Hatching Asynchrony in the Yellow-headed Blackbird (Xanthocepha!lIs xantlhocephalus)
Bommarito, Aaron Jonathan
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Hatching asynchrony of nidicolous altricial birds leads to a distinct size hierarchy among siblings. Only 1 egg hatches per day which results in a large difference in body size between the first and last egg hatched. Theory predicts that if food is scarce, parents may find it advantageous to reduce the size of the brood by one or two individuals so as to ensure the nutritional needs of the other brood members. Typically the younger hatchlings will suffer the most from brood reduction because they are the smallest and cannot compete for food as well as their older and larger siblings. However, when the youngest hatchling survives to approximately day 5 it has an increased chance of survival. During the 1996 breeding season we studied Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) in the prairie pothole region of South Dakota to develop a better understanding of the impact of hatching asynchrony on hatchling physiology and growth during the nestling period. Originally 104 eggs were measured in 31 nests with average clutch sizes of 3 or 4 eggs. However, nest abandonment caused us to delete 10 nests and 27 eggs. Of the remaining 77 eggs, 54 hatched with only 20 individuals surviving to the point of fledging. Nestlings that hatched later than other siblings increased in size rapidly at about day 5 posthatching and almost reached the size of their older siblings by the time of fledging. This rapid growth may have enhanced their ability to compete for food and perhaps escape brood reduction. A reasonable explanation for the increased growth of all hatchlings after day 5 may be related to the achievement of an enlarged gape enabling them to eat an increased amount of food and convert it to biomass for growth.