Estimation of moult duration in birds with suspended moults: the case of the Red Crossbill and its relation to reproduction
Suspended moult is a relatively common phenomenon in birds, but it has remained relatively ignored for a long time in studies dealing with moulting process. Furthermore, the new and increasing number of models used to estimate moult duration systematically omit the fact that suspended moults can occur and that they significantly alter parameter estimates. Taking suspended moults into account is critical to obtain reliable moult-associated parameter estimates, which is fundamental given the demographic and evolutionary consequences of moult in birds. The main goal of this work is to develop a standardised method useful for characterising the main moulting parameters (such as the duration and starting date) of bird species that perform suspended moults. Additionally, with the aim of delving into our understanding of when and why suspended moults happen, we study their relationship with the occurrence of second breeding attempts in summer. We used data obtained from a Red Crossbill population in the Pyrenees during a period of 17 years. We observed that the percentage of crossbills undergoing suspended moult was approximately 50% during summer (July-August) and that moult suspension ultimately gives rise to a lengthened moulting period. Underhill-Zucchini models assume a time-linear replacement of feathers, overestimating the moult duration. Based on these same modelling approaches, we proposed a method to calculate the real moult duration after removing the time during which the moult was suspended. We also obtained evidence supporting the idea that crossbills suspend their moult as a strategy that could increase their breeding output in summer. The method proposed here can be used as a tool for working with species that undergo suspended moults.