Wild fledgling tits do not mob in response to conspecific or heterospecific mobbing calls

Mobbing, where birds harass a predator through a combination of vocalizations and stereotyped behaviours, is an effective anti-predator behaviour for many species. Mobbing may be particularly important for juveniles, as these individuals are often more vulnerable than adults. Although the component behaviours of mobbing are often considered to be un-learned, there are few confirmatory data, and the developmental trajectory of mobbing is unknown. In this study, we tested whether conspecific or heterospecific mobbing calls initiated mobbing behaviour in juvenile Blue Tits Cyanistes caeruleus. We located wild adult and recently fledged juvenile Blue Tits and presented them with playback recordings of adult conspecific (Blue Tit) and heterospecific (Great Tit Parus major) mobbing alarm calls. Although adult birds readily mob in response to these types of playbacks, juveniles did not exhibit characteristic mobbing behaviour. Some juveniles did, however, exhibit individual components of mobbing behaviour found in mobbing, despite not producing adult-like mobbing behaviour in response to either conspecific or heterospecific playback. These results suggest that, although birds might be capable of mobbing as juveniles, the associations between the non-vocal stereotyped mobbing behaviours and mobbing calls may be learned.     

Cost of mobbing call to breeding pied flycatcher, Ficedula hypoleuca

Mobbing signals advertise the location of a stalking predatorto all prey in an area and recruit them into the inspectionaggregation. Such behavior usually causes the predator to moveto another area. However, mobbing calls could be eavesdroppedby other predators. Because the predation cost of mobbing callsis poorly known, we investigated whether the vocalizations ofthe mobbing pied flycatcher, Ficedula hypoleuca, a small holenesting passerine, increase the risk of nest predation. We usedmobbing calls of pied flycatchers to examine if they could lurepredators such as the marten, Martes martes. This predator usuallyhunts by night and may locate its mobbing prey while restingnearby during the day. Within each of 56 experimental plots,from the top of one nest-box we played back mobbing sounds ofpied flycatchers, whereas blank tapes were played from the topof another nest-box. The trials with mobbing calls were carriedout before sunset. We put pieces of recently abandoned nestsof pied flycatchers and a quail, Coturnix coturnix, egg intoeach of the nest-boxes. Nest-boxes with playbacks of mobbingcalls were depredated by martens significantly more than werenest-boxes with blank tapes. The results of the present studyindicate that repeated conspicuous mobbing calls may carry asignificant cost for birds during the breeding season.     

Cooperative nest‐defence behaviour and territory quality in a resident and socially monogamous passerine

Nest defence is a fundamental aspect of parental care in secondary cavity‐nesting birds, and predation or competition for nesting sites can involve different defensive behaviours. Because habitat quality determines breeding success, we were interested in whether breeding pairs of the Eurasian nuthatch, Sitta europaea, established in more favourable environment also manifest higher probability of cooperative behaviour during their nest‐site defence. To explore this relationship, we quantified behavioural displays of both parents and analysed activity budget ethogram data from simulated territorial intrusions performed in the chick‐feeding phase with one conspecific and two different heterospecific stimuli (dummies of nuthatch, starling and woodpecker). We found that paired individuals shared their roles during nest‐site defence to a considerable extent. Males had a significantly higher number of attacks on intruders than females, and females performed more threat displays and controls of the brood than males. Multinomial analysis of the cooperative behaviour suggested that pairs in a high‐quality territory had higher probability of reciprocal substitution of different roles towards a balance between attacks, threat displays and nest controls. Contrary to this, pairs in a low‐quality territory had less likely pairwise combinations of simultaneous behavioural states that are associated with effective nest‐site defence. The difference in response probability according to territory quality was, however, highly variable in view of the stimulus that was used in simulated territorial intrusion. Because individual roles and the complex behavioural repertoire of pairs altered in response to territory quality and potential nest‐site competitor or brood predator, our results suggest that the cooperative nest‐defence behaviour could be linked to the breeding success of this year‐round territorial species living in a heterogeneous forest habitat.     

