Bird Study (2004) 51, 41–47
Importance of competition for food and nest-sites
in aggressive behaviour of Collared Flycatcher
Ficedula albicollis
MILOS KRIST
Laboratory of Ornithology, Faculty of Science, Palacky University, Tr. Svobody 26, 771 46 Olomouc, Czech
Republic and Museum of Natural History, Republiky 5, 771 73 Olomouc, Czech Republic
Capsule Using an experimental approach, this study disentangles effects of two important sources on
the elicitation of aggressive interspecific behaviour.
Aims To investigate experimentally whether competition for nest cavities and/or food can explain
aggressive behaviour between competing species.
Methods The Collared Flycatcher’s Ficedula albicollis responses to mounts of Great Tit Parus major
(nest-site and food competitor), Chaffinch Fringilla coelebs (food competitor) and Dunnock Prunella
modularis (control species) were tested. Trials were performed near flycatchers’ nests during nest
building, incubation and care of nestlings.
Results The intensity of dive attacks and frequency of contact attacks showed that the aggression of
Collared Flycatchers decreased in the direction: Great Tit > Chaffinch > Dunnock. The difference in
aggressiveness was greater between Great Tit and Chaffinch than between Chaffinch and Dunnock.
Aggression directed toward Great Tits increased from the nest building to the incubation stage and then
decreased markedly in the nestling stage. Males were more aggressive than females.
Conclusion These results suggest that competition for nest-sites, and to a lesser extent for food, may be
of an interference nature and that the reproductive value hypothesis can only partly explain differences
in the Collared Flycatcher’s defensive behaviour found between sexes and stages of the breeding cycle.
Food and nest-sites are the most important resources in
interspecific competition in birds (Newton 1998).
Interspecific competition for food is mostly considered
to be exploitation (i.e. competition without direct
behavioural interactions between competing individuals) whereas that for nest-sites is considered to
be interference (i.e. competition with direct behavioural interactions between competing individuals).
However, departures from this rule are quite common,
at least in the case of food competition. Substantial
aggression directed toward heterospecifics competing
for food can eventually lead to interspecific territoriality
(Orians & Willson 1964) which is very clear evidence
of interference. However, interspecific territoriality is
costly and so preferable only in specific conditions.
Among those is life in a structurally simple habitat
(Reed 1982) or exploiting stratified food sources
(Dearborn 1998), both of which preclude niche differEmail: krist@prfnw.upol.cz
© 2004 British Trust for Ornithology
entiation. Newton (1998) listed 27 studies in which at
least 32 species pairs maintained interspecific territories.
In the case of nest-sites, there is little doubt about
the nature of competition. However, there is no
support from experimental research that is directly
suggestive of interference. So far, most studies have
explored changes in local densities of secondary cavitynesting birds after manipulation of nest-site availability – usually after nest boxes were provided.
Newton (1998) listed 34 such studies. In all of them
local densities of secondary cavity-nesting birds
responded in the expected direction to manipulation
and the same conclusion can be drawn – cavities are
the limiting resource and thus subject to intensive
intraspecific and interspecific competition. The latter
was documented by complementary changes in the
breeding densities of two or more competing species
(Minot & Perrins 1986, Gustafsson 1988).
