (1972— 73 and 1974-75) but resident in only one habitat during each of two years of low seed production (1973-74 and 1975-76). Similarly, Vesper Sparrows were absent from the study site in all years except 197475, the year of the greatest seed production when they were resident in three different habitats. Seed production in arid grasslands is closely correlated with late summer rainfall (Pulliam and Parker, 1979) and Figure 1 shows the overall relationship between the number of habitats occupied by each species and total summer (JuneOctober) rainfall. In years of high seed production, relatively more sparrows stay in the northern portion of their wintering range (Pulliam and Parker, 1979). My study sites at The Research Ranch in southern Arizona are near the northern edge of the wintering range of a large population of Chipping Sparrows. The wintering range of the Western Chipping Sparrow (Spizella passerina arizonae) consists of two disjunct regions, one on the far west coast, extending from central California to northern Baja California and the second beginning in central Arizona and extending south into the Sierra Madre mountains in Mexico. The entire wintering population that inhabits the Sierra Madre Occidental of northwestern Mexico probably migrates through southern Arizona or southwestern New Mexico. Grass seed production in the oak woodland site was only 1 kg ha~' during the fall of 1973 as compared to 24 kg ha" 1 in the same site during the fall of 1974. Chipping Sparrow density in the winter of 1973-74 was only 0.8 individual ha~' as compared O Chipping Sparrow I3 A Brown Townee • Grasshopper Sparrow X Vespers Sparrow 200 225 -LXA250 275 Summer precipitation 300 L_ 325 FIG. 1. The range of habitats occupied by sparrow species expands during years of high rainfall and high seed production. to 5.1 in 1974-75. During February 1974, Parker and I censused sparrows along a 15 km route in Nonoava, Chihuahua (27°28'N) which is in the southern portion of the winter range of 5. passerina arizonae and along nearly 34 km near Mesa Tres Rios, Sonora (29°52'N) in the central portion of the wintering range. We relocated and recensused the same routes in February of 1975. Because the habitats are quite different in the different portions of the wintering range, I do not compare sparrow densities between sites but only at the same sites, comparing sparrow populations in years of high and low seed production. The migratory habit and winter distribution of Chipping Sparrows respond to variation in seed production in the northern portion of the wintering range. As shown in Figure 2, in contrast to the increase in the density of Chipping Sparrows in the northern portion of the wintering range, when seed production was high, the relative density of Chipping Sparrows in the southern portion of the wintering range was sharply reduced. I compared the number of 0.5 km segments of the census routes that had more sparrows in 1974 to the number that had more in 1975 and found the difference between years at Nonoava statistically significant. There was, however, no statistically difference in the relative Chipping Sparrow densities for the two years at Mesa Tres Rios in the central portion of the wintering range. The overall pattern indicates a shift 74 H. RONALD PULLIAM 10 >, <7i c 8 CD The Reseach Ranch, Arizona |f 6 (31° 2 8 ' ; I? 4 High seed production (1974-75) search of a suitable habitat when food is scarce. In years of high seed production, sparrows occupy a greater variety of habitats and tolerate greater population densities in each habitat in the northern portions of the wintering range. In years of low seed production individuals are apparently more selective and subsequently travel farther south in search of an appropriate habitat. DIETARY EXPANSION Sparrows wintering at The Research 125 Ranch consume seeds produced in the late summer and early fall and the abundance 100 of these seeds declines during the winter Mesa Tres (Pulliam and Brand, 1975). As discussed 75 (32) Rios, Sonora below, Chipping Sparrows consume a (29°52') 50 greater variety of seed species towards the end of winter after they have depleted the 25 supply of preferred seed species. Almost (35) 0 all of the seeds fall from the plants onto Feb. 1975 the ground by late fall and the sparrows at I 300 the ranch forage almost exclusively on the w ground, only rarely taking seeds from seed 250 c heads still on the plants. Nonoava, I 200 In mid-January and again in early March Chihuahua o (27° 28') of 1975,1 caught Chipping Sparrows in an 150 oak woodland at the ranch and gave each an oral emetic to induce regurgitation of 100 the crop contents (Pulliam, 1980). On the (29) day following each emetic