Interspecific reciprocity explains mobbing behaviour of the breeding chaffinches,Fringilla coelebs

When prey animals discover a predator close by, they mob it while uttering characteristic sounds that attract other prey individuals to the vicinity. Mobbing causes a predator to vacate its immediate foraging area, which gives an opportunity for prey individuals to continue their interrupted daily activity. Besides the increased benefits, mobbing behaviour also has its costs owing to injuries or death. The initiator of mobbing may be at increased risk of predation by attracting the predator's attention, especially if not joined by other neighbouring prey individuals. Communities of breeding birds have always been considered as temporal aggregations. Since an altruist could not prevent cheaters from exploiting its altruism in an anonymous community, this excluded any possibility of explaining mobbing behaviour in terms of reciprocal altruism. However, sedentary birds may have become acquainted since the previous non-breeding season. Migrant birds, forming anonymous communities at the beginning of the breeding season, may also develop closer social ties during the course of the breeding season. We tested whether a male chaffinch, a migrant bird, would initiate active harassment of a predator both at the beginning of the breeding season and a week later when it has become a member of a non-anonymous multi-species aggregation of sedentary birds. We expected that male chaffinches would be less likely to initiate a mob at the beginning of the breeding season when part of an anonymous multi-species aggregation of migratory birds. However, their mobbing activity should increase as the breeding season advances. Our results support these predictions. Cooperation among individuals belonging to different species in driving the predator away may be explained as interspecific reciprocity based on interspecific recognition and temporal stability of the breeding communities.     

Long-lasting mobbing of the pied flycatcher increases the risk of nest predation

Mobbing behavior may provide real benefits because mobbing preyindividuals often cause a predator to leave the vicinity. However,mobbing calls of prey can attract acoustically oriented predatorsthus increasing nest predation. Therefore, a real value of mobbingas a type of adaptive behavior may depend on its duration. Inthis experimental study, we tested whether mobbing durationby the pied flycatcher Ficedula hypoleuca, a small hole-nestingpasserine, increases the risk of nest predation. From the topof one nest-box within each of 78 experimental plots, we playedback long calls of pied flycatchers, whereas recordings of shortmobbing calls were played back from the top of another nearbynest-box. The nest-boxes were arranged in pairs, and each ofthem contained a quail Coturnix coturnix egg. Long-call nest-boxeswere depredated by martens Martes martes significantly moreoften than short-call nest-boxes. This predator usually huntsby night and may eavesdrop on the calls of their mobbing preywhile resting nearby during the day. The results of the presentstudy show that long-lasting conspicuous mobbing calls may carrya significant cost for the breeding birds.     

Reconsidering the importance of harvested forests for the conservation of tree-dwelling bats

Intensively managed forests are often seen as of low priority to preserve forest bats. The main conservation strategy recommended, i.e. saving unmanaged “habitat islands” from logging to preserve some suitable habitat, detracts conservationists’ attention from ameliorating conditions for bats in harvested sites. We studied the threatened bat Barbastella barbastellus, mostly roosting in snags, in two beech forests: an unmanaged forest—the main maternity site—and a nearby, periodically logged area. We compared roost availability, roost use, capture rates, food availability and movement between these areas. The managed forest had a greater canopy closure, fewer dead trees, a smaller tree diameter and trees bearing fewer cavities than the unmanaged one. These differences helped explain the larger number of bats recorded in the unmanaged forest, where the sex ratio was skewed towards females. Prey availability was similar in both areas. We radiotracked bats to 49 day roosts. Five individuals caught in the managed area roosted in the unmanaged one at 6.7–8.2 km from the capture site. Few bats roosted in the managed forest, but those doing so proved flexible, using live trees and even rock crevices. Therefore, bats utilise areas in the matrix surrounding optimal roosting sites and sometimes roost there, highlighting the conservation potential of harvested forests. Besides leaving unmanaged patches, at least small numbers of dead trees should be retained in logged areas to favour population expansion and landscape connectivity. Our findings also question the validity of adopting presence records as indicators of forest quality on a site scale.     