Although such correlations can suggest the existence
of competition, they cannot reveal its nature. To inves-
42
M. Krist
tigate whether interference or exploitation is acting it
is necessary to pay attention to a proximate level of
interspecific relationships, i.e. to document aggressive
behaviour in at least one species involved. So far the
evidence of such aggression is mainly observational
(Slagsvold 1975, Gustafsson 1988, Winge & Järvi
1988, Meek & Robertson 1994, Merilä & Wiggins
1995). Experimental studies are few in this area. Järvi
et al. (1978) tested mutual aggressive responses of Pied
Flycatchers Ficedula hypoleuca and Great Tits Parus
major to playback of the other species’ song. Král &
Bicík (1992) investigated behaviour of Collared
Flycatchers Ficedula albicollis towards a dummy of the
Great Tit placed on their nest boxes. Both studies concluded that the observed aggression was selected due to
nest-site competition. However, there was no appropriate control treatment in either of these studies. Ficedula
flycatchers compete strongly with tits not only for nestsites but also for food (Slagsvold 1975, Gustafsson
1987, Sasvári et al. 1987). Thus, trials without controls
for the possible confounding effect of food competition
cannot adequately explain the function of observed
interspecific aggression. Similarly, Martin & Martin
(2001) revealed mutual aggression between two
warbler species which compete for food as well as for
nest-sites but their study also was not specifically
designed to conclude which source caused this aggression. I know of no other study investigating the role
of nest-site competition in aggressive behaviour of
birds.
The main aim of the present study was to investigate
whether competition for nest cavities and/or food can
explain aggressive behaviour between competing
species. To test these ideas, the aggressive responses of
Collared Flycatchers to the Great Tit (nest-site as well
as food competitor), the Chaffinch Fringilla coelebs,
(food competitor) and the Dunnock Prunella modularis,
(neither food nor nest-site competitor) were compared.
A prediction of decreasing aggression in the direction
Great Tit > Chaffinch > Dunnock was made. In addition, changes in aggression of Collared Flycatchers
during the breeding cycle and differences between the
sexes were examined. Aggression directed toward nestsite competitors should increase during the breeding
cycle, because of the increasing reproductive value of
the brood. Changes in aggression caused by competition for food are more difficult to predict because both
food availability and food demands presumably
increase during the breeding cycle. Thus, it is not clear
in which stage of the breeding cycle food competition
should be most severe.
© 2004 British Trust for Ornithology, Bird Study,
51, 41–47
METHODS
Study area and subjects
The study was conducted in Velky Kosir area (49°32′N,
17°04′E, 370–450 m asl), central Moravia, Czech
Republic, during 1997–2000. Experiments were conducted on two nest-box plots. Both are about 12 ha in
area, each provided with approximately 60 nest boxes.
The vegetation of these plots consisted of structurally
very simple managed spruce Picea abies forest. The
spruces were about 90 years old and 25 m tall. Other
species of tree were very scarce, represented mainly by
pine Pinus sylvestris and birch Betula pendula. The shrub
understorey was not developed. The herb level, consisting mainly of Calamagrostis villosa, covered only
about 40% of the ground.
The Collared Flycatcher is a small (about 13 g), cavity-nesting, migrant passerine species with sexually
dimorphic plumage. It easy adopts nest boxes for breeding (about 40 Collared Flycatcher pairs nested each
year on the study plots). Collared Flycatchers forage
mainly in the canopy, less in the shrubs or on the
ground. They obtain food by sallying out from a perch
after flying prey or picking up directly from leaves and
twigs (Cramp & Perrins 1993). Diet brought to
nestlings consists mainly of Lepidoptera larvae,
Diptera, Hymenoptera and Araneida (Bures 1986,
Cramp & Perrins 1993). The Chaffinch (an opennesting species) and Great Tit (a cavity-nesting species)
forage in a way similar to that of the Collared Flycatcher
and also the food brought to their young overlaps
broadly with that of the Collared Flycatcher (Cramp &
Perrins 1993, 1994). Moreover, food competition has
been directly proven between the Collared Flycatcher
and the Great Tit (Gustafsson 1987, Sasvári et al. 1987)
and between the Great Tit and the Chaffinch (Reed
1982). The Dunnock (an open-nesting species), on the
other hand, differs from all three species mentioned
above largely in its foraging techniques and the food
brought to young. This species is mainly a ground forager
and its diet consist mainly of harvestmen (Opilionidea),
colembolan (Entomobryidae), Auchenorrhyncha and
Lepidoptera larvae occurring near the ground
(Geometridae) (Cramp 1988, Kristín 1989). All three
bird species chosen for experiments are of similar size
(Great Tit 20 g, Chaffinch 23 g and Dunnock 21 g) and
were present on the study plots. Stuffed specimens of two
males of the Great Tit, two males of the Chaffinch and a
Dunnock (sexually monomorphic species) of unknown
sex were used for experiments. All specimens were
stuffed in a similar posture.
