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EMU Vol. 91, 164-171, 1991
O Royal Australasian Ornithologists Union 1991
0158-4197/91/03164 + 7 $2.00
Received 25-6-1990, accepted 9-11-1990
Anti-predator Behaviour in the
Bell Miner Manorina melanophrys
Aldo Poiani
Department of Zoology, La Trobe University, Bundoora, Vic. 3083
Summary: Data on anti-predator and anti-competitor behaviour in the Bell Miner Manorina rnelanophrys were collected opportunistically during two breeding seasons at
Healesville, south-eastem Victoria, and show that: (a) nest
predators were mobbed by more Bell Miners than were competitors; (b) in general, nest predators were mobbed by more
birds within a dense than within a less dense colony; (c) this
difference in the mobbing response between colonies does
not appear to lead to an increase in reproductive success in
dense colonies; (d) intruders were mobbed by more birds
early than late in the season; and (e) potential nest predators
were mobbed by slightly more Bell Miners than were competitors when an active nest was close by. Mobbing behaviour in the Bell Miner may decrease the risks to individuals engaged in anti-predator defence and helpers' involvement in mobbing may benefit breeders through the 'dilution'
of those risks.
Introduction
Miner. Swainson (1970) reported that 3-12 birds participated in a distraction display against a human intruder
at an active nest. Smith & Robertson (1978) showed
that small intruders (presumably potential competitors)
are pursued by a single Bell Miner, often beyond the
boundaries of its territory, while potential nest predators
such as currawongs Strepera spp., Laughing Kookaburras Dace10 novaguineae and ravens Corvus spp. are
mobbed by up to 12 birds from more than one breeding
unit. Clarke (1984) confirmed observations made by
Smith & Robertson (1978) that bird species which forage in the understorey are not attacked, while a Laughing Kookaburra was mobbed by a group of nine Bell
Miners from more than one breeding unit.
Here I present field observations on mobbing and
other anti-predator behaviour and on attacks against potential competitors by the Bell Miner. My aim was to
test whether the number of Bell Miners engaged in an
attack against any intruder (predator or competitor) varied according to intrinsic and extrinsic factors such as
how dangerous was the intruder for the Bell Miners and
the period of the breeding season.
Anti-predator behaviour in birds can take the form of
either attack (Curio 1975) or distraction display (Armstrong 1956) by a single individual, or mobbing by
more than one bird of the same or different species
(Curio 1978).
The pattern of anti-predator behaviour is affected by
the kind of predator being mobbed, the season (e.g.
breeding vs non-breeding), the period within the season
(e.g. early vs late), the spatial distribution of potential
mobbers with respect to the predator (e.g. colonial vs
non-colonial species), the developmental stage of the
nest contents (when attacks or displays are part of the
nest defence) among others (Curio et al. 1978; Shalter
1978; Robinson 1985; Zimmermann & Curio 1988).
The participants in anti-predator defence may accrue
benefits in terms of enhanced present and/or future fitness by evicting a predator from their breeding areas or
territories (Curio 1978). However, they are also running
risks (of injury or death) which represent the costs of
the anti-predator behaviour (Curio & Regelmann 1985;
see also Montgomery & Weatherhead 1988). Even
though Hennessy (1986) has recently questioned the
costs of anti-predator defence, both direct and indirect
evidence in its favour are increasing (Curio & Regelmann 1986; Poiani & Yorke 1989; see Sordahl 1990 for
a recent review).
The Bell Miner Manorina melanophrys is a cooperatively breeding, multi-brooded honeyeater that lives in
loose colonies of variable sizes (Smith & Robertson
1978; Clarke 1988). There have been few studies of
mobbing and other anti-predator behaviour in the Bell
Methods
Data were collected opportunistically during a study of
a population of Bell Miners in the Sir Colin Mackenzie
Zoological Park at Healesville, 60 km east of Melbourne, between November 1988 and April 1990,
which covered nearly two breeding seasons (August to
April at Healesville).
The observations suffer from two major biases.
First, a predator's or competitor's intrusion into the Bell
Poiani: Bell Miner Anti-predator Behawour
165
Miners' territory may not have been noticed unless the
intruder was mobbed or otherwise chased away. The
presence of raptors, however, was generally evident to
the observer even when they elicited no response from
the Bell Miners. Second, the effort to record anti-predator behaviour was spread unevenly through the breeding
season. Hence, the data were analysed for two periods,
early-mid and late season, instead of month by month.