Management of reedbeds: mosaic reed cutting does not affect prey abundance and nest predation rate of reed passerine birds

Reed passerine birds are strict habitat specialists inhabiting reedbed habitats. In Europe, many of these species are threatened due to loss and degradation of natural reedbeds. Another important factor that can negatively affect the abundance of reed passerines is commercial reed harvesting. Previous studies have shown negative impacts of large-scale winter reed cutting on passerine breeding assemblages and arthropod communities. The effect of reed cutting on a small scale, however, has not been studied experimentally to date. The aim of this study was to investigate whether and how small-scale, mosaic reed cutting influences prey abundance and nest predation rate of reed passerines. In June, after the reed had reached maturity, we conducted nest predation experiments with artificial nests and arthropod sampling using pan traps in cut reed patches, adjacent uncut reed patches and unmanaged reedbed. We found no differences in the risk of egg predation between three types of reedbeds. In contrast, the abundance of arthropods in cut and adjacent uncut reed patches was significantly higher than that in unmanaged reedbed. We assume this was caused by habitat heterogeneity, small size of cut patches and their rapid recolonization by arthropods from adjacent uncut patches. Our results suggest that in contrast to large-scale reed cutting, small-scale, mosaic reed cutting has no negative effect on nest survival and food abundance of reed passerine birds. However, given that we performed all experiments in June, i.e., when the reed was mature, our findings cannot be generalized to whole breeding period of all reed passerine birds. Therefore, temporal variation in nest predation rate and arthropod abundance in managed and unmanaged reedbeds during the entire breeding season should be examined in future studies.     

Functional traits determine heterospecific use of risk‐related social information in forest birds of tropical South‐East Asia

In birds and mammals, mobbing calls constitute an important form of social information that can attract numerous sympatric species to localized mobbing aggregations. While such a response is thought to reduce the future predation risk for responding species, there is surprisingly little empirical evidence to support this hypothesis. One way to test the link between predation risk reduction and mobbing attraction involves testing the relationship between species’ attraction to mobbing calls and the functional traits that define their vulnerability to predation risk. Two important traits known to influence prey vulnerability include relative prey‐to‐predator body size ratio and the overlap in space use between predator and prey; in combination, these measures strongly influence prey accessibility, and therefore their vulnerability, to predators. Here, we combine community surveys with behavioral experiments of a diverse bird assemblage in the lowland rainforest of Sumatra to test whether the functional traits of body mass (representing body size) and foraging height (representing space use) can predict species’ attraction to heterospecific mobbing calls. At four forest sites along a gradient of forest degradation, we characterized the resident bird communities using point count and mist‐netting surveys, and determined the species groups attracted to standardized playbacks of mobbing calls produced by five resident bird species of roughly similar body size and foraging height. We found that (1) a large, diverse subcommunity of bird species was attracted to the mobbing calls and (2) responding species (especially the most vigorous respondents) tended to be (a) small (b) mid‐storey foragers (c) with similar trait values as the species producing the mobbing calls. Our findings from the relatively lesser known bird assemblages of tropical Asia add to the growing evidence for the ubiquity of heterospecific information networks in animal communities, and provide empirical support for the long‐standing hypothesis that predation risk reduction is a major benefit of mobbing information networks.     

Brood defence and parental roles in a biparental cichlid fishLamprologus toae in Lake Tanganyika

Brood defence of a cichlid fish,Lamprologus toae, was investigated in its natural habitat in Lake Tanganyika. Both parents guarding a brood attacked both conspecific and heterospecific intruder fishes at different locations. The heterospecific intruder fishes could be classified into three groups on the basis of the locations at which the attacks against each species took place. The distinction of groups by the parents seemed to be primarily based on food habits and feeding behaviour of the intruder fishes. The piscivorous species which were more dangerous for the brood were attacked by both parents at more distant locations from the brood. Parental defence of breeding territory changed with the development of the young. The frequency of attacks against each group decreased after the young reached the size too large for the fishes of the group to prey on. Division of labour in the territorial defence was recognized between male and female. The male parent mainly defended the peripheral region of the territory and the female parent defended the inner region. Significance of the selective attack against intruders and the division of labour between the two sexes in brood defence is discussed.     

