Laying order, hatching asynchrony and nestling body mass in Tree Swallows Tachycineta bicolor

We studied the reproductive biology of a box-nesting population of Tree Swa llows Tachycineta bicolor in southeastern Wisconsin, USA. We were intereste d particularly in the relationship between laying order and hatching order and the extent to which each was a predictor of nestling body mass. We foun d that laying order was a significant predictor of hatching order. Laying a nd hatching order were related to nestling mass at 4 days of age and to a l esser extent at 12 days of age. In addition, we investigated the effects of natural variation in hatching asynchrony. Hatching asynchrony was positive ly related to the range of nestling body masses within a brood at days 4 an d 12. The probability that brood reduction occurred was also positively rel ated to the degree of hatching asynchrony, though this effect was significa nt only at day 4. Our results suggest that laying order and hatching order have their greatest effects on nestling Tree Swallows early in the nestling period.