47.4 Thursday, Jan. 5 Egg pigmentation varies with laying order and differs in populations at different latitudes ALDREDGE, R.A.*; SOCKMAN, K.W.; Univ. of North Carolina at Chapel Hill; Univ. of North Carolina at Chapel Hill email@example.com
Development time can vary widely both within and among organisms. In birds the incubation period is shorter for open-cup than cavity nesting birds, decreases as the season progresses and is shorter for temperate than for tropical breeding birds. Recent evidence suggests that photoperiod might partially explain why incubation periods are shorter for nests exposed to more hours of light per day (longer photophases). Females may be able to regulate this photoacceleration by altering certain egg properties, such as eggshell thickness or degree of pigmentation. We investigated 1) whether egg pigmentation differs between three populations of house sparrows (Passer domesticus) at different latitudes and 2) whether egg pigmentation varies over the laying sequence of the clutch. Pigmentation was greatest at our high latitude site, which experiences the longest photophases, declined at our geographically intermediate site, and was lowest at our tropical site, where photophases are the shortest. Additionally, the density of egg pigmentation decreased in last-laid eggs relative to earlier-laid eggs, regardless of site. Although we do not know the functional significance of these results, they raise the hypothesis that egg pigmentation increases the similarity in incubation periods across populations, a pattern that is unexpected given the latitudinal difference in avian incubation periods and increases the similarity in hatching synchrony within clutches.