Biology session

Natal dispersal, i.e. the movement from the hatching site to the first breeding site, can reduce the risk of inbreeding and kin competition, while promoting the detection of favorable habitats. Several internal and external factors might influence natal dispersal patterns of species and populations. Beyond that, the difficulty of tracing individuals between hatching and breeding sites also make it challenging to study dispersal. Nonetheless, information obtained from such studies can be valuable in species conservation.

At the end of the 1980s only 15 to 25 pairs of eastern imperial eagle (Aquila heliaca) were nesting in Hungary, but the population has multiplied notably since then. Despite the fast growth from a small population size, high levels of inbreeding have not been detected, implying some kind of kin avoiding mechanism, such as sex-biased natal dispersal. In our study the effect of sex and the position of territory on dispersal distance were examined.

Identification of individuals based on nine microsatellite loci and molecular sex determination of 537 eastern imperial eagle nestlings and 223 breeding birds were performed using feathers of chicks banded between 2011 and 2016, and shed feathers of nesting birds collected between 2014 and 2019. Subsequently, we searched for matching genotypes between the two age groups. Dispersal distances were calculated using nest coordinates, and local density was determined by counting breeding pairs around each nest in a radius twice the median of nearest neighbour distances (NND) in that year.

Altogether 460 nestlings (222 males, 238 females) and 209 breeding birds (62 males, 147 females) were genotyped on at least eight microsatellite loci. Full match of alleles was found in 30 pairs (8 males, 22 females). The median of natal dispersal was 30.36 km (SD = 13.27 km) in males and 46.50 km (SD =55.90 km) in females. Local density at the breeding site compared to the hatching site was lower for most individuals. Neither sex, nor density had significant effect on dispersal distance.

The observed 1:1 sex ratio of nestlings shifted towards a heavily female-biased ratio (1:2.3) in breeding birds, which can be explained by the lower sampling probability of males due to behavioural differences, i. e. males spend notably less time on and around the nest from where most feathers are collected.

Differences in distribution of male and female dispersal distances suggest different strategies, furthermore, the decreasing tendency of density between hatching and breeding sites might indicate negative density dependence. Statistical analyses were complicated by the small number of matches which, besides our strict identity criteria, could also be explained by possible emigration of fledglings or low probability of successful settling down. Therefore, it would be important to continue the research with a larger marker set whilst considering the mortality and breeding success of fledglings.