Jukka T. Forsman

Evolution of heterospecific attraction: using other species as cues in habitat selection

We analyzed the ecological conditions that may favor a habitat selection process in which later arriving individuals (colonists) use the presence of earlier established species (residents) as a cue to profitable breeding sites (heterospecific attraction). In our model, colonists assessing potential breeding patches could select between high-quality source and low-quality sink patches. A proportion of the source patches were occupied by residents. Colonists could either directly sample the relative quality of the patches (termed samplers) or, alternatively, they could also use residents as a cue of patch quality (cue-users). Cue-users gained benefit from lowered costs when assessing occupied source patches. The cue-using strategy is an efficient way to choose the best possible patch not only when interspecific competition is intense, but also when benefits from social aggregation exceed the effects of competition. High relative cost of sampling empty patches increases the fitness of the cue-using strategy relative to samplers. The strongest attraction to heterospecifics was predicted when the benefit from aggregating with residents exceeded the effects of competition, and approximately half of the landscape consisted of occupied, high-quality source patches.

Positive interactions between migrant and resident birds: testing the heterospecific attraction hypothesis.

We experimentally tested the conditions where heterospecific attraction is more likely to occur. The heterospecific attraction hypothesis predicts that colonizing or migrant individuals use the presence of resident species as a cue for profitable breeding sites. In other words, increasing resident densities will result in increased migrant densities until the costs of interspecific competition override the benefits of heterospecific attraction. The experiment consisted of a reference and a manipulation year. In the reference year, resident titmice were permitted to breed at intermediate densities whilst in the manipulation year, resident densities were manipulated in nine study plots. Three treatments were performed as low, intermediate and high resident densities and migrant density responses were measured in both years. Relative between-year migrant and resident densities were analyzed by regression analysis. Migrant foliage gleaning guild densities responded linearly and positively, as did densities of habitat generalists, in particular Chaffinch (Fringilla coelebs),. The ground-foraging guild did not show a response. This study provides support for predictions of the heterospecific attraction hypothesis and suggests that information on habitat quality with reference to both food availability and safe breeding sites are important in using heterospecifics as cues. Based on Chaffinch response data, artificially increased resident densities were not high enough for competitive effects between residents and migrants to decrease heterospecific attraction. It seems unlikely that in northern environments natural resident densities will reach high levels where competitive effects would occur, therefore heterospecific attraction will always be beneficial. This study again shows the importance of heterospecific attraction in migrant habitat selection and as a process promoting species diversity in northern breeding bird assemblages.

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

Abstract 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.

Mixed-species foraging aggregations and heterospecific attraction in boreal bird communities

We investigated whether different species of birds associated with each other while foraging during the breeding season. On average 50% of foraging observations were in mixed-species foraging aggregations, but with extensive interspecific variation. The occurrence of mixed-species foraging aggregations was relatively constant from the end of May until early July, with no indication that the appearance of fledglings increased the frequency of aggregations. These mixed-species foraging aggregations consisted of a wide variety of species in terms of foraging niche : typical flycatchers, specialized arboreal insectivores, and generalists were observed foraging together. The titmice species (Parus spp.) seemed to be preferred as foraging companions. The sites where foraging aggregations were observed did not differ from an equal number of randomly chosen sites with respect to forest vegetation structure. The dispersion of individuals in study areas was analysed using a 50 m x 50 m quadrat as the sampling unit. Altogether, 96 quadrats were surveyed. The dispersion of individual birds with respect to individuals of other species was non-random, and significantly clumped within quadrats. We used two playback experiments to test whether the aggregations were due to active search for heterospecific companions. In the first experiment, 20 randomly assigned sites received two 10-min treatments : willow tit (Parus montanus) song and silence as control. In the second experiment, we had two control treatments at 30 playback sites for the willow tit song, the song of the olive-sided flycatcher (Contopus borealis) and classical music. More species were observed during willow tit song than during any of the controls. This suggests that aggregated dispersion pattern and the existence of mixed-species foraging aggregations were due to active search for heterospecific companions, and also that recognition of neighbouring species was involved. We suggest that birds actively aggregate in mixed-species foraging groups to gain protection against predators and/or enhance feeding efficiency. The results support the view that positive interactions have an importance in structuring northern breeding bird communities.

New behavioural trait adopted or rejected by observing heterospecific tutor fitness

Animals can acquire behaviours from others, including heterospecifics, but should be discriminating in when and whom to copy. Successful individuals should be preferred as tutors, while adopting traits of poorly performing individuals should be actively avoided. Thus far it is unknown if such adaptive strategies are involved when individuals copy other species. Furthermore, rejection of traits based on tutor characteristics (negative bias) has not been shown in any non-human animal. Here we test whether a choice between two new, neutral behavioural alternatives—breeding-sites with alternative geometric symbols—is affected by observing the choice and fitness of a heterospecific tutor. A field experiment replicated in four different areas shows that the proportion of pied flycatcher females matching the choice of the tit tutor consistently increased with increasing number of offspring in the tit nest, to the extent of nearly complete prevalence in one of the areas when tit fitness was highest. Notably, all four replicates demonstrate rejection of the behaviour of lowest-fitness tutors. The results demonstrate both acquisition and avoidance of heterospecific behavioural traits, based on the perceived (lack of) tutor fitness. This has potential implications for understanding the origin, diversity and local adaptations of behavioural traits, and niche overlap/partitioning and species co-occurrence.