Interspecific aggression
Experimental design
Experiments were conducted in three stages of the
breeding cycle: nest building, incubation and care for
nestlings. The dummy was placed either directly on the
nest box, or on the wooden stick about 50 cm high
placed at a distance of 2 m from the nest-box.
Experiments were conducted on 94 nests. Each of them
was tested with all three dummies, placed at a single
distance (nest box or 2 m), on the same day. The order
of presentation was random. Successive presentations
were separated by an interval at least 45 min (75 min
in the case of the former contact attack).
After the dummy had been installed, focal nest
observations were made at a distance about 25 m. After
the male or female had approached to a close vicinity
of the nest and so presumably noticed the dummy, the
observational interval began. It lasted 10 min for each
focal bird. For each individual the number of dive
attacks against the dummy and the latency to first dive
attack were recorded. In cases when an individual did
not attack the dummy at all, the latency was stated as
600 s (i.e. the length of observational interval). This
method should hold type I errors below the stated level
(α = 0.05) and results may be a bit conservative in this
respect.
Sometimes physical contact was made with the
dummy when it was attacked. In this case the trial was
stopped and the dummy removed to prevent it from
destruction. Such trials were classified as ‘contact
attack occurred’ and that was the only information
involved in statistical analysis, because of the artificially shortened observational interval.
the two models test the same hypothesis, a sequential
Bonferroni adjustment (Rice 1989) was applied to
reduce the type I error rate.
In both models the following factors with their twoway interactions were included as fixed effects: species,
sex, breeding stage and distance. Since several trials
were performed on individual nests, nest was treated as
a random factor in both models. To select the best
model, a two-step method was used. First, in the fixed
part of the model, non-significant interactions were
eliminated by backwards selection. Second, the random part of the model (covariance structure) was
selected according to Akaike’s information criterion
(AIC). In the case of dive attacks, the best model contained random slopes (within nests) for sex and
distance. In the case of contact attacks, inclusion of
random factors caused an increase in AIC and therefore
the final model contained only the fixed part.
Denominator degrees of freedom were computed using
Satterthwaite’s method (Littell et al. 1996). Dive
attacks were analysed using PROC MIXED, contact
attacks using the GLIMMIX macro for SAS. Whenever a
factor with more than two levels was significant,
Tukey’s HSD tests were computed. All tests were computed in SAS (Littell et al. 1996).
RESULTS
As judged by dive attacks, both male and female
Collared Flycatchers behaved most aggressively against
the Great Tit, less aggression was directed toward
Chaffinch and the Dunnock was the least attacked
dummy (Fig. 1 & Table 1; Tukey’s HSD tests: Great Tit
1.25
Statistical analysis
1.00
0.75
Response
Two response variables (dive attacks and contact
attacks) were used for the statistical analyses. Dive
attacks was the first principal component from the
principal component analysis (PCA) performed on two
original variables (number of dives and the latency to
first dive attack) which were highly correlated. Before
PCA was performed, variables were ln- (number of
attacks) or box-cox- (latency) transformed to achieve
better approximation to the required normal distribution. The composite variable has eigenvalue 1.84 (92%
variance explained) and the following factor loadings:
number of dive attacks (0.71), and latency to first
dive attack (–0.71). The second response variable was
binomial (contact attack occurred yes/no). The two
response variables were analysed separately. Because
43
0.50
0.25
0.00
–0.25
–0.50
–0.75
Great Tit
Chaffinch
Dunnock
Figure 1. Least squares means ± se for the response of male (■
■)
and female (●) Collared Flycatcher to the three species studied. The
response is the first principal component comprising the number of
dive attacks and latency to the first dive attack. A higher value
represents more dive attacks and a shorter latency to the first dive
attack.
