Seasonal declines in replacement egg-layings in a long-lived, Arctic seabird: costs of late breeding or variation in female quality?

1. In many species of birds, the proportion of females that relays after lo sing their first clutch varies. Replacement clutches can make important con tributions to an individual's lifetime reproductive success, so the decisio n on whether or not to relay may have significant consequences for fitness. 2. Many studies report that female birds of 'high quality' (e.g. older and more experienced) were more likely to relay following clutch loss, and that the proportion of females that relaid declined as the breeding season prog ressed. High quality females tend to nest early, and may be less likely to lose their clutches. As a result, it is unclear whether few females relay l ate in the season because there are prohibitive costs associated with late breeding, or because those females that lay and lose their eggs late are of low quality, and therefore incapable of relaying, 3. We examined the roles of dare and quality in causing seasonal declines i n replacement layings in Brunnich's guillemot [Uria lomvia (Linnaeus)], a l ong-lived, Arctic marine bird that lays a one-egg clutch. At two low-arctic colonies in eastern Canada in 1996 and 1997, the single egg was experiment ally removed from samples of early-laying pairs (presumably those of high q uality) at 3-day intervals beginning on the median laying date and ending 1 2 days later. 4. In the general population, the proportion of pairs that relaid following natural egg loss declined with the date of loss. In contrast, the proporti on of experimental pairs that relaid remained high irrespective of the date their egg was removed. These results support the quality hypothesis, but n ot the date hypothesis. We conclude that seasonal declines in replacement l ayings occurred primarily because increasing proportions of low quality fem ales lost eggs as the season advanced. 5. The experimental pairs also suffered no overt reduction in reproductive success as a result of the delay, at least up to the time that their chicks departed to sea. This suggests that timing of breeding had little effect o n the immediate success of more capable pairs. 6. Our results may have impo rtant implications about the seasonality of low-Arctic marine environments, and the life histories of birds that inhabit them.