Meeting Abstract
Finding links between hormonal phenotypes and fitness has proven difficult because of considerable within- and among-individual variation in hormone concentrations. One valuable approach is to quantify within-individual hormonal flexibility along an environmental gradient and to relate the resulting reaction norm to fitness-related traits. In wild great tits (Parus major), we experimentally varied work load during the breeding season to manipulate cellular oxidative stress. We trapped 46 birds of both sexes when their nestlings were 7 days old. All birds were blood-sampled to determine baseline corticosterone levels, oxidative damage and antioxidant compounds. Before immediately releasing the birds, we symmetrically clipped three primary feathers of 13 males (11 males served as controls). Males with clipped primaries are known to decrease offspring provisioning rates, which induces a compensatory increase in work load in their female mates. When nestlings were 12 days old we re-trapped and re-sampled all focal adults. Provisioning rates were determined from video-recordings. We quantified proxies of reproductive success like lay date, clutch size, hatching success, fledging success, and fledgling body condition. The data will be analyzed using mixed effect models to test the following predictions: 1) individuals will differ in their glucocorticoid responses to increased work load, 2) parameters describing oxidative status will explain among-individual variation in glucocorticoid responses, 5) the interplay between oxidative status and glucocorticoid flexibility will be related to reproductive investment and to fitness.
