Abstract
Colour polymorphism may be maintained within a population by disruptive-selection. One hypothesis proposes that different morphs are adapted to different ambient light conditions, with lighter morphs having a selective advantage in bright conditions and darker morphs having advantages in darker conditions. The mechanism for this advantage is proposed to be through enhanced crypsis via background-matching. We explore this hypothesis in a polymorphic raptor, the black sparrowhawk Accipiter melanoleucus, which exhibits a discrete dark and white-morph. We use GPS-tracking data to contrast the foraging behaviour and habitat selection of morphs. As predicted, we found that light-levels influenced foraging behaviour in different ways for morphs: Dark-morphs showed a decrease in foraging with increasing light-levels; whereas no relationship was found for white-morphs. Furthermore, we found differential-degrees of habitat selection, with dark-morphs selecting more enclosed habitats compared to white-morphs. This suggests that different morphs may be better adapted to foraging under different light-conditions, potentially playing a role in maintaining colour polymorphism in this species. Our results may also help explain why dark-morphs predominate in this study region, which experiences high rainfall and lower light-levels during the breeding-period. This study suggests that avian morphs may allocate/partition foraging activity by weather conditions/habitat, which maximise their concealment from prey.
Highlights
Colouration has been widely recognised to play an important role in a variety of ecological processes, from camouflage[1] to intraspecific communication[2] and mate choice[3], all of which are likely to be under considerable selective pressure from both natural and sexual selection[4]
During the active breeding season (April–October, with peaks in May and August), we found that overall foraging activity was greater in dark morphs than white morphs (χ21 = 16.5, P < 0.001)
After controlling for breeding stage, there was a significant interaction between morph and light levels (χ21 = 18.2, P = 0.01); for white morph birds, foraging activity was not influenced by light conditions (χ21 = 8.7, P = 0.8; Fig. 1), whereas activity levels for dark morphs declined substantially and significantly as light levels increased (χ21 = 4.8, P < 0.001; Fig. 1)
Summary
Colouration has been widely recognised to play an important role in a variety of ecological processes, from camouflage (crypsis)[1] to intraspecific communication[2] and mate choice[3], all of which are likely to be under considerable selective pressure from both natural and sexual selection[4]. Colouration and patterning in their plumage is likely to play a vital role in camouflage and crypsis via background matching[1, 4]. According to this principle, the more similar to the visual background the colours and geometry of patterns of an individual are, the more concealed it should be[2]. As ambient light condition strongly influences the level of crypsis in certain colours[20, 21], and affects the ability for prey to visually detect predators, or for predators to detect prey[7], light could play an important role in driving disruptive selection in polymorphic species. Lighter morphs may be less detectable in brighter conditions and may forage more successfully as a result under these conditions (i.e. on sunny days, in the middle of the day, or in open habitats)[18]
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