© 2004 British Trust for Ornithology, Bird Study,
51, 41–47
44
M. Krist
Table 1. Type III tests of fixed effects. Response was the composite
variable derived from a principal component analysis on latency to
first dive attack and number of dive attacks. Backwards elimination
procedure; only interactions significant after applying a Bonferroni
correction were retained in the presented model.
NDF
DDF
F
P
2
1
2
1
2
621.0
94.1
671.0
69.6
615.0
22.8
13.4
2.7
8.3
5.9
< 0.001
< 0.001
0.069
0.005
0.003
Table 2. Results of GLIMMIX model performed on contact attacks.
Probability of a contact attack occurring was modelled. Backwards
elimination procedure; all interactions were non-significant and
therefore deleted. Dunnock, male, nestling stage, and the distance
of 2 m are the reference levels for the factors species, sex, breeding stage, and distance, respectively.
a. Type III analysis of effects
Species
Sex
Breeding stage
Distance
Species*sex
NDF, numerator degrees of freedom; DDF, denominator degrees of
freedom.
Species
Sex
Breeding stage
Distance
DDF
F
855
855
855
855
41.0
26.1
7.6
16.3
P
<
<
<
<
0.001
0.001
0.001
0.001
b. Analysis of REML estimates
vs. Chaffinch, P < 0.001; Great Tit vs. Dunnock, P <
0.001; Chaffinch vs. Dunnock, P = 0.005). The occurrence of the most aggressive behaviour (contact
attacks) supported this pattern (Fig. 2 & Table 2). The
Great Tit was contacted significantly more frequently
than either Chaffinch (P < 0.001) or Dunnock (P <
0.001). There was no significant difference between
Chaffinch and Dunnock in this respect (P = 0.19).
Males were the more aggressive sex. They contacted
dummies more often (Table 2) and performed stronger
dive attacks (Table 1). In the latter case this difference
was highest when the Great Tit was presented, less
when the Chaffinch presented and virtually disappeared when the Dunnock presented (Fig. 1).
Incubation was the stage when the probability of
attacking the dummy physically was highest (Table 2;
Tukey’s HSD tests: incubation vs. nestling, P < 0.001;
incubation vs. nest building, P = 0.020; nest building
vs. nestling P = 0.55). Similarly to contacts, dive
attacks were strongest at the incubation stage too.
However, because the result of the mixed model was
Contact attacks (%)
35
30
262
25
20
15
10
295
305
5
0
Great Tit
Chaffinch
Dunnock
Figure 2. Least squares means ± 95% confidence limits of occurrence of contact attacks against the three species studied. Values
are adjusted for sex, breeding stage and distance. The sample size
for each species is indicated above the bar.
© 2004 British Trust for Ornithology, Bird Study,
51, 41–47
Factor
Intercept
Species
Sex
Breeding stage
Distance
Level
Parameter estimate
se
–4.32
2.65
0.55
–1.30
0.20
1.09
1.16
0.437
0.367
0.426
0.255
0.336
0.280
0.287
Great Tit
Chaffinch
Female
Nest building
Incubation
Nest box
DDF, denominator degrees of freedom; REML, restricted maximum
likelihood.
marginally non-significant (Table 1), multiple comparisons were not computed.
DISCUSSION
Food and nest-site competition
In research on interspecific competition it is commonly
assumed that competition for nest-sites is of an interference nature. However, hitherto there has been no
support from experimental research for this idea. This
study revealed that dive attacks decreased in the
direction Great Tit > Chaffinch > Dunnock as predicted under the hypothesis that nest-site competition
as well as food competition contribute to aggressive
behaviour of Collared Flycatcher. The difference in the
intensity of dive attacks was greater between the Great
Tit and the Chaffinch than between the Chaffinch and
the Dunnock. The same was true for the occurrence of
contact. These results imply that competition for nestsites contributes more to aggressive behaviour of
Collared Flycatchers than competition for food.