A predator (or competitor) may perceive the intensity of the mobbing response in many ways. For instance,
it may be perceived as more intense if many of the potentially available mobbers actually attack the predator.
Once the mobbing group has formed, a predator may
perceive a further increase in the intensity of mobbing
as the mobbers get closer or increase the frequency of
swoops, and/or the frequency of alarm calls. In this
study I measured the intensity of the mobbing response
only by the number of mobbers engaged in the attack.
For each occasion on which one or more Bell Miners attacked an intruder (whether a predator or competitor) data were recorded for the following six variables
and their effects analysed: (a) period of the breeding
season (early-mid or late); (b) density of the colony
where the event occurred (dense or sparse); (c) number
of Bell Miners involved in the attack; (d) whether the
intruder was flying or perched; (e) whether the attack
took place within 15 m of an active nest; and (f) identity
of the intruder.
Unless otherwise specified data were analysed by a
two-way Analysis of Variance to examine differences
between means. A d(x + 0.5) transformation of data
was used to make the variance independent of the mean.
Statistical analyses follow Sokal & Rohlf (1981) and
Conover (1980), tests are all one-tailed; means standard deviations are given throughout.
+
Results
Table 1 lists the intruding species considered in the present study, classified according to their potential relationship with the Bell Miners, i.e. nest predators, predators of adult birds and competitors. Competitors had
feeding habits similar to those of Bell Miners, a high
probability of being harassed by Bell Miners (Smith &
Robertson 1978; Loyn et al. 1983), and a positive response towards a Bell Miner's abandonment of a former
territory (Clarke 1984).
Period of the breeding season
The breeding season of the Bell Miner at Healesville
Table 1 Potential predators and competitors of Bell Miners considered in this study.
Species
Common name
Family
(a) POTENTIAL NEST PREDATORS
Gymnorhina tibicen
Australian Magpie
Cracticidae
Strepera graculina
Pied Currawong
Cracticidae
Cracticus torquaius
Grey Butcherbird
Cracticidae
Corvus coronoides
Australian Raven
Cowidae
Dace10 novaeguineae
Laughing Kookaburra Alcedinidae
Varanus varius
Lace Monitor
Varanidae
Antechinus stuartii
Stuart's Antechinus
Dasyuridae
(b) POTENTIAL PREDATORS OF ADULT BIRDS
Accipiter cirrhocephalus
Collared Sparrowhawk Accipitridae
A. fasciatus
BrownGoshawk
Accipitridae
Aquila audax
Wedge-tailed Eagle
Accipitridae
Falco longipennis
Australian Hobby
Falconidae
(c) POTENTIAL COMPETITORS
Alisterus scapularis
King Parrot
Psittacidae
Platycercus eximius
Eastern Rosella
Platycercidae
F elegans
Crimson Rosella
Platycercidae
Anthochaera carunculata
Red Wattlebird
Meliphagidae
extended from August to April. The frequency distribution of nests built showed a maximum between November and February, while virtually no nests were built between May and July. The sample sizes for early and
mid-season separately were small, so I compared the
data for the period August to February with data from
the late season (March to April). Data from the 1988-89
and the 1989-90 breeding seasons were pooled in order
to increase the sample size.
The number of mobbers in the early-mid and late
season was compared. Colony density did not have an
independent effect on the number of Bell Miners attacking any of the three kinds of intruders considered (Tables 2 & 3), while the time of the breeding season had
an independent effect only for Pied Currawongs (P <
0.01, Table 3b), mobbed by more Bell Miners in the
early-mid periods of the season. Also a marginally significant interaction between colony density and season
is shown for the Pied Currawongs (Table 3b) with the
difference between early-mid and late season being
greater in the sparse colony than in the dense colony.
When all intruders are considered together (Table 3d),
time during the breeding season is the only factor statis-
EMU Vol. 91, 1991
166
Table 2 The number of Bell Miners mobbing Laughing Kookaburras, Pied Currawongs and competitors at different colony density
and at different times of the breeding season. Data shown are
mean f s.d. (sample size).
Early-mid season
Late season
(a) LAUGHING KOOKABURRA
Table 3 Two-way Analysis of Variance for the effect of time during
the breeding season and colony density on the number of Bell
Miners mobbing Laughing Kookaburras, Pied Currawongs and
psittaciform competitors.
Source of variation
d.f.
M.S.