sample, soil sam50 o (I) ples were collected for subsequent analysis Feb. 1975 0 Feb. 1974 of seed densities in the soil. Table 3 lists FIG. 2. The geographic wintering range of Chipping the common seed species identified in the Sparrows expands during years of low seed produc- diet samples. The list accounts for approxtion. Seed production in the fall of 1974-75 at The imately 95% of the seeds consumed in both Research Ranch was more than ten times greater than it was in the fall of 1973-74. More Chipping Sparrows January and March. For the January diet over-wintered at the ranch and fewer showed up in sample, the weighted average seed size was the southern portions of the wintering range when 0.2 mg and the usable energy content was seed production was high at the ranch. approximately 0.8 cal per seed. Because an active Chipping Sparrow in mid-winter uses upwards of 15 kcal day"1, each individual southward of the sparrow population when bird requires about 20,000 seeds per day. seed production is low in the northern end Based on 10 hr of available daylight in midwinter, a Chipping Sparrow must locate of the wintering distribution. and consume one seed every 1.5 to 2.0 secThe southward shift in sparrow populations during years of low seed production onds just to meet its daily energy requireappears also to occur for species other than ments. Chipping Sparrow (Pulliam and Parker, Chipping Sparrows may meet their daily 1979). The shift is apparently the result of requirements either by consuming very individuals having to travel farther in large numbers of small, very abundant 75 NICHE EXPANSION AND CONTRACTION TABLE 3. Characteristics of seeds eaten by Chipping Sparrows. Weight (mg) Husking time (sec) I. Unarmoured seeds Amaranlhus sp. Eragrostis lehmanniana Mollugo verticillata Portulaca oleracea P. retusa 0.05 0.11 0.06 0.12 0.19 0.2 0.6 0.6 0.9 0.6 3.6 19.3 37.6 1.5 28.6 II. Armoured seeds Aristida divaricata A. hamulosa Bouleloua chrondrosiodes B. curtipedula B. gracilis Hackelochloa granularis Leptochloa dubia Panicum obtusum Schkuhria wislezeni Trichachne californica 0.62 0.54 1.00 0.63 0.39 0.63 0.52 1.00 0.81 0.49 3.2 2.8 1.1 3.9 0.9 2.3 0.1 1.3 -— 1.4 1.3 — 1.2 4.5 4.7 2.3 Density (x ± SE) ± ± ± ± ± 2.5 4.7 15.0 0.4 9.2 ± 0.4 ± 2.5 ± 0.6 ± 0.6 ± 0.1 ± 0.6 0.0 0.9 ± 0.3 1.7 ± 0.7 0.1 ± 0.1 Variance/mean 48.8 32.7 168.0 2.5 83.5 4.0 45.1 11.4 3.8 0.9 7.7 — 3.0 8.6 1.0 * Unarmoured seeds are small and have very short husking times. Armoured seeds are larger, have much heavier husks and require more time to husk. The seed density data is for the January sample and seed density is given in seeds per 100 cm8. seeds or by searching for less abundant, but more profitable, larger seeds. Table 3 divides the seeds eaten by Chipping Sparrows into two categories. The so-called "armoured" seeds are all relatively large (0.4 to 1.0 mg) and all have heavy husks and often have conspicuous hairs, awns or other projections from the husks. Armoured seeds are always individually husked and each seed requires 1 to 5 seconds husking time. The "unarmoured" seeds are all very small (0.1 to 0.2 mg) and most are very abundant. Handling times for unarmoured seeds are very small (0.2 to 0.9 sec) and some species are frequently swallowed unhusked (see Pulliam, 1980). As shown in Table 3, most seed species have a clumped dispersion pattern (variance to mean ratio greater than 1.0). This is particularly true of unarmoured seeds that tend to be found in dense clumps at the base of the plants that produced them. Furthermore, different species of unarmoured species tend to co-inhabit disturbed areas with the result that their seeds are positively associated in the soil samples. For example, Mollugo verticillata and Portulaca retusa, the two most common unarmoured seeds, were common in the same soil samples (r = 0.87, n = 28, P < 0.01). The armoured seeds are more widely dispersed, probably because they are borne on relatively tall stalks and more prone to wind dispersal. Small, unarmoured seeds account for 93% by number and 68% by weight of all seeds found in Chipping Sparrow diets in January. As shown in Figure 3, by March Chipping Sparrows had shifted towards eating more large, armoured seeds. In the March sample, the unarmoured seeds still accounted for 89% of the seeds in the diet by number but only 39% of the seeds by weight. The data presented in Figure 3 suggest that by March, some individuals were specializing on armoured seeds. As shown in Figure 4, there were strong positive correlations between pairs of armoured seeds in the sparrow diets. Interestingly, none of these seeds species were significantly correlated in the soil samples. These data indicate that, whereas some individual