DENSITY-DEPENDENT FECUNDITY BY HABITAT HETEROGENEITY IN AN INCREASINGPOPULATION OF SPANISH IMPERIAL EAGLES

We report on a 32-yr study of a population of Spanish Imperial Eagles, Aquila adalberti, which increased during the first 16 yr of study but remained stable during the last 16 yr. We analyzed changes in the mea n and variance of fecundity in relation to population density to test predictions of two hypotheses of density-dependent fecundity. Accordin g to the ''interference'' hypothesis, as density increases, frequency of agonistic encounters increases, resulting in a relatively uniform d ecrease in habitat quality. Consequently, mean fecundity decreases, an d no relationship is expected between density and variance in fecundit y. For the ''habitat heterogeneity'' hypothesis, however, as density i ncreases, a greater proportion of individuals are forced to occupy low er quality habitats. Thus, mean fecundity decreases and fecundity vari ance must increase. Additionally, for this hypothesis, fecundities in good sites are expected to be equal in both low- and high-density situ ations. An inverse relationship between fecundity and population size was found in this eagle population. Annual variance in productivity sh owed significant increases over the study period. This trend was inver sely related to mean productivity. Variance in productivity was relate d to the year of pair establishment, being higher in recently occupied territories. Mean and variance of the longer term territories remaine d constant during the study period. These results are in accordance wi th the habitat heterogeneity hypothesis.