F
P
(a) LAUGHING KOOKABURRA
Sparse colony
1.84f1.14(13)
1.88f1.36 (9)
Colony dens~ty
1
0.23
3.38
> 0.05
Dense colony
2.92 f 2.69 (13)
2.00 f 1.80 (9)
Stage of the BS1
1
0.14
2.05
> 0.10
0.13
1.91
> 0.10
Two-way interaction colony
density x stage of BS
(b) PlED CURRAWONG
Sparse colony
3.44 f 1.42 (9)
1.66 f 2.08 (3)
Dense colony
3.44 f 2.65 (9)
2.33 f 0.57 (3)
(c) PSllTAClFORM COMPETITORS
Residual
(b) PlED CURRAWONG
1
0.00
0.00
> 0.75
1
0.72
12.00
c 0.001"*
2.00 f 1.41 (7)
1.OO f 0.00 (2)
Stage of the BS
Dense colony
2.14 f2.19 (7)
1.50 f 0.70 (2)
Two-way interaction colony
density x stage of BS
Sparse colony
2.37 f 1.44 (29)
Dense colony
2.89 2.52 (29)
+
0.068
Colony dens~ty
Sparse colony
(d) LAUGHING KOOKABURRAS, PlED CURRAWONGS AND
PSITTACIFORM COMPETITORS
1
172
Residual
1
92
0.19
3.16 0.10 > P >0.05
0.06
1.71 f 1.38 (14)
(c) PSITTACIFORM COMPETITORS
2.00f 1.46 (14)
Colony density
1
0.00
0.00
> 0.75
Stage of the BS
1
0.11
1.83
>0.10
1
0.03
0.50
> 0.25
68
0.06
tically significant, indicating that more mobbers participated in anti-predator attacks in the early-mid than in
the late season.
Effect of colony density
Data were recorded within two zones (A and B) of the
Park, with zone A occupied by denser colonies than
zone B. Nests of different females in zone A were built
closer to each other than in zone B (Mann-Whitney Utest T I = 9.11, n = 21, m = 16, P < 0.001, Conover
1980). Females in zone A tended to have more attendants at nests with chicks 2-5 days old than females of
zone B (2, = 5.5 f 4.0 v s fB = 3.6 f 1.2
attendantsJnest); Mann-Whitney U-test T I = 1.23, n = 6,
m = 6, 0.11 > P > 0.10. Only attendants accumulated
during the first two hours of observation were included
in order to keep the observation effort constant. None of
the attendants considered was included in more than
one nest.
The number of Bell Miners engaged in each attack
against an intruder was compared between zones for the
early-mid and late parts of the breeding season. All the
effects of colony density and kind of intruder plus the
interaction are significant ( P c 0.01, Tables 4a & 5a).
This indicates that both predators and competitors were
mobbed by more Bell Miners in a dense colony than in
Two-way interaction colony
density x stage of BS
Res~dual
d) LAUGHING KOOKABURRAS, PlED CURRAWONGS AND
PSITACIFORM COMPETITORS
Colony density
Stage of the BS
Two-way mteraction colony
density x stage of BS
Residual
> 0.10
1
1
0.15
0.80
2.14
11.42
< 0.01"
1
0.00
0.00
> 0.75
340
0.07
a less dense one. In this case all the species included in
Table 1 were considered. Nest predators were mobbed
by more Bell Miners than were competitors, and competitors were mobbed more intensely when intruding
into a sparse colony whereas nest predators were
mobbed more intensely in a dense colony. However, the
percentage of nests lost by predation was greater in the
denser colony than in the less dense one (41.2%, 68
nests laid in vs 30.0%, 60 nests laid in) but this difference is not significant (G test for independence: Gad,=
1.70, P > 0.10).
Mobbing vs attack by a single bird
The frequencies of mobbing episodes and attacks by a
Poiani: Bell Miner Anti-predator Behaviour
167
Table 4 The effect of: (a) colony density; (b) intruder perched or
flying; and (c) vicinity of an active nest on the number of Bell Miners mobbing nest predators and competitors. Data shown are
mean f s.d. (sample size).
Nest predator
Table 5 Analysis of Variance for effects of: (a) colony density; (b)
intruder perched or flying; and (c) vicinity of active nest on the
number of Bell Miners mobbing nest predators and competitors.
Source of variation
d.f.
MS.
