Causes and consequences of territory change and breeding dispersal distance in the black kite

Factors affecting individual variation in between-year territory change and dispersal distance were measured in a long-term study of Black Kites (Milv us migrans) living in an area of similar to 100 000 ha in and around Donana National Park in southern Spain. Adult birds (N = 210) and fledlings (N = 3061) were individually marked, and breeding birds were monitored annually during 1989-1996 to detect banded individuals. Of these 3271 birds, 652 bre eding birds were resighted, and 164 of them were sexed through copulatory b ehavior. Nests were checked annually to monitor breeding performance. Frequencies of return between years were 83.1% for breeding males and 89.5% for breeding females. Frequencies of territory change were 25.6% and 32.8% , respectively. Females >8 yr old rarely changed territories. Dispersing bi rds departed significantly more from low-quality territories (evaluated thr ough breeding success variables). Breeding failure and mate loss (divorce o r death) favored breeding dispersion, both in males and females, but female s changed territories more frequently after nest predation. We did not dete ct effects of resource availability, conspecific breeding density, or mate quality on the frequency of territory change. When all the variables were i ncluded into a Generalized Linear Model (GLM), only breeding success, mate loss, and the interaction between the two had significant effects on territ ory change. Dispersal distance of birds that had changed territory was simi lar for males and females (median = 302 m). Ninety percent of individuals m oved <5 km, and none moved >10 km. Distance moved depended on the bird's ag e and territory quality, but a GLM model retained only age as a significant variable: older birds (males and females) dispersed shorter distances. These results indicate that breeding territory change and dispersal distanc e are different individual decisions and are determined by different select ive pressures. Dispersal consequences were significant only for females: th ey tended to change to territories with a lower risk of predation and obtai ned higher breeding success in the new location. Moreover, individuals in t he new territories had higher breeding success than those occupying abandon ed territories the same year. Dispersal distances were not correlated with the new territory quality and probability of breeding failure. Black Kites remained close to their former breeding places; individual competitive abil ities may determine the final dispersal distance. Short-distance movements in dispersal are probably related to benefits of philopatry (ecological and genetic) and dispersal costs associated with finding a new territory in a saturated population, such as the one studied.