Heterospecific female mimicry in Ficedula flycatchers

Mimicry is a widespread phenomenon. Vertebrate visual mimicry often operates in an intraspecific sexual context, with some males resembling conspecific females. Pied flycatcher (Ficedula hypoleuca) dorsal plumage varies from the ancestral black to female‐like brown. Experimental studies have shown that conspecific and heterospecific (collared flycatcher, F. albicollis) individuals of both sexes respond, at least initially, to brown individuals as if they were female. We quantified the perceptual and biochemical differences between brown feathers and found that brown pied flycatcher males are indistinguishable from heterospecific, but not from conspecific, females in both aspects. To our knowledge, this is the first evidence of a visual mimetic signalling system in a sexual context where the model is heterospecific to the mimic. By only mimicking heterospecific females, brown pied flycatcher males can establish territories next to the more dominant collared flycatcher in sympatry, suffer less aggression by darker conspecifics in allopatry and preserve within‐species sexual recognition throughout the breeding range. A closer look at the evolutionary history and ecology of these two species illustrates how such a mimetic system can evolve. Although likely rare, this phenomenon might not be unique to Ficedula flycatchers.     

Heterospecific attraction and food resources in migrants' breeding patch selection in northern boreal forest

We studied experimentally how heterospecific attraction may affect habitat selection of migrant passerine birds in Finnish Lapland. We manipulated the densities of resident tit species (Parus spp.). In four study plots residents were removed before the arrival of the migrants in the first study year, and in four other plots their densities were increased by releasing caught individuals. In the second year the treatments of the areas were reversed, allowing paired comparisons within each plot. We also investigated the relative abundance of arthropods in the study plots by the sweep-net method. This allowed us to estimate the effect of food resources on the abundance of birds. The heterospecific attraction hypothesis predicts that densities of migrant species (especially habitat generalists) would be higher during increased resident density. Results supported this prediction. Densities and number of the most abundant migrant species were significantly higher when resident density was increased than when they were removed. On the species level the redwing (Turdus iliacus) showed the strongest positive response to the increased abundance of tits. Migrant bird abundances seemed not to vary in parallel with relative arthropod abundance, with the exception of the pied flycatcher (Ficedula hypoleuca) which showed a strongly positive correlation with many arthropod groups. The results of the experiment indicate that migrants can use resident tit species as a cue to a profitable breeding patch. The relationship between the abundance of the birds and arthropods suggests that annual changes in food resources during the breeding season probably do not have a very important effect on bird populations in these areas. The results stress the importance of positive interspecific interactions in structuring northern breeding bird communities. Received: 1 September 1997 / Accepted: 22 January 1998     

Territorial breakdown of black-capped chickadees, Poecile atricapillus, in disturbed habitats?

