An assessment of possible intraspecific brood parasitism in ring-billed gulls

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Abstract
Ring-billed gull (Larus delawarensis) egg colour, egg dimensions, egg shape, egg-laying intervals, nest initiation dates, and fertility were documented at several different colonies over a 6-year period. Significantly greater variability in egg dimensions and egg shape, lower fertility, and atypical laying intervals in mixed-coloured clutches compared with even-coloured clutches suggest that more than one female laid eggs in mixed-coloured clutches. We present evidence that some odd-coloured eggs were laid by females that parasitized nests.