Bird eggs come in an extraordinary range of colours and patterns, from neutral greys and whites to startling blues and reds, and from completely plain to densely speckled. It is thought that this huge variety has evolved in order to help parent birds to recognise their own offspring, and importantly, to distinguish them from the eggs of brood parasites such as the cuckoo, whose success relies on mimicry. Although there is considerable evidence to support that the more closely cuckoo eggs resemble those of their host, the more sensitive the perceptive processes of the species become to detecting the presence of foreign eggs, it is not clear whether mimicry by parasites also drives the evolution of egg patterns to make them easier to recognise.
A recent study by Stoddard and colleagues therefore sought to establish whether this was the case. The research examined eight European species parasitised by the common cuckoo, making use of a new computer tool called NaturePatternMatch, which assesses the ‘recognisability’ of egg patterns based on known visual and cognitive processes. There are three key features that are thought to contribute towards making patterns easily distinguishable from an invading brood parasite:
- reduced pattern variation within each clutch of eggs (known as intraclutch variation), so that it is easier to spot an odd one out.
- increased pattern variation between eggs laid by different females (known as interclutch variation), making it difficult for brood parasites to evolve to closely resemble all patterns at the same time.
- high complexity of patterning, making it difficult for the brood parasite to accurately replicate.
The results support that host species with more recognisable egg patterns are better at rejecting foreign eggs from their nest, and that hosts that are mimicked most closely by their cuckoo parasites have the most recognisable egg patterns. Interestingly, however, they found that the three features expected to contribute to improved recognisability were rarely all present in combination, and in fact, observed that highly complex patterns were detrimental, presumably because it then became difficult to identify distinguishing features. Instead, patterns with intermediate complexity in which features were well spaced out were most easily distinguishable. The results indicate that each species that produces highly recognisable egg patterns appears to achieve this in a different way. For example, while brambling egg patterns have widely spaced features that make them easy to identify, garden warbler patterns show low spatial dispersion of features, but instead have extremely low intraclutch variation. In this way, host species that are targeted by the common cuckoo are evolving to fight back in the ongoing evolutionary arms race.
Ref: Stoddard M. C., Kilner R. M. & Town C. (2014) Pattern recognition algorithm reveals how birds evolve individual egg pattern signatures. Nature Communications 5: 4117.
Aidala Z. & Hauber M. E. (2010) Avian Egg Coloration and Visual Ecology. Nature Education Knowledge 3: 53.
Photo: Reed Warbler Nest 18.05.11 by nottsexminer