The propensity of males to behave territorially may be greatly affected by the perceived quality of the habitat. In suboptimal habitats, the cost of defence may be prohibitively large compared to the associated benefits, and territorial behaviour may be expected to decline. We tested this hypothesis in a population of black-capped chickadees breeding in adjacent habitats: one habitat consisted of an 85-ha patch of early seral forests regenerating after clear-cut logging (disturbed site), and the other consisted of an 85-ha patch of mature mixed-wood forests (undisturbed site). Breeding success of pairs in the disturbed site was significantly lower than in the undisturbed site, suggesting a difference in the relative quality of the two habitats. During the spring of 2000 and 2001, we mapped the actively defended areas and song posts of colour-marked chickadees and found that pairs occupying the disturbed site were subject to more territorial intrusions than pairs in the undisturbed mature woodlands. Five pairs of chickadees in the disturbed habitat and five pairs in the undisturbed habitat were radiotagged and we conducted repeated focal observations on the movement patterns of the birds. All five pairs in the disturbed habitat regularly intruded into areas actively defended by neighbouring birds; only one of the five pairs in the undisturbed habitat ever intruded onto the known territory of another pair during an observation period. Our results suggest that the quality of the disturbed habitat may have been sufficiently low so as to make normal levels of exclusive defence cost inefficient. Conversely, birds in disturbed habitat may have intruded more frequently into neighbouring territories because resource levels in their own exclusively defended territory were insufficient to meet energetic requirements during the breeding season.     

Climate change and fitness components of a migratory bird breeding in the Mediterranean region

The increase in spring temperatures in temperate regions over the last two decades has led to an advancing spring phenology, and most resident birds have responded to it by advancing their onset of breeding. The pied flycatcher (Ficedula hypoleuca) is a long‐distance migrant bird with a relatively late onset of breeding with respect to both resident birds and spring phenology in Europe. In the present correlational study, we show that some fitness components of pied flycatchers are suffering from climate change in two of the southernmost European breeding populations. In both montane study areas, temperature during May increased between 1980 and 2000 and an advancement of oak leafing was detected by using the normalized difference vegetation index (NDVI) to assess tree phenology. This might result in an advancement of the peak in availability of caterpillars, the main prey during the nestling stage. Over the past 18 yr, the time of egg laying and clutch size of pied flycatchers were not affected by the increase in spring temperatures in these Mediterranean populations. However, this increase seems to have an adverse effect on the reproductive output of pied flycatchers over the same period. Our data suggest that the mismatch between the timing of peak food supply and nestling demand caused by recent climate change might result in a reduction of parental energy expenditure that is reflected in a reduction of nestling growth and survival of fledged young in our study populations. The data seem to indicate that the breeding season has not shifted and it is the environment that has shifted away from the timing of the pied flycatcher breeding season. Mediterranean pied flycatchers were not able to advance their onset of breeding, probably, because they are constrained by their late arrival date and their restricted high altitude breeding habitat selection near the southern border of their range.     

Positive feedback between ecological and reproductive character displacement in a young avian hybrid zone

Character displacement can reduce costly interspecific interactions between young species. We investigated the mechanisms behind divergence in three key traits-breeding habitat choice, timing of breeding, and plumage coloration-in Ficedula flycatchers. We found that male pied flycatchers became expelled from the preferred deciduous habitat into mixed forest as the superior competitor, collared flycatchers, increased in numbers. The peak in food abundance differs between habitats, and the spatial segregation was paralleled by an increased divergence in timing of breeding between the two species. Male pied flycatchers vary from brown to black with brown coloration being more frequent in sympatry with collared flycatchers, a pattern often proposed to result from selection against hybridization, that is, reinforcement. In contrast to this view, we show that brown male pied flycatchers more often hybridize than black males. Male pied flycatcher plumage coloration influenced the territory obtained in areas of co-occurrence with collared flycatchers, and brown male pied flycatchers experienced higher relative fitness than black males when faced with heterospecific competition. We suggest that allopatric divergence in resource defense ability causes a feedback loop at secondary contact where male pied flycatchers with the most divergent strategy compared to collared flycatchers are favored by selection.     

