A paper has been published that genetic factors in soaring birds are very important too as compared with social (cultural) factors. Authors compared the autumn journeys and wintering ranges of two closely related large raptorial bird species, the greater spotted eagle Clanga clanga and the lesser spotted eagle Clanga pomarina, and hybrids between them. The timing of migration in hybrids was similar to that of one parental species, but the wintering distributions and home range sizes were similar to those of the other. Tracking data were extended by habitat suitability modelling, based on GPS fixes and ring recoveries. These results suggest a strong genetic influence on migration strategy via a trait-dependent dominance effect, although the contribution of social interactions may be important as well.
If social influence predominated in spotted eagles, one would expect hybrids to follow C. pomarina to their wintering range because hybrids started their migration as early as C. pomarina, while most C. clanga individuals departed their breeding grounds significantly later. This was not the case, as the wintering ranges of most hybrids were similar to those of C. clanga, and the median latitude was intermediate between those of the two species. Additionally, there was substantial variation among wintering longitudes in hybrids, similar to observations in C. clanga. Finally, the wintering strategy of hybrids also resembled the strategy of C. clanga in that they often used a restricted home range instead of travelling long distances over the course of winter, which is a typical behaviour in C. pomarina.
Differences in routes and destinations between the first two years of life were discovered, suggesting the importance of learning. Genetic determination is further supported by two examples of studied families verified by genetic analyses. First, tracking data and ring recoveries indicated that four studied brothers of C. clanga from four different years migrated to southwestern Europe; second, a tracked hybrid female wintered in Sicily, and her brother was identified by a colour ring in Sardinia. However, a tracked hybrid juvenile migrated to east Africa, but offspring from the same nest migrated at least twice to west Africa.
Only weak support for an additive genetic effect was found, which would result in an intermediate wintering strategy in hybrids, and no support for sex-linked inheritance. The results indicated relationships among various climatic factors and the wintering ranges of spotted eagles. The ranges of both C. clanga and hybrids seemed to be determined by precipitation, but this factor was not included in the distribution model for C. pomarina, which indicated a lack of overlap not only with the wintering range of C. clanga, but also with that of hybrids.
Ülo Väli; Paweł Mirski; Urmas Sellis; Mindaugas Dagys; Grzegorz Maciorowski; Genetic determination of migration strategies in large soaring birds: evidence from hybrid eagles, Proc. R. Soc. B (2018) 285 (1884): 20180855.