Competition for nest cavities has been considered to
be low in primeval forests in the past because of high
availability of cavities. Further it has been considered
that at present we can observe competition for cavities
Interspecific aggression
in nature only due to decrease in hole numbers caused
by intensive silvicultural management (Walankiewicz
1991). However, the life-history traits of some North
American (Martin 1993a) as well as European
(Mönkkönen & Martin 2000) secondary cavitybreeding birds are best explained by the limited breeding opportunities hypothesis (Martin 1993b). This fact
suggests that competition for tree cavities played an
important role in the evolutionary past of these species.
Nest defence behaviour is possibly less apt to reflect the
situation hundreds of years ago, because of its potential
for rapid evolution through learning and cultural transmission (cf. Maloney & McLean 1995). On the other
hand, studying this behaviour is the only way to reveal
underlying mechanisms involved in competition.
Aggressive behaviour directed specifically toward the
nest competitor in this experimental study, although it
cannot be treated as evidence of long-acting selection
pressure, proves the interference nature of competition
for nest cavities.
Ficedula flycatchers are not usually assumed to defend
large feeding territories (Cramp & Perrins 1993).
However, in contrast to this traditional view, older
males of the Pied Flycatcher were found to possess
territories with a greater abundance of invertebrate
food than subordinate yearlings (Huhta et al. 1998).
This suggests that the abundance of food can influence
intraspecific and so possibly also interspecific behaviour. The latter was proved in this study on the
Collared Flycatcher when the food competitor was
attacked more strongly than the non-competing
species. However, the fact that the differences in
aggression were greater between the Great Tit and
Chaffinch than between Chaffinch and Dunnock
indicates that food competition plays only a minor role
in aggressive interactions between tits and flycatchers.
Moreover, the effect of food on aggression may be
specific for a simple habitat such as that on my study
plots. As Orians & Willson (1964) stated, a simple
habitat can prevent niche differentiation, consequently
competition for food is more pronounced and can lead
to interspecific territoriality among competing bird
species. Reed (1982), for example, found Great Tit to
be interspecifically territorial with Chaffinch on
Scottish islands whereas such a pattern did not emerge
in the more structured habitats on the mainland.
Sex differences
Males were the more aggressive sex in this study. They
contacted dummies more frequently and performed
45
stronger dive attacks compared to females. In the case
of dive attacks, the difference between sexes was greatest with the Great Tit, less with the Chaffinch and
almost non-existent with the Dunnock.
The role of sex in nest defence against nest competitors has been investigated just once (Král & Bicík
1992). Unlike this study, Král & Bicík (1992) did not
find any differences in the nest defence between sexes.
These contrasting results can be explained by a different assessment of the intensity of defence. Král &
Bicík (1992) combined records of watchfulness and
aggressiveness whereas here only aggression was
recorded, making the interpretation more straightforward. Overall, higher aggression of males against
food competitors (Great Tit and Chaffinch) is not
surprising. As in most other birds, males in Collared
Flycatcher are the sex predominantly engaged in
territory defence (Cramp & Perrins 1993). The finding
that the largest difference in aggression between sexes
was seen with the Great Tit can be explained in terms
of residual reproductive value, because the Great Tit, in
contrast to Chaffinch and Dunnock, can destroy a
flycatcher’s nest. Greater intrasexual competition
in males, as suggested by a small proportion of yearling
males in the breeding population in contrast to
a great one in females (Král & Pithart 1995, M. Krist,
unpubl. data), results in their lower renesting potential and residual reproductive value. Consequently,
males should defend the brood more aggressively
(Montgomerie & Weatherhead 1988). In addition, if
the territory defence and residual reproductive value
alone are responsible for the sex differences in aggressiveness, as suspected, then aggression toward a
non-competing bird (Dunnock) should be low for both
males and females, as was found.