Response of Captive Raptors to Avian Mobbing Calls: the Roles of Mobber Size and Raptor Experience

The ‘move on’ hypothesis for avian mobbing proposes that mobbing induces stress in potential predators, thereby provoking them into moving elsewhere. We tested whether mobbing is stress inducing by subjecting captive owls, hawks, and falcons to the mobbing calls of four species of co‐occurring passerine birds that vary considerably in body size. Test subjects comprised 15 individuals of seven species of birds of prey that were housed at a wildlife rehabilitation center in northwestern Pennsylvania, USA. Playback treatments included mobbing calls of the black‐capped chickadee (Poecile atricapillus), blue‐headed vireo (Vireo solitarius), blue jay (Cyanocitta cristata), and American crow (Corvus brachyrhynchos), which vary in size from 12 g (chickadee) to 500 g (crow). A non‐mobbing vocalization (song of the black‐capped chickadee) was included in the playback treatments as a control. Playback treatment produced pronounced effects on three behavioral indicators of probable stress: head orientation toward the playback speaker, raising of feathers or spreading of wings, and changing of perch positions. Test subjects responded more vigorously to mobbing calls of the large passerines (jays and crows) than to those of the smaller passerines (chickadees and vireos). In addition, raptors that had entered the rehabilitation facility as adults generally responded more vigorously to the mobbing calls of jays and crows than did naive individuals that had entered the facility as fledglings or young juveniles. Our results are therefore consistent with the ‘move on’ hypothesis and suggest (1) that the mobbing calls of large passerines are more effective in provoking stress and altering the behavior of potential predators than are the mobbing calls of small passerines and (2) that a raptor's previous experience with mobbing in the wild can exaggerate the strength of its response, particularly to the mobbing calls of large passerines.     

Fear factor: prey habitat selection and its consequences in a predation risk landscape

Predation risk influences prey use of space. However, little is known about how predation risk influences breeding habitat selection and the fitness consequences of these decisions. The nest sites of central-place foraging predators may spatially anchor predation risk in the landscape. We explored how the spatial dispersion of avian predator nests influenced prey territory location and fitness related measures. We placed 249 nest boxes for migrant pied flycatchers Ficedula hypoleuca , at distances between 10 and 630 m, around seven different sparrowhawk nests Accipiter nisus . After closely monitoring flycatcher nests we found that flycatcher arrival dates, nest box occupation rates and clutch size showed a unimodal relationship with distance from sparrowhawk nests. This relationship suggested an optimal territory location at intermediate distances between 330 and 430 m from sparrowhawk nests. Furthermore, pied flycatcher nestling quantity and quality increased linearly with distance from sparrowhawk nests. These fitness related measures were between 4 and 26% larger in flycatcher nestlings raised far from, relative to those raised nearby, sparrowhawk nests. Our results suggest that breeding sparrowhawk affected both flycatcher habitat selection and reproductive success. We propose that nesting predators create predictable spatial variation in predation risk for both adult prey and possibly their nests, to which prey individuals are able to adaptively respond. Recognising predictable spatial variation in perceived predation risk may be fundamental for a proper understanding of predator-prey interactions and indeed prey species interactions.     

Syntax manipulation changes perception of mobbing call sequences across passerine species

Many species approach predators to harass them and drive them away. Both the intensity of this antipredator strategy and its success are positively related to the size of the group that carries out this mobbing. To recruit individuals to the mob, members of prey species produce mobbing calls. In some songbirds—the Japanese tit, Parus minor, and the southern pied babbler, Turdoides bicolor—mobbing calls are structurally complex and it has been suggested that they convey information by means of compositional syntax, when meaningful items are combined into larger units. These two species combine alert and recruitment calls into an alert and recruitment sequence when attracting conspecifics to cooperate in mobbing a predator. Whether this rudimentary, two‐call, compositional structure is used by other bird species in mobbing calls and how it can alter the ability of heterospecifics to adequately recognize mobbing calls is not well understood. Heterospecifics’ responses to mobs are critical to the success of the mobbing strategy, so it is of great importance to understand whether and how syntax influences these responses. To address these questions, we conducted two playback experiments. Firstly, we investigated whether the great tit, Parus major, extracts different meanings from different individual motifs (i.e., component calls), and from combined motifs in both natural and artificially reversed order. We found that great tits extract different meanings from the two motifs involved in mobbing calls and that they also discriminate for motif order reversal in the mobbing call sequence. Secondly, we investigated whether heterospecifics (the coal tit, Periparus ater, and the common chaffinch, Fringilla coelebs) are sensitive to syntax alteration of great tit mobbing calls. While chaffinches did not respond to great tit mobbing calls, coal tits were sensitive to mobbing call sequence reversal although they did not react in the same way as conspecific subjects. Overall, whereas our results indicate that tits are sensitive to call reversal, this is not to say that tits actually use compositional syntax to increase the information content.     

