Laying date depends on territorial breeders’ age, climatic conditions and previous breeding success: a long-term study (2004–2021) with Bonelli’s eagle in Spain

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Abstract
Adjustment of reproductive timing within the annual cycle is a crucial issue for both offspring and parents’ survival, and breeding success. Early laying date is closely related to successful breeding outcome and better survival. Obtaining long datasets on threatened long-lived species’ breeding performance can contribute to their conservation. Using field observations and telemetry information, here we analyse how laying date of an endangered long-lived raptor varies with nest characteristics, individual factors and climatic variables during an 18-year (2004–2021) study period in eastern Spain. To this end, we estimated how environmental and individual variables affect laying date. Our results showed that average laying date in our study area took place on February 17th. Laying date showed no trend during the study period. Nests placed in higher elevations showed delayed laying dates. Territories occupied by adults with previous successful experience and high brood size showed earlier laying dates. Current year laying date was highly correlated with previous year laying date. Colder territories delayed laying date and the minimum temperature of the previous December were negatively correlated to previous laying date. Our results highlight the importance of experienced birds able to adjust timing of reproduction to maximise biological fitness. High adult mortality allows sub-adults to settle as breeders and jeopardises species’ persistence. Therefore, the reduction of mortality of territorial birds is of the outmost importance to improve the conservation status of declining long-lived species such as the Bonelli’s eagle (Aquila fasciata).
Funding Information
  • Conselleria de Innovación, Universidades, Ciencia y Sociedad Digital, Generalitat Valenciana (ACIF/2020/051)
  • Generalitat Valenciana
  • Fundación Iberdrola España
  • Universitat de València

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