sparrows were avoiding the armoured seeds altogether, those that ate any armoured seeds tended to eat a lot of several different species. The shift away from specializing on clumps of unarmoured seeds and towards eating more armoured seeds was accompanied by an increase in the variety of seeds 76 H. RONALD PULLIAM IO JANUARY ."2 0 0 10 20 30 40 50 60 70 80 90 100 10 JANUARY SAMPLE MARCH (n = l6) 0 10 20 30 40 50 60 70 80 Percent unormored seeds in diet 90 100 FIG. 3. Chipping Sparrows in Oak woodland habitat ate mostly unarmoured seeds; however, after the unarmoured seeds were depleted, they consumed more armoured seeds. consumed. Table 4 lists seven seed species that were eaten in March that were not found in the January diet. At least two of these species, Hackelochloa granularis and Schkuhria wislezeni were very abundant in the soil samples on both dates and are unlikely to have been missed due to sampling error alone (see Pulliam, 1980). This dietary expansion coincides with a decline in the abundance of seeds in the soil. The density of all seeds known to be eaten by sparrows in the January soil samples was 105.2 seeds 100cm~2. The density of these same seeds in the March soil samples was only 69.4 seeds 100 cm~2, a decline of 34%. During this period the density of unarmoured seeds declined by 40% as compared to a decline of only 5% in armoured seeds. Unarmoured seeds of the more common species disappeared more rapidly from areas with dense concentration of these seeds than from areas were the same seed species occurred in lesser concentrations. This pattern of seed disappearance presumably resulted from patch choice by sparrows. Figure 5 shows the distribution of the number of seeds per soil sample for the two most common seed species, Por- MARCH SAMPLE FIG. 4. Sparrows that ate a lot of any one species of armoured seeds tended to eat a lot of other species as well. The numbers are correlation coefficients between the abundance of one species of seed and the abundance of a second species in the sparrow diet samples. None of these seed species are significantly correlated in the soil samples. tulaca retusa and Mollugo verticillata. In both cases, the soil samples with the most seeds of the given species contained a far smaller proportion of the total seeds of that type in March than in January. Since the seeds of the unarmoured seed species tended to be positively associated in the soil samples, I also looked at the frequency distribution of unarmoured seeds of all types combined in the soil samples. In January, 6 of 28 soil samples contained more than 150 unarmoured seeds each and 60% of the unarmoured seeds occurred in samples with more than 150 unarmoured seeds each. In March only 1 of 28 samples contained more than 150 unarmoured seeds and only 19% of the unarmoured seeds occurred in samples with more than 150 seeds each. NICHE EXPANSION AND CONTRACTION 77 TABLE 4. Seed species eaten by Chipping Sparrows in March but not found in the January diet samples. Bouteloua gracilis Croton sp. cf. Cyperus sp. Hackelochloa granularis Paspalum sp. Schkuhria wislezeni Talinum sp. These data indicate that Chipping Sparrows consumed a greater variety of seed species as the density of seed declined. One way of assessing the magnitude of such a dietary expansion is to compare the energetic value of the seeds being consumed to the rate of food intake required to maintain a positive energy balance. Seed value is measured as the usable energy content (cal) or mass (mg) of the seed divided by the time (sec) required to husk the seed (handling time). An "energetically profitable" seed is one whose seed value (cal sec"1 or mg sec"1) exceeds the rate of food intake required to meet the animals' 24 hr energy requirements. If all profitable seeds encountered are consumed, the diet is said to be "fully expanded." Pulliam (1980, 1985) calculates that during a typical 10 hr mid-winter day, a Chipping Sparrow requires approximately 0.15 mg sec"1 seed intake while foraging in order to meet its 24 hr energy requirements. In the March dietary sample, Chipping Sparrows were consuming 14 of the 15 seed species with known seed value exceeding 0.15 mg sec"1 and none of the 4 seed species with seed value of less than 0.15 mg sec"1. This suggests that by the end of the season the diets of Chipping Sparrows were fully expanded in the sense of eating all energetically profitable seeds. The dietary expansion observed for Chipping Sparrows indicates that sparrow diets are flexible and respond in a predictable fashion to variation in food supply. In a year of moderately low seed production (~ 25 kg• ha"•), Chipping Sparrow diets were fully expanded by the end of the season. Though there is no