Tibetan Birds Lay Larger Eggs at the Cost of Egg Number
Life history theory predicts that when breeding in harsh environments, selection favors organisms to allocate more energy into fewer offspring at the expense of offspring number. However, it is unclear whether such a trade-off remains evident in the presence of parental care, which might compromise the maternal investment to offspring quality. We address this question using a comparative approach for phylogenetically-paired passerines breeding in the Tibet Plateau vs. adjacent lowlands, the two
... t lowlands, the two systems that sharply differ in environmental hardness. While total biomass of eggs within a clutch became lighter towards Tibet, clutch size decreased and egg size increased. Tibetan birds were more time-constrained in breeding duration but had longer incubation and nestling periods than their lowland counterparts. Despite so, nestlings reared in Tibet were of similar body mass at fledging to those reared in lowlands. Therefore, more investment in fewer eggs as well as in incubation and provisioning young suggests that Tibetan birds are making the best of a bad job to ensure offspring survival under the hard conditions. This research supports a role of adaptive reproductive allocation by animals with parental care in the evolution of life history strategy along elevational gradients.