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

Abstract

. Factors affecting individual variation in between-year territory change and dispersal distance were measured in a long-term study of Black Kites (Milvus migrans) living in an area of —100000 ha in and around Doñana National Park in southern Spain. Adult birds (N = 210) and fledglings (N = 3061) were individually marked, and breeding birds were monitored annually during 1989—1996 to detect banded individuals. Of these 3271 birds, 652 breeding birds were resighted, and 164 of them were sexed through cop­ ulatory behavior. 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 birds departed significantly more from low-quality territories (evaluated through breeding success variables). Breeding failure and mate loss (divorce or death) favored breeding dispersion, both in males and females, but females changed territories more frequently after nest predation. We did not detect effects of resource availability, conspecific breeding density, or mate quality on the fre­ quency of territory change. When all the variables were included into a Generalized Linear Model (GLM), only breeding success, mate loss, and the interaction between the two had significant effects on territory change. Dispersal distance of birds that had changed territory was similar for males and females (median 302 m). Ninety percent of individuals moved <5 km, and none moved >10 km. Distance moved depended on the bird’s age and territory quality, but a GEM 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 distance are different individual decisions and are determined by different selective pressures. Dispersal conse­ quences were significant only for females: they tended to change to territories with a lower risk of predation and obtained higher breeding success in the new location. Moreover, individuals in the new territories had higher breeding success than those occupying aban­ doned territories the same year. Dispersal distances were not correlated with the new ter­ ritory quality and probability of breeding failure. Black Kites remained close to their former breeding places; individual competitive abilities may determine the final dispersal distance. Short-distance movements in dispersal are probably related to benefits of philopatry (eco­ logical and genetic) and dispersal costs associated with finding a new territory in a saturated population, such as the one studied