Can intraspecific brood parasitism be detected using egg morphology only?

Intraspecific brood parasites lay their eggs in the nests of conspecifics. There are a number of methods for detecting intraspecific brood parasitism (IBP) in birds based on egg morphology. Here we test Eadieʹs (1989) method, which calculates the Euclidean distances between eggs in a given clutch in a three-dimensional space (weight, length and width). A parasitised clutch is predicted to contain an egg (or eggs) that is significantly different from the clutchʹs other eggs. Data from three species were analysed. Our captive zebra finch Taeniopygia guttata clutches did not include any instances of IBP, the wild jackdaw Corvus monedula data were unlikely to contain any, and for the goldeneye Bucephala clangula data set we had an observational estimate of IBP. We simulated IBP in the zebra finch, jackdaw and goldeneye data to test whether the method reliably detects an experimentally ‘parasitised’ clutch. We show that the distributions of the test statistics greatly overlap in ‘parasitised’ and unmodified clutches, and are dependent on the clutch size. We therefore conclude that the method can only be used with caution, after calibrating it for a given population.