F
P
Competitor
(a) COLONY DENSITY
(a) COLONY DENSITY
Dense
3.40 f 3.27 (42)
1.38 It 1.57 (18)
Sparse
2.09 f 1.42 (42)
1.55 k 1 .09 (18)
(b) INTRUDER PERCHED OR FLYING
Colony density
1
0.83
Kmd of intrude
1
3.29
1
0.97
476
0.07
Two-way interaction colony
density x kind of intruder
Perched
2.23 f 1.37 (21)
2.00 ? 1.58 (9)
Residual
Flying
1.66 f 1.49 (21)
1.OO ? 0.70 ( 9)
(b) INTRUDER PERCHED OR FLYING
(c) VICINITY OF ACTIVE NEST
Close to nest (2 15 m)
3.88 f 4.37 (18)
Farfromnest(>15m)
2.61f2.14(18)
1.46 ? 1.68 (15)
1.66?1.11 (15)
single bird were compared for nest predators and competitors using a Binomial test. Nest predators were more
frequently mobbed than attacked by a single bird: 55 instances vs 23 instances (P < 0.05) while there was no
difference for competitors: 15 vs 14 (P > 0.05).
Effect of the intruder's behaviour
I compared the number of mobbers engaged in attacks
against flying or perching intruders (nest predators or
competitors). Intruders were mobbed more intensely
when they perched than when they flew through the
colony (P < 0.01, Tables 4b & 5b). This trend is not affected by the kind of intruder (nest predators or competitors, interaction: P > 0.05, Table 5b).
Effect of the vicinity of an active nest
The numbers of mobbers engaged in attacks against intruders more or less than 15 m from an active nest were
compared. This was the distance that an avian predator
might perch in a tree just above a nest in the understorey. Nest vicinity by itself did not have an effect on
the number of mobbers (P > 0.05, Table 4c & 5c). However, an almost significant interaction between the two
factors suggests that the difference in the number of
Bell Miners mobbing nest predators and competitors
was slightly greater when an active nest was close by
than when the nest was far away (Table 5c).
Kind of potential predator
Predators were divided into: (a) predators of adult birds
(e.g. raptors); (b) avian predators of nests (e.g. curra-
Perched or flying
1
0.84
Kind of intruder
1
0.31
Two-way interaction perched1
flying x kind of intruder
1
0.00
Residual
0.04
236
(c) VICINITY OF ACTIVE NEST
Vicinity
1
0.00
Kind of intruder
1
2.68
1
0.39
260
0.11
Two-way interaction
vicinity x kind of intruder
Residual
wongs); and (c) terrestrial predators of nests (e.g. monitor lizards).
I did not carry out statistical analysis for raptors and
terrestrial predators of nests since for the former the
Bell Miners' behaviour was invariable and for the latter
I had insufficient data. It was possible to carry out a
one-way Analysis of Variance (with unequal sample
sizes) for some of the avian predators of nests.
Potential avian predators of nests were mobbed by
an average of 2.73 Bell Miners (range 1-15) while raptors were never mobbed. I recorded 18 instances in
which a raptor intruded into a colony, especially flying
at or below the level of the canopy, but on only one occasion I recorded a Brown Goshawk being dive-bombed (once) by a single Bell Miner. The Goshawk was
perching about 20 m from the ground. In all other cases
the Bell Miners uttered an alarm call in chorus until the
raptor was out of sight. They did not call if the raptor
was flying well above the canopy nor when a Wedgetailed Eagle flew level with the top of the canopy on 11
March 1990.
There was no significant difference in the number of
EMU Vol. 91. 1991
168
Table 6 One-way Analysis of Variance for the effect of the nest
predator's species (Laughing Kookaburra, Pied Currawong and
Australian Raven) on the number of Bell Miners engaged in mobbing. F = 0.65, P > 0.50
,,,,
Species of predator
Mean k s.d. (sample size)
Australian Raven
7.30 f 5.50 (3)
Laughing Kookaburra
2.13 f 1.79 (52)
Pied Currawong
4.25 ? 5.33 (28)
mobbers engaged in attacks against Laughing Kookaburras, Pied Currawongs and ravens (one-way Analysis of Variance. Table 6. P > 0.50).
