Differential maternal investment across European starling (Sturnus vulgaris) clutches

Abstract

Most passerine species begin incubating their clutches once the penultimate egg is laid, allowing for asynchronous hatching of nestlings. This results in delayed development by 24 hours or more of the nestling hatching from the last-laid egg. The brood survival hypothesis postulates that maternal investment into this last-laid egg increases such it will be larger relative to other eggs in the clutch to compensate for delayed development initiated by asynchronous hatching. Alternatively, the brood reduction hypothesis states that maternal investment into the last-laid egg decreases such that it will be smaller relative to the rest of the clutch, allowing this smallest nestling to act as an insurance policy whose survival depends on food availability during the breeding season. Ample food will allow this nestling to survive, while food scarcity will cause it to die without risking the entire brood. European starlings (Sturnus vulgaris) are cavity-nesting, groundfeeding passerines who eat terrestrial invertebrates. They exhibit biparental care of asynchronously-hatched young and typically have two clutches in a season. The objective of this study was to determine whether female European Starlings laid a larger or smaller last-laid egg relative to the rest of the clutch. I predicted that females would follow a brood reduction strategy due to unpredictable food availability. I studied 189 clutches over five years and found that, as predicted, the last-laid eggs laid by females were significantly smaller than other eggs in the clutch. Late clutches were significantly smaller than early-season clutches, but last-laid eggs did not differ in mass from last-laid eggs in early clutches, nor did the mean mass of all other eggs differ. Adoption of the brood reduction strategy in European starlings is likely due to fluctuating food availability; a decrease in clutch size may reflect the typical downward trend of food abundance throughout the breeding season.