The niche variation hypothesis and the evolution of colour polymorphism in birds: a comparative study of owls, nightjars and raptors

Abstract

We studied the evolution of colour polymorphism in diurnal raptors, owls and nightjars, the avian taxa in which this trait is most widespread, in relation to species ecological niche width and diet. Two main mechanisms have been put forward to explain the maintenance of polymorphism, namely apostatic selection and disruptive selection. The niche variation hypothesis states that species with broader ecological niches should be more variable compared with those with narrow niches because of the action of disruptive selection; the apostatic selection hypothesis conversely suggests that intraspecific colour variation should be promoted in predators by prey forming an avoidance image for the more common colour morph. Our aim was to determine if colour polymorphism occurrence was associated with broad ecological niches as predicted by the niche variation hypothesis, or with predation on intelligent and sharp-sighted prey as predicted by the avoidance image hypothesis. Pairwise comparisons were made between pairs of closely related species differing in variables expected to influence the occurrence of polymorphism. We found that polymorphic species of all three groups showed wider and more continuous distribution ranges, frequented many different habitats, both open and closed, and lived in seasonally alternating dry/wet climates. Polymorphic species were more migratory compared with monomorphic ones, and they showed an activity pattern covering both day and night. Conversely, colour polymorphism was not higher in species preying on birds and mammals. All these findings support the hypothesis that colour polymorphism evolved in bird species with wider niche breadth and not in species preying on intelligent prey. Therefore, we propose that disruptive selection may be the main mechanism maintaining colour polymorphism in these bird groups by favouring different morphs in different environmental conditions.

Climate fluctuations and differential survival of bridled and non‐bridled Common GuillemotsUria aalge

“…(1) Different color patterns may be adaptive under different environmental conditions by providing behavioral or physiological advantages (Galeotti and Cesaris 1996, Galeotti and Rubolini 2004, Galeotti et al 2009). In the Common Guillemot genes for bridling may have a pleiotropic effect, such that color dimorphism may be associated with another property that is the real target of selection.…”

“…Furthermore, different color patterns may be adaptive under different environmental conditions by providing behavioral or physiological advantages to their bearers (e.g., Cesaris 1996, Galeotti andRubolini 2004). Galeotti et al (2009) also suggested that the equilibrium between gene frequencies could vary under different conditions and habitats, if the lifetime fitness of different morphs differed between habitats, and that this equilibrium could be disrupted by climate change through direct and indirect effects on fitness.…”

Global changes and animal phenotypic responses: melanin-based plumage redness of scops owls increased with temperature and rainfall during the last century

“…A comparative study suggested that polymorphic species of owls showed a wider niche than monomorphic ones, since they frequented many different habitats, both open and closed, lived in seasonally alternating dry/ wet climates and were active during both day and night (Galeotti & Rubolini 2004). Such results suggest that different colour patterns may be adaptive in different environmental conditions by providing behavioural or physiological advantages to their bearers.…”

Comparative support for the niche variation hypothesis that more generalized populations also are more heterogeneous

“…Although the NVH is intuitively appealing and has some empirical support (1,(3)(4)(5)(6)(7), it also has been heavily criticized on both empirical and theoretical grounds. A reanalysis of some of the data that first inspired the NVH found that island populations of common chaffinches (Fringilla coelebs) were in fact not more variable than their mainland counterparts (8).…”