Not all terrestrial predators of nests were mobbed in
the same way. Stuart's Antechinus Antechinus stuartii
were attacked by four (n = 1 observation) or one (n = 2)
Bell Miners. These attacks occurred when it was climbing a tree very close (0.3 and 2 m) to an active nest. I
observed anti-predator behaviour against a Lace Monitor Varanus varius only once. The encounter did not
occur close to an active nest. Two Bell Miners perched
about 2 m from the ground as the Lace Monitor was
passing below them. The birds then uttered an alarm
call but did not leave the bush they were perching on.
When a human approached a nest with nestlings a variable number of Bell Miners (5-20) usually surrounded
the intruder and performed broken-wing displays while
uttering loud distress calls. I was once repeatedly attacked by a female Bell Miner while measuring a
nestling; she swooped so resolutely and close that I was
able to catch her with an entomological net! This happened inside the zoo area of the Park where Bell Miners
may be more habituated to human presence.
Discussion
It has been suggested that the intensity of mobbing
against potential nest predators (here measured as the
number of birds engaged in mobbing) will vary as the
breeding season progresses because late in the season
the probability of renesting decreases (Carlisle 1982;
Winkler 1987). Decreased renesting with time occurs in
the Bell Miner population at Healesville where the birds
show a humpback distribution of nesting activity with a
maximum between November and February and virtually no nests built between May and July. This contrasts
with the findings of Clarke (1988) who showed that
Bell Miners can nest year-round in the more coastal site
of Janefield, near Melbourne. The higher seasonality of
Bell Miners at Healesville may be explained by the different annual precipitation regimes in the two sites:
1000 mm at Healesville vs 691 mm at Melbourne.
For a bird such as the Bell Miner which lives in
loose colonies (sensu Dow 1975), an additional factor
may complicate the possible effects of time in the season on anti-predator behaviour. It might be expected
that as the breeding season comes to an end, colonial
birds would suffer relatively more from increased competition for food than from nest predation (Haas 1985),
assuming (a) that the bird has a limited amount of time
to allocate to either anti-predator or anti-competitor attacks; (b) that costs of foraging increase towards the
end of the breeding season; and (c) that there is a tradeoff between benefits accruing to the bird through the
current reproductive event and the probability of the
bird's survival during the non-breeding season. Unfortunately, we lack data to test the above assumptions for
Bell Miners, although the non-breeding season at
Healesville may indicate that assumptions (b) and (c)
may hold true for this population. In which case competitors should be mobbed more intensely late in the
season (especially in the dense colony) while predators
should be mobbed more intensely in the early-mid season.
My data do not support this prediction. Although
Bell Miners mobbed Pied Currawongs more intensely
in the early-mid season within a dense colony a able
2b), Laughing Kookaburras and psittaciform competitors were mobbed as intensely early or late in the season, within a dense or a sparse colony (Table 2a,c). This
trend was also valid if we consider that the number of
active nests in the colony was greater in the early-mid
season in the dense colony (unpubl. data).
In general, intruders were mobbed more intensely in
a dense than in a less dense colony (Table 4). Coloniality may have increased the efficiency of anti-predator
defence (e.g. through an increased recruitment of mobbers) and it might therefore increase reproductive success (Gotmark & Anderson 1984; Shields 1984; Robinson 1985; Raveling 1989). Nest predators (all species
from Table 1) were mobbed more intensely in a dense
colony and more than competitors (Table 4). However,
the increased number of mobbers within dense colonies
did not coincide with a decreased rate of predation upon
nests and preliminary data suggest that the percentage
of nests lost by predation was slightly greater in the
denser colony than in the less dense one although this
difference was not significant (see Results, 'Effect of
Poiani: Bell Miner Anti-predator Behaviour
colony density'). This lack of coincidence might be due
to denser colonies attracting more predators because of
their vicinity to the zoo area, although the kinds of
predators were similar in both dense and sparse zones.
Increased mobbing in the denser zone might have had
an effect in deterring predators if we consider that if
that level of mobbing had not occurred, predation might
have been even higher. Therefore, mobbers in dense
colonies may be making the best of a bad job. Nevertheless, they still do slightly worse than birds in sparse
colonies in terms of nest failures.
In comparison to competitors, nest predators were
more likely to be mobbed than to be chased by a single
bird (P < 0.005). But if we consider that predator species are the same all over Healesville and that the addition of one or two extra mobbers (which is the average
unit size difference between zones A and B, see above)
to a mobbing group composed of 3-4 individuals (unit
size at B) lacks a significant effect as a deterrent to
those predators (at least within the limits of colony density differences considered in this study), why should
that extra bird join an already 'efficient' mobbing
group? In other words, although it is possible that the
difference between one and more than one mobber was
significant in terms of nest protection, if the efficiency
of anti-predator defence reached a saturation point at a
mobbing group size of 3-4 individuals, what was the
functional reason for an extra bird increasing the mobbing group up to 5 or 6 individuals?