Aggregate dispersion of birds after encountering a predator: experimental evidence

Animals may join flocks to gain protection against predation. In this experiment we investigated how predation risk affects the spatial distribution of forest birds during breeding time. We manipulated the perceived risk of predation by showing stuffed avian predators and by playing the warning signals of some of the passerine species in experimental areas. The spatial distribution of the bird individuals in both experimental and control areas was investigated by censusing birds and marking the locations of all individuals on maps both before and after the simulated appearance of a predator. We predicted that the distances between heterospecific individuals would be reduced in the experimental areas compared with those in control plots because of a perceived increased risk of predation. After predator presentation individuals in experimental areas were closer to heterospecifics than in control areas. Predation risk is one possible cause of clumped distribution of species and mixed-species foraging flocks in boreal breeding bird communities.

Indirect cues of nest predation risk and avian reproductive decisions.

Current life-history theory predicts that increased mortality at early stages of life leads to reduced initial investment (e.g. clutch size) but increased subsequent investment during the reproduction attempt. In a field experiment, migratory pied flycatchers Ficedula hypoleuca perceived differences in mammalian nest predation risk and altered their reproductive strategies in two respects. First, birds avoided nest sites manipulated to reflect the presence of a predator. Second, birds breeding in risky areas nested 4 days earlier and laid 10 per cent larger clutches than those in safe areas, a result that runs counter to the prevailing life-history paradigm. We suggest that the overwhelming importance of nest predation to individual fitness reduces the value of collecting other information on habitat features leading to expedited onset of nesting, and, consequently, to larger clutch size.

How does variation in the environment and individual cognition explain the existence of consistent behavioral differences

According to recent studies on animal personalities, the level of behavioral plasticity, which can be viewed as the slope of the behavioral reaction norm, varies among individuals, populations, and species. Still, it is conceptually unclear how the interaction between environmental variation and variation in animal cognition affect the evolution of behavioral plasticity and expression of animal personalities. Here, we (1) use literature to review how environmental variation and individual variation in cognition explain population and individual level expression of behavioral plasticity and (2) draw together empirically yet nontested, conceptual framework to clarify how these factors affect the evolution and expression of individually consistent behavior in nature. The framework is based on simple principles: first, information acquisition requires cognition that is inherently costly to build and maintain. Second, individual differences in animal cognition affect the differences in behavioral flexibility, i.e. the variance around the mean of the behavioral reaction norm, which defines plasticity. Third, along the lines of the evolution of cognition, we predict that environments with moderate variation favor behavioral flexibility. This occurs since in those environments costs of cognition are covered by being able to recognize and use information effectively. Similarly, nonflexible, stereotypic behaviors may be favored in environments that are either invariable or highly variable, since in those environments cognition does not give any benefits to cover the costs or cognition is not able to keep up with environmental change, respectively. If behavioral plasticity develops in response to increasing environmental variability, plasticity should dominate in environments that are moderately variable, and expression of animal personalities and behavioral syndromes may differ between environments. We give suggestions how to test our hypothesis and propose improvements to current behavioral testing protocols in the field of animal personality.

Observed Fitness May Affect Niche Overlap in Competing Species via Selective Social Information Use

Social information transmission is important because it enables horizontal spread of behaviors, not only between conspecifics but also between individuals of different species. Because interspecific social information use is expected to take place among species with similar resource needs, it may have major consequences for the emergence of local adaptations, resource sharing, and community organization. Social information use is expected to be selective, but the conditions promoting it in an interspecific context are not well known. Here, we experimentally test whether pied flycatchers (Ficedula hypoleuca) use the clutch size of great tits (Parus major) in determining the quality of the observed individual and use it as a basis of decision making. We show that pied flycatchers copied or rejected a novel nest site feature preference of great tits experimentally manipulated to exhibit high or low fitness (clutch size), respectively. Our results demonstrate that the social transmission of behaviors across species can be highly selective in response to observed fitness, plausibly making the phenomenon adaptive. In contrast with the current theory of species coexistence, overlap between realized niches of species could dynamically increase or decrease depending on the observed success of surrounding individuals.

The effects of small-scale disturbance on forest birds: a meta-analysis

Small-scale disturbance is a significant process in all major forest biomes. Some silvicultural practices, particularly group selection harvesting, intend to emulate natural small-scale disturbance by harvesting small clearcuts in the continuous forest. We conducted a meta-analysis on the effects of small-scale harvesting on North American breeding forest birds. We extracted species richness and relative abundance of several functional bird groups and guilds from published studies and compared them between gap-dominated and unlogged forest as a function of forest type and the size and age of the gap. The abundance of many bird groups was higher in the gap-dominated than in the continuous forest. Species preferring interior parts of the forest had the most negative association with the presence of gaps but this relationship was not statistically significant. Abundances of many bird groups increased with increasing gap size, while its effect on abundance of some bird groups disappeared quickly. Our review s...