Breeding cycle
The frequency of contact attacks decreased in the
direction incubation > nest building > nestling stage.
Due to the very low frequency of contacts against the
Chaffinch and the Dunnock (Fig. 2) this pattern is
driven by flycatchers’ responses toward the Great Tit.
Parental investment is predicted to increase in the
course of the breeding cycle due to an increase in both
the reproductive value of the current brood
(Andersson et al. 1980) and a feedback stimulus of the
nest contents to a parent (McLean & Rhodes 1992). A
large number of studies of nest defence against predators have confirmed this prediction (Montgomerie &
Weatherhead 1988). In contrast to the large number of
© 2004 British Trust for Ornithology, Bird Study,
51, 41–47
46
M. Krist
studies investigating breeding-cycle changes of nest
defence against predators, just two such studies were
done with nest competitors, although nest competitors
might pose the same threat for the brood as nest
predators. In the first, Winge & Järvi (1988) found no
evidence that past parental investment influenced the
success of nest defence in Great Tits. However, all their
observations were performed during the nest building
stage, where the amount of variation in individuals’
residual reproductive values was probably too small to
answer this question adequately. In the second, the nest
defence of Collared Flycatchers against the Great Tit
was observed to be stronger later in the breeding cycle
(Král & Bicík 1992). However, their results may reflect
watchfulness rather than aggression, the former surely
being very low at the egg-laying stage.
The pattern of contact attack variation in this study
provided only partial support for a parental investment/
feedback hypothesis. Aggression against nest competitors increased from the nest building to incubation
stage as predicted but then decreased markedly. This
decrease can probably be explained as follows.
Although both nest competitors and nest predators
pose theoretically the same danger for the brood, they
probably behave in a different way. Nestlings are
presumably more valuable prey for nest predators than
eggs, because of their greater energy content. For nest
competitors, on the other hand, the presence of young
can complicate the take-over of a nest hole for two reasons. First, it may be difficult for them to build their
own nest on that with living young because of their
activity. Second, if young are much more valuable for
parents than eggs, increased aggression from nest hole
owners will deter an intruder to try to take over such a
hole. The latter also concerns nest predators but the
difference is in the duration of experiencing such
aggression. Predation can happen very quickly, whereas
take-over is necessarily a lengthy act and thus costs of
suffering aggression from hole owners would be probably much greater. Such conditions could favour an
increase in defence against nest predators occurring
near the nest with nestlings, as has been repeatedly
confirmed (Montgomerie & Weatherhead 1988), since
the presence of a predator near such a nest means a
high risk of nest predation. However, the presence of a
nest-site competitor near a nest containing nestlings is,
in contrast to a nest containing eggs, probably only a
random event and does not mean that it is trying to
take over the hole. This hypothesis explains why the
take-over of cavities occurs just before incubation
(Slagsvold 1975, Meek & Robertson 1994), while
© 2004 British Trust for Ornithology, Bird Study,
51, 41–47
aggression directed toward the nest competitor occurring in the vicinity of nest was found to be low during
the nestling stage.
Acknowledgements
I thank Stanislav Bures, Tomás Grim, Tomás Herben, Václav
Pavel, Vladimír Remes, Emil Tkadlec and Karel Weidinger
for valuable comments or discussion on the manuscript and
Honza Str̆íteský for exceptional willingness in field cooperation. The secondary grammar school in Kromeríz kindly
provided me dermoplastic preparates. Forest enterprise Cechy
pod Kosírem allowed me to work on their nest-box plots.
This study was supported by a grant from the Czech Ministry
of Education (MSM 153100012). In the final part of the
project I was supported also by a grant from GACR (No.
206/03/0215). I thank Kacenka for everyday support.
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(MS received 6 January 2003; revised MS accepted 14 April 2003)
© 2004 British Trust for Ornithology, Bird Study,
51, 41–47