Abstract
The direction and strength of selection for prey colouration by predators vary in space and time and depend on the composition of the predator community. We tested the hypothesis that bird selection pressure on prey colouration changes through the season due to changes in the proportion of naïve juvenile individuals in the bird community, because naïve and educated birds differ in their responses to prey colours. Bird predation on caterpillar-shaped plasticine models in two boreal forest sites increased sevenfold from early summer to mid-summer, and the time of this increase coincides with the fledging of juvenile birds. In early summer, cryptic (black and green) models were attacked at fivefold higher rates compared with conspicuous (red and yellow) models. By contrast, starting from fledging time, cryptic and conspicuous models were attacked at similar rates, hinting at a lower selectivity by naïve juvenile birds compared with educated adult birds. Cryptic models exposed in a group together with conspicuous models were attacked by birds at a threefold lower rate than cryptic models exposed singly, thus supporting the aposematic commensalism hypothesis. However, this effect was not observed in mid- and late summer, presumably due to the lack of avoidance of conspicuous prey by the juvenile birds. We conclude that selection pressure on prey colouration weakens considerably when naïve birds dominate in the community, because the survival advantages of aposematic colouration are temporarily lost for both the conspicuous and their neighbouring cryptic prey.
Highlights
Birds use visual cues to find their prey, and bird predation is an important factor driving the evolution of colouration in various prey species (Lindstedt et al 2011; Chouteau et al 2016)
Two major defensive strategies have evolved in organisms to reduce attacks by predators through colouration: crypsis, when prey avoids detection by adopting colours of their backgrounds, and aposematism, when prey is signalling about its defences through conspicuous colour patterns (Ruxton et al 2018)
Attack rates on plasticine models significantly differed between the observation periods (Table 1): they were lowest in early summer, started to increase during the third week of June, and remained high through mid- and late summer, followed by a decrease in September (Fig. 1)
Summary
Birds use visual cues to find their prey, and bird predation is an important factor driving the evolution of colouration in various prey species (Lindstedt et al 2011; Chouteau et al 2016). The direction of selection pressure may differ between localities, depending on the species composition of the bird community (Nokelainen et al 2014). It may differ within a locality, because bird communities change seasonally with respect to the proportion of naïve juvenile birds (Mappes et al 2014). Adult birds generally avoid aposematic colours and patterns, because they have memorised previous unpleasant experiences with chemically defended or unpalatable prey usually possessing conspicuous colouration (Stevens and Ruxton 2012; Skelhorn et al 2016). Juvenile birds have no such experience and usually must learn to avoid unpalatable prey based on colouration, unlearnt biases against certain colours can exist (Lindström et al 1999; Halpin et al 2020)
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