data on Chipping Sparrow diets in a year of very low seed production, it is reasonable to assume that I 5 10 15 20 Sample number (rank ordered) 25 FIG. 5. The distribution of the number of seeds per sample of the two most common unarmoured seed species show that they tended to occur in dense clumps. Notice that the tails of the two curves are very similar, but that fewer soil samples in March contain large numbers of seeds. This pattern suggests that sparrows concentrate their foraging effort on the larger clumps of seeds. dietary expansion would occur even more rapidly and the sparrows would consume all energetically profitable seeds encountered for most of the winter months. DISCUSSION According to optimal foraging theory, animals should specialize on the most profitable prey types when these are abundant but accept less profitable prey when more profitable ones are scarce. If, during periods of scarcity, all food types are roughly equally reduced in abundance, each consumer species should eat a greater variety of prey, resulting in greater dietary 78 H. RONALD PULLIAM overlap between consumer species. However, as pointed out by Schoener (1974, 1982), if during periods of abundant food, some prey types become "superabundant" relative to others, several consumer species may all specialize on the same superabundant prey, leading to potentially greater overlap during periods of plenty. The dietary preferences of sparrows in nature appear to be influenced by both the energetic values of seeds and by seed abundance and spatial distribution. In laboratory preference experiments, Chipping Sparrows prefer seeds weighing about 0.5 mg to seeds of all other sizes (Pulliam, unpublished data). However, seeds of this size are not particularly well represented in Chipping Sparrow diets in nature, whereas much smaller seeds (0.1 to 0.2 mg) are over-represented in natural diets. Though these small unarmoured seeds have relatively low energetic value individually, Chipping Sparrows can apparently achieve high rates of intake by concentrating on dense patches of them. The abundance of small, unarmoured seeds varies widely from year to year. Table 1 gives an indication of the magnitude of this variation, since most of the small, unarmoured seeds are forbs and most of the larger, armoured seeds are grasses. Though forb seeds are usually most abundant in years of high seed production, this is not always the case. Most of the common forbs flower earlier than do the most common grasses. If the summer rains start early, forb seed production is very high but late summer rains can result in low forb seed production and relatively high grass seed production. Very early rains followed by a relatively dry late summer and early fall can result in superabundant forb seed production and relatively low grass seed production. Dietary overlap between Chipping Sparrows and other sparrow species is likely to be particularly low during years of superabundant forb seed production. This is because most forb seeds are too small to be profitably consumed by larger sparrow species. For example, Pulliam (1985) calculates that under typical mid-winter conditions, White-crowned Sparrows (23 g) cannot profitably consume seeds smaller than 0.25 mg. Chipping Sparrows (13 g) on the other hand can profitably consume seeds as small as 0.1 mg and, as already discussed, appear to specialize on these small seeds when they are abundant. During years of abundant seed production two or more sparrow species occupy oak woodland habitat at The Research Ranch, but as shown in Table 2, during years of low seed production, only Chipping Sparrows are resident in oak woodlands. Chipping Sparrows are particularly well adapted to oak woodlands because they can consume virtually the entire range of seed sizes produced there. Chipping Sparrows can profitably consume most seeds in the size range of 0.1 to 1.0 mg (Pulliam, 1985) and only about 5% of the grass and forb seeds produced in oak woodland weigh more than 1.0 mg. Thus, Chipping Sparrows can profitably consume most of the seeds available to larger sparrow species, in addition to being able to consume seeds too small to be profitably eaten by the larger sparrow species. I postulate that during periods of food scarcity, each sparrow species specializes on a few habitats where it can outcompete all other sparrow species. The suitability of a particular habitat depends, in part, on the sizes of seeds produced there, as discussed for Chipping Sparrows in oak woodland. Habitats, no doubt, differ in other ways that make them more or less suitable for a particular sparrow species. Chipping Sparrows, for example, require habitats where foraging sites are in close proximity to trees