One possibility is that the addition of one or more
mobbers in the denser colonies decreased the frequency
of visits by each predator (Curio 1978) but, as those
colonies might also be visited by more predators, the effects of mobbing on nest success were relatively small.
On the other hand, evidence is increasing that attacks
against predators can be dangerous for the mobbers
(Curio & Regelmann 1986); even nest predators such as
Laughing Kookaburras can represent a risk for adult
Bell Miners in some circumstances (Poiani & Yorke
1989). Mobbing in Bell Miners may be a result of a
strategy to minimise the risks to individuals engaged in
anti-predator attacks (Curio 1978). This hypothesis is
supported by the fact that competitors, less dangerous to
adult Bell Miners than predators, are attacked by a single bird more than they are mobbed. Smith & Robertson
(1978) also observed that potential competitors of Bell
Miners are attacked by single birds while potential nest
predators are mobbed.
Since the Bell Miner is a cooperatively breeding
bird, the recruitment of helpers during mobbing might
169
be a cost the helpers pay to be allowed to remain in the
parental territory (Gaston 1978, Emlen 1982, Hunter
1985). The parents may benefit through the 'dilution effect' of mobbing (Curio 1978). However, an effect of
group mobbing on the reproductive success of breeders
cannot be ruled out since we do not know whether mobbing by a pair of solitary breeders is less successful than
mobbing by breeders plus one or more helpers.
The perils that Bell Miners face during anti-predator
defence are clearly shown when we compare their responses towards nest predators and raptors. Raptors
were never mobbed while nest predators were mobbed
by an average of three birds. In addition, intruders were
mobbed more intensely when they were perched than
when they were flying (Table 4b). On the other hand,
small Stuart's Antechinus were fiercely attacked by one
or more Bell Miners, while large Monitor Lizards and
humans were not approached closely. This contrasts
with mobbing behaviour of the Noisy Miner Manorina
melanocephala which has been observed mobbing
Monitor Lizards at close range (Dow 1975).
Laughing Kookaburras, Pied Currawongs and Australian Ravens are known nest predators of Bell Miners
(Falconer 1984; Poiani 1991) and, as they are roughly
the same size, it is not surprising that they are mobbed
with similar intensity (Table 6).
The vicinity of an active nest was another variable
which affected mobbing behaviour. Bell Miners tended
to mob nest predators more intensely than competitors
when a nest was close by (0.10 > P > 0.05), suggesting
that they do not chiefly rely on nest concealment to escape nest predators (Zimmermann & Curio 1988). This
is not surprising since the great activity of birds (breeders and helpers) provisioning chicks is likely to make
the nest more easily located by predators (see also Dow
1977). This was further supported by observations of
brooding females leaving the nest immediately after
other Bell Miners uttered an alarm call (e.g. in the presence of Pied Currawongs), instead of crouching in the
nest. It is possible that those breeding females left the
nest to join the mobbing group.
Mobbing behaviour has been studied in other cooperatively breeding species. As in the Bell Miner, raptors
are not mobbed by the Florida Scrub Jay Aphelocoma c.
coerulescens (Francis et al. 1989), and cooperative
breeding appears to be a factor enhancing the intensity
of the mobbing response in both the Scrub Jay and the
Mexican Jay Aphelocoma ultramarina (Cully & Ligon
1976). Other aspects of mobbing behaviour important in
cooperative breeders are the effect of kinship on the risk
170
taken during mobbing (Payne et al. 1988), effect of age,
sex and social status (Payne et al. 1985; Francis et al.
1989), and the occurrence of specialised anti-predator
behaviours such as the 'sentinel system' of the Florida
Scrub Jay (McGowan & Woolfenden 1989).
Acknowledgements
I am very grateful to R.A. Zann, A.A. Martin, M.F. Clarke,
E. Curio, K. Rogers and E. Russell for their very helpful
comments and criticisms which greatly improved the
manuscript. Many thanks also to my wife Marisa for her
very valuable help in the field and constant encouragement.
This work was financed by the Department of Zoology, La
Trobe University, the Sir Colin Mackenzie Zoological Park
and the M.A. Ingram Trust.
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