Group Mobbing Behaviour and Nest Defence in a Cooperatively Breeding Australian Bird

Cooperatively breeding noisy miners (Manorina melanocephala) are well known in Australia for their persistent and very vocal group mobbing of heterospecifics. Here I investigated the nature of this extraordinary behaviour, in particular its role in nest defence, in a colour banded population of noisy miners in south‐east Queensland, Australia. I focused on two questions. First, did the intensity of mobbing vary according to factors such as the threat to the nest, or the ‘value’ of a clutch? Secondly, what role did group mobbing play in the success of a nest? To answer these questions, I experimentally manipulated the nest defence behaviour by placing one of three stuffed models near active noisy miner nests. The response of noisy miners to intruders was not indiscriminate. However, I found that the number of birds that mobbed a model did not simply reflect the potential threat posed. The response of noisy miners to raptors and other potential nest predators may have reflected their rarity as well as the threat posed. The number of mobbers did not vary with the age or size of a brood. In this study, the fate of nests was independent of the number of mobbers or visitors at nests. Finally, up to 80% of mobbers were never seen to make any other type of contribution to a nest, and many could not be related to the brood that they were ‘defending’. Hence, for some noisy miner ‘helpers’ the benefits that they accrued were probably not wholly dependent on the survival of the broods. I suggest that, in this gregarious species, mobbing behaviour at the nest may be a display of social status or individual quality. This hypothesis warrants further investigation.     

Nonrandom dispersal drives phenotypic divergence within a bird population

Gene flow through dispersal has traditionally been thought to function as a force opposing evolutionary differentiation. However, directional gene flow may actually reinforce divergence of populations in close proximity. This study documents the phenotypic differentiation over more than two decades in body size (tarsus length) at a very short spatial scale (1.1 km) within a population of pied flycatchers Ficedula hypoleuca inhabiting deciduous and coniferous habitats. Unlike females, males breeding in the deciduous forest were consistently larger than those from the managed coniferous forest. This assortment by size is likely explained by preset habitat preferences leading to dominance of the largest males and exclusion of the smallest ones toward the nonpreferred coniferous forest coupled with directional dispersal. Movements of males between forests were nonrandom with respect to body size and flow rate, which might function to maintain the phenotypic variation in this heritable trait at such a small spatial scale. However, a deeply rooted preference for the deciduous habitat might not be in line with its quality due to the increased levels of breeding density of hole‐nesting competitors therein. These results illustrate how eco‐evolutionary scenarios can develop under directional gene flow over surprisingly small spatial scales. Our findings come on top of recent studies concerning new ways in which dispersal and gene flow can influence microevolution.     

Mobbing behaviour in a passerine community increases with prevalence in predator diet

Mobbing behaviour against predators is well documented but less is known about the factors influencing variation in behavioural response between prey species. We conducted a series of playback experiments to examine how the mobbing responses of prey species differed according to their relative risk of predation by the Eurasian Pygmy Owl Glaucidium passerinum, a predator of passerines. We found that mobbing among 22 passerine prey species was positively correlated with their prevalence in the Pygmy Owl diet. To compare mobbing behaviour between two seasons, we conducted playback experiments during spring (breeding season) and autumn (non‐breeding season). Contrary to previous studies, we found that mobbing intensity was greater during autumn than in spring. Our study shows a differential mobbing response of 22 species to the calls from one predator species and underscores the importance of considering seasonal variation in mobbing behaviour. Mobbing response differences observed among bird species strongly suggest different cooperation behaviour at the community level.