and shrubs to which they retreat when disturbed by predators. Grasshopper Sparrows, on the other hand, are most common in open grasslands with widely scattered trees and shrubs. Pulliam and Mills (1977) argue that differences in crypticity and social behavior adapt different sparrow species to habitats with very different vegetative characteristics. If each sparrow species is best adapted to a few habitats where it can outcompete all other sparrow species, why do they occupy other habitats during periods of food abundance and only specialize on favored habitats during periods of food NICHE EXPANSION AND CONTRACTION shortage. As previously discussed, The Research Ranch is near the northern end of the wintering distribution of several sparrow species. When seed production is high, more individuals can be supported in prime habitats in the northern portion of the wintering distribution. If seeds are sufficiently abundant, feeding rates will be more than adequate to meet energetic and nutritional requirements of sparrows in a variety of otherwise suboptimal habitats. In this case the benefit to be derived from finding a slightly better wintering habitat farther south might not compensate for the cost of migration. The result would be a greater concentration of individuals in a greater variety of habitats in the northern end of the wintering distribution. During periods of low seed production, Chipping Sparrows consume virtually all of the seeds produced in occupied habitats at The Research Ranch. Greater densities of overwintering sparrows in the northern end of the wintering distribution would undoubtedly result in over-exploitation of the available food and high mortality of sparrows. In this case the cost of migration is bound to be more than compensated for by an increase in survival probability. As pointed out by Pulliam and Parker (1979), rainfall, and presumably seed production, is more reliable and the density of potential competitors is lower in the southern portion of the wintering range of Chipping Sparrows. Sparrow population in arid grasslands appear to be food limited only during occasional years of unusually low seed production (Pulliam and Parker, 1979). As discussed in this paper, each sparrow species appears to occupy a narrow range of habitats and to consume a wide variety of seeds within each habitat during periods of food scarcity. Since sparrows appear to specialize on particularly profitable seeds when food is abundant, an estimate of average dietary overlap would underestimate the potential for competition for food within habitats. On the other hand, an estimate of average habitat overlap would overesti- 79 mate the potential for competition, since habitat overlap appears to be lowest when food is scarce. For many species of animals, resource utilization is highly flexible and responds adaptively to changes in resource abundance and distribution. For all such species, average niche overlap may be a poor index of competitive interactions. Resource utilization must be studied under a variety of conditions with special attention paid to the utilization of limiting resources during periods of resource scarcity. ACKNOWLEDGMENTS Portions of the research reported here were supported by DEB-8206936 and previous grants to the author from the National Science Foundation. I thank Craig Benkman, Mary Price, Tom Schoener, Gary Grossman, John Glasser and Graham Pyke for useful comments on an earlier version of this paper. Finally, I express my appreciation to the sparrows and woodlands at The Research Ranch. REFERENCES Krebs.J. R.( D. W. Stephens, and W. J. Sutherland. 1983. Perspectives in optimal foraging. In A. H. Brush and G. A. Clark, Jr. (ed.), Perspectives in ornithology. Cambridge University Press. Pulliam, H. R. 1980. Do Chipping Sparrows forage optimally? Ardea 68:75-82. Pulliam, H. R. 1985. Foraging efficiency, resource partitioning and the co-existence of sparrows. Ecology 73. (In press) Pulliam, H. R. and M. R. Brand. 1975. The production and utilization of seeds in plains grassland in southeastern Arizona. Ecology 56:1158-1166. Pulliam, H. R. and G. S. Mills. 1977. The use of space by sparrows. Ecology 58:1393-1399. Pulliam, H. R. and T. A. Parker, III. 1979. Population regulation of sparrows. Fortschritte der Zoologie 25:137-147. Pyke, G. H., H. R. Pulliam, and E. L. Charnov. 1977. Optimal foraging: a selective review of theory and tests. Quart. Rev. Biol. 52:137-154. Schoener, T. W. 1974. Resource partitioning in ecological communities. Science 185:27-38. Schoener, T. W. 1982. The controversy over interspecific competition. Amer. Sci. 70:586-595. Wiens, J. A. 1977. On competition in a variable environment. American Scientist 65:590-597.