Cooperative Fish and Wildlife Research Units Program: Wyoming
Education, Research and Technical Assistance for Managing Our Natural Resources

The young of some altricial bird species hatch asynchronously, sometimes leading to considerable size differences among siblings. Asynchronous hatching may comprise a bet-hedging strategy whereby adults can maximize the probability of successfully fledging at least one young from the nest even when resources are limited. Despite the potential importance of intrabrood variation in nestling size to the breeding strategy of altricial birds, the extent of this phenomenon remains largely unclear for many species. We weighed 453 nestlings from 148 nests of three sympatric, sagebrush-associated songbird species in Wyoming, USA to describe the magnitude of intrabrood size differences for these species. Intrabrood differences in nestling mass were greatest for the largest species, the Sage Thrasher (Oreoscoptes montanus), for which the smallest nestling in a brood was on-average 26.2% smaller than the largest. The smaller Vesper Sparrow (Pooecetes gramineus) and Brewer’s Sparrow (Spizella breweri) exhibited similar intrabrood size ratios, with the smallest nestling being 17.4% and 18.4% smaller on average than the largest for the two species, respectively. For each additional nestling within a brood, the smallest nestling was an additional 6.6 ̶ 13.6% smaller than the largest nestling, depending on species. Understanding the extent of intrabrood variation in nestling traits has important implications for the productivity of sensitive species facing increasingly unpredictable environments.