Individual variation in parental tradeoffs between the number and size of offspring at the pre‐ and post‐natal stages

Abstract

Life‐history theory predicts that parents refer to the resources they hold to determine their breeding strategy. In multi‐brooded species, it is hypothesized that single‐brooded parents produce larger clutches and raise offspring with a brood survival strategy, whereas multi‐brooded parents only do this under good breeding conditions. Under poor conditions, they produce smaller clutches and raise offspring with a brood reduction strategy. We tested this hypothesis in the Brown‐cheeked Laughing Thrush Trochalopteron henrici, which can breed twice a year on the Tibetan Plateau, by investigating the life‐history traits and provisioning behaviours of single‐ and double‐brooded parents. Single‐brooded parents laid larger clutches of smaller eggs and produced more and larger fledglings than double‐brooded parents in their first brood. Double‐brooded parents produced smaller clutches of larger eggs but fledged larger nestlings in their first brood than in their second brood. As single‐brooded parents only need to raise one brood a year, then producing and raising as many offspring as possible (i.e. the brood survival strategy in a large brood) can maximize their reproductive success. For double‐brooded parents, producing and raising fewer offspring in the first brood (i.e. the brood survival strategy in a small brood) can ensure their nesting success during a short breeding cycle. Additionally, producing more offspring but raising larger nestlings in the second brood (i.e. the brood reduction strategy in a large brood) can select for offspring of higher quality within the brood. Our findings indicate that different tradeoffs between single‐ and double‐brooded parents in egg‐laying and nestling‐raising may be an adaptation to the seasonal variation in environmental conditions.