We report on an 11 yr study of floater interference in a population of Spanish Imperial Eagles Aquila adalberti. We analyzed changes over the years in the productivity of 15 territories to test predictions of two hypotheses of density dependent fecundity in relation to the presence of floaters. According to the "interference" hypothesis, as density increases, the frequency of floater intrusion increases, resulting in a decrease in fecundity. Thus, in a high density population a negative relationship between floater intrusions and productivity of the territory is expected. In contrast, on the "habitat heterogeneity" hypothesis, as density increases a greater proportion of individuals are forced to occupy lower quality habitats and, at least up to a certain level, no relationship between floater intrusions and fecundity is expected. Results showed that floater eagles tend to visit natal populations during certain months which coincide with the beginning of the breeding cycle. Among floater eagles, males made significantly more intrusions per day than females, but females stayed in the natal population for longer each year than males. Floater intrusions and fecundity were highly positively correlated, supporting the "habitat heterogeneity" hypothesis; individuals are apparently able to assess the quality of a territory and, at the frequencies observed, their interference with the breeding pair had no obvious depressing effect on fecundity.