Matching Multiple Backgrounds: Egg Camouflage Across Different Habitats in a Shorebird

For species found in multiple habitats, the problem of camouflage against visually different backgrounds can be challenging. This is particularly so for bird eggs in open nests, as the scope for movement or alternative defensive strategies is limited. We studied egg camouflage in a small shorebird, the Kentish plover Anarhynchus alexandrinus, in two different coastal habitats in Cádiz province, Spain: sandy beaches and saltmarshes. Using calibrated photographs taken in situ and neurophysiologically plausible models of colour and pattern vision, we assessed the predicted discriminability of egg colour and patterning from those of backgrounds for likely nest predators (avian and mammalian carnivore) and, for comparison, humans. The findings suggest that at close range A. alexandrinus eggs are more susceptible to detection by visual predators based on their patterns (aka visual texture) rather than their colours, but at distances beyond which individual pattern elements can be resolved, they are highly cryptic. Although the colours and patterns of the saltmarsh and beach nest sites differ, the colours and surface patterning of eggs do not, suggesting that there is no local adaptation. However, the colours of eggs are similar to the types of background colours that overlap between the beach and saltmarsh. This suggests that, although the gross visual appearances of beach and salt marsh are quite different, egg camouflage in Kentish plovers relies on behavioural nest-site selection and a good colour match to the average location type. The maculation on the eggs does not appear to represent background matching in terms of pattern, so its function remains speculative.