Seasonal declines in incubation periods of Brunnich's Guillemots Uria lomvia: testing proximate causes

In many species of bird, eggs laid late in the laying period hatch after a shorter incubation period than do eggs laid early. These seasonal declines in incubation period are generally thought to confer evolutionary advantage s, but the proximate mechanisms that underlie them are poorly understood. S easonal declines in incubation period are usually attributed to: (1) season al increases in ambient air temperatures; (2) seasonal changes in the behav iour of incubating birds; and/or (3) seasonal declines in egg size. In a pr evious study, Common Guillemot Uria aalge incubation periods declined with laying date at a low-Arctic colony. As there was no support for hypotheses 1 or 2, it was suggested that this occurred because egg size declined with laying date, but eggs were not measured in that study. We recorded similar seasonal declines in the incubation periods of the single eggs laid by Brun nich's Guillemots Uria lomvia at two low-Arctic colonies in four years. Nei ther seasonal variation in ambient air temperatures, nor in the behaviour o f incubating adults, appeared to cause the declines. As predicted for Commo n Guillemots, incubation period increased with egg size among Brunnich's Gu illemots, in one of two years. However, incubation period declined with lay ing date in the absence of corresponding declines in egg size. We conclude that none of the three commonly proposed proximate mechanisms adequately ex plains the seasonal variation in guillemot incubation periods. Several test able, alternative mechanisms are explored.