Differential predation of eggs in clutches of Glaucous-winged Gulls Larus glaucescens

Differential predation of eggs of Glaucous-winged Gulls Larus glaucescens was studied on Mitlenatch Island, British Columbia. The predation of single eggs from 3-egg clutches by conspecifics and Northwestern Crows Corvus caurinus was recorded to assess whether the predators selected eggs at random or showed a preference for a specific egg in the laying sequence. It was found that small and light-coloured eggs, characteristically the last-laid egg in the clutch, were taken significantly more often than the other two eggs in the clutch. As chicks derived from this third egg are least likely to fledge because of their small size and the effects of asynchronous hatching, the lightness of the third egg is adaptive in that it helps to focus attention on the small egg.