Assessing the role of body mass and sex on apparent adult survival in polygynous passerines: a case study of cetti's warblers in central Portugal
David Monticelli, Pedro M. Araújo, James E. Hines, Paulo Q. Tenreiro, Luis P. Silva, Jaime A. Ramos
Adult survival, an important fitness component, is usually 1) lower in lighter individuals due to their reduced ability to survive winter conditions compared to heavier ones, especially in resident species at northern temperate latitudes and 2) lower in females compared with males due to higher reproductive costs incurred by females. In this paper, a capture–mark–recapture dataset of 649 cetti's warblers Cettia cetti ringed seasonally at two wetlands in central Portugal over an 11-yr period (2000–2010) was modelled in a multi-state framework to examine the influence of these individual covariates on apparent adult survival, while controlling for the presence of transient individuals in our study area. The probability of change in mass state (ψLight→Heavy, ψHeavy→Light) during the annual cycle was also estimated. Overall, birds survived better during spring–summer (breeding/moulting periods) compared with autumn–winter, but there was no effect of body mass on apparent adult survival probability. The modelling detected a significant interaction between sex and season, in which resident females survived better than resident males in spring–summer (ϕRF= 0.857 ± 0.117 and ϕRM= 0.698 ± 0.181) while the opposite pattern was found in autumn–winter (ϕRM= 0.440 ± 0.086 and ϕRF= 0.339 ± 0.084). In addition, cetti's warblers had a tendency to lose mass in spring–summer (ψHeavy → Light= 0.560 ± 0.063) and to regain mass in autumn–winter (ψLight→Heavy= 0.701 ± 0.069). This pattern of body mass change during the annual cycle may reflect energetic costs to reproduction and moulting, and/or a response to increased starvation risk during winter. High body mass, however, did not increase adult survival in this population presumably due to the relatively mild winter weather prevailing in central Portugal. Survival estimates are more likely to be explained by important ecological and behavioural differences between the two sexes in polygynous passerines. Our results highlight that studies aiming to identify the main factors shaping survival and individual fitness in polygynous species should be conducted during different phases of their annual cycle.