Breeding biology of Western Bonelli's Warblers Phylloscopus bonelli in the Mediterranean region

Capsule: The Western Bonelli's Warbler Phylloscopus bonelli has a nest success of only 25% in the core of its range in western Europe.Aims: To investigate the breeding biology of Western Bonelli's Warbler P. bonelli, focusing on possible altitude effects and potential reproductive problems.Methods: Three Western Bonelli's Warbler populations were monitored during the 2012 and 2013 breeding seasons in the massif range of Sierra Nevada, Spain. We determined all the breeding parameters and calculated daily survival and success rates for each reproductive period.Results: The three studied populations did not differ in any breeding parameters. Altitude showed a positive relationship with clutch size and duration of incubation period, but a negative relationship with nestling tarsus growth and body mass gain. Daily survival rates during incubation and nestling periods were similar to those of common warblers, but the species presented a low breeding success of 25%.Conclusion: The absence of differences among the three populations suggests that the information provided here could be representative of its distribution in the woodlands of Sierra Nevada. The novel and detailed information reported is crucial not only for expanding our understanding of this species but also to draw attention to the potential risks that it might face in the near future, considering the reduction that this species has suffered in Sierra Nevada during recent decades.