Effects of altitude and prey availability on the laying date of Bonelli´s Eagles (Hieraaetus fasciatus) in Granada (SE Spain).
Authors: GIL-SÁ SÁNCHEZ, J. M.
Published: Volume 47(1), June 2000. Pages 1-8.
Original Title: Efecto de la altitud y de la disponibilidad de presas en la fecha de puesta del Aguila-Azor Perdicera Hieraaetus fasciatus en la provincia de Granada (SE de España)
The influence of altitude on laying date was studied in a Bonelli´s Eagle population breeding in south-eastern Spain (Granada province, a typical Mediterranean area). We dated 96 reproductions of 28 eagle pairs between 1994 and 1999. In addition, the influence of prey availability on laying date was studied in 1997. Laying date was calculated by means of the pterilar growth of the chicks (Appendix 1), considering an incubation period of 39 days. The altitude of the nesting sites was obtained from 1:25000 maps. Prey availability (rabbits Oryctolagus cuniculus and Red-legged Partridges Alectoris rufa) was estimated from census itineraries carried out for each nesting territory (8.2 km/territory on average) during March and April. The relationship between laying date and four independent variables (altitude, density of rabbits+Partridges, density of rabbits and density of Partridges) was tested by means of stepwise linear regressions. Laying date varied between 21 January and 17 March (mean: February 10; Fig. 1), a result that agreed with previous reports for Bonelli´s Eagles in south-eastern Spain. There was a positive relationship between altitude and laying date when data from the six study years were considered together (Fig. 2), and this relationship was significant for three out of the six years when we tested the within-year correlations (Table 1). During 1997, altitude was the only variable selected by the regression analysis (R 2 = 0.395, P = 0.022, d.f. = 12; Fig. 3). There was a slight negative relationship between laying date and prey availability (partial correlation = -0.211), with a stronger association with rabbit availability (partial correlation = –0.228) than with Red-legged Partridge availablity (partial correlation = –0.121, Fig. 3). High prey availabilities as compared to other regions and/or microclimatic factors may be the causes of the marginal effect of trophic resources on the laying date of Bonelli´s Eagles in Granada.