Geographic variation in body size and plumage colour according to diet composition in a nocturnal raptor

Abstract

Predator-prey interactions are amongst the strongest selective forces that promote the evolution of local phenotypes in both predators and prey. However, intraspecific spatial covariation in phenotypic traits between predators and prey has been rarely investigated, especially at a large geographic scale. Here, we studied the covariation between prey composition and some phenotypic traits, such as wing length, bill length and plumage colour, of a widely-distributed nocturnal predator, the western barn owl Tyto alba. By using 3100 specimens collected across its entire range of distribution, spanning from Europe to Middle East and Africa, we showed that wing length positively covaries with prey size, but not with taxonomic composition. This finding suggests that larger prey might have selected for larger body size and/or that larger individuals might be more selective in hunting large prey. In addition, we also found that paler-plumaged populations generally hunt larger prey. Paler barn owls might be thus better specialized in capturing averagely larger prey and/or mainly hunt in habitats where larger prey are more abundant. In addition, considering that paler individuals are generally larger than brownish ones, it is possible that paler plumage colour might have evolved as a by-product of selection towards a large body size, which in turn have emerged in response to prey size composition. However, irrespectively of the direction of causality and the phenotypic target of selection, we showed that predator-prey interactions can affect spatial phenotypic variation by promoting the evolution of local adaptations.

Where and when local adaptation happens: lessons from the European barn owl (Tyto alba)

“…These results emphasise the potential link between phenotypic variation and the local adaptation of the barn owl. A previous study identified a significantly positive relationship between body size and the hunted prey size, suggesting that larger owls might be more competitive in hunting larger prey (Romano et al 2021). (Romano et al 2020) also highlighted a significant geographical and climatic trend in the diet of this raptor species, with an increase in the proportion of small prey in the diet of colder environments (including Central and Northern Europe).…”

“…(Romano et al 2020) also highlighted a significant geographical and climatic trend in the diet of this raptor species, with an increase in the proportion of small prey in the diet of colder environments (including Central and Northern Europe). Finally, this variation in body size also co-varies with colour polymorphism (with smaller owls being redder, (Romano et al 2021), a trait known to be maintained by selection (Antoniazza et al 2010), and linked to hunting behaviour (Roulin 2004; Romano et al 2021). Taken together, these results suggest an association between the size of the barn owl, their colour, their behaviour and their diet, all possibly linked to the local adaptation of the barn owl to heterogeneous climatic conditions in Europe.…”

Island volcanism predicts pheomelanin‐based plumage colouration in a cosmopolitan raptor

“…We recorded information on plumage colour of thousands of barn owl skins collected by several museums and private citizens (see Romano et al, 2020Romano et al, , 2021aRomano et al, , 2021bRomano, Séchaud, et al, 2019).…”

“…This categorization was used in several previous studies (e.g. Romano et al, 2021aRomano et al, , 2021bRomano, Séchaud, et al, 2019;Roulin et al, 2018) and has been shown to be highly correlated with reflectance in the visible light when compared to spectrophotometric measures (Dreiss & Roulin, 2010). Moreover, this categorization is correlated with pheomelanin, but not eumelanin (Roulin et al, 2008), thus indicating that it is a good proxy of pheomelanin content.…”

“…When distinct selective pressures act in different populations, but also when the same selective pressures vary spatially due to differences in climate, environmental conditions and community composition as well as in breeding behaviour and sexual selection, the presence and/or distribution of alternative colour morphs may differ among populations (Cuthill et al, 2017;Hill & McGraw, 2006). Under such circumstances, polymorphism may vary spatially with populations being differently pigmented along geographical gradients, as shown in several model systems at different scales (Cerezer et al, 2020;Delhey et al, 2019Delhey et al, , 2020Passarotto et al, 2022;Romano et al, 2021b;Romano, Séchaud, et al, 2019).…”