Why some bird eggs are patterned while others are plain has long intrigued scientists, but a recent study of hirundine eggs has offered some interesting explanations.
Previous researchers have suggested egg spotting (or maculation) as having potential functions in camouflage, structural support, mimicry, egg or clutch recognition and signalling. However, there has been little consensus on the main factors influencing diversity among bird eggs.
A recent study, published in IBIS, investigated the relationships between eggshell maculation, social breeding behaviour and nest type in the family Hirundinidae (swallows and martins) to test for correlated evolution between these traits.
Maculation, as seen on these Swallow eggs, was found to be commoner among open-nesting hirundines (Kati Fleming via Wikimedia Commons).
Investigating hirundine eggs
The team led by, Iris Levin, looked at 61 swallow and martin species, considering whether each is solitary or breeds in colonies, eggshell maculation and nest type (having an open cup or closed/cavity nest). The diverse Hirundinidae family offered the chance to explore whether eggshell patterns act as an identity signal for recognition during nesting in crowded colonies or in reaction to brood parasitism.
The results showed a correlation between social breeding, open-cup nesting and the laying of maculated eggs. Five out of eight species with documented conspecific brood parasitism turned out to lay maculated eggs. These results back up previous studies that indicate a lower occurrence of patterned eggs in cavity-nesting birds.
The researchers suggested that unmarked white eggs could help parents birds find them in the poor light conditions associated with cavity nests, while maculation in better-lit nests made of an open cup could aid recognition. The correlation between social breeding and eggshell patterns suggests that maculation, along with low variation within a clutch, helps birds pinpoint their own nests in bustling colonies.
While maculation may assist in recognising foreign eggs in cases of conspecific brood parasitism, the team highlighted that there was little evidence of egg rejection in the relevant species.
Levin and her colleagues say there would be much to learn if other researchers performed similiar studies on other bird families, adding to our understanding of the adaptive value of eggshell markings.
Reference
Levin, I I, Kaufman, S L, Knaysi, S E, and Rataezyk, O G. 2023. Correlated evolution of eggshell maculation with social breeding and nest type in Hirundinidae, IBIS, DOI: 10.1111/ibi.13118
