Egg-size variation in the bluethroat (Luscinia s. svecica): constraints and adaptation

We examined inter- and intra-clutch egg-size variation in the bluethroat (Luscinia s. svecica), an open-nesting passerine breeding in the sub-alpine region in southern Norway. By removing first clutches shortly after egg-laying, we induced laying of a repeat clutch. Females significantly reduced the number of eggs from the first to the second nesting attempt, but increased mean egg size. Females in good condition laid significantly larger eggs than those in poor condition. Consistent with predictions of the “brood survival” hypothesis, assuming an adaptive investment in last eggs to ensure survival of all eggs in the clutch, we found that the size of the last eggs in first clutches was generally larger than the mean egg size of the clutch, and that the relative size of the last egg increased with clutch size. However, a large last egg reflected a general increase in egg size throughout the laying sequence rather than a specific investment in the last egg only. Egg size was not significantly influenced by sex or paternity (within-pair versus extra-pair) of the embryo. In repeat clutches the last egg was not consistently larger than the mean for the clutch. We conclude that female bluethroats face resource limitations during egg formation early in the season, and that the patterns of increase in egg size with laying order for first clutches, and from first to repeat clutches, can largely be explained by proximate constraints on egg formation.