Sunday, 16 January 2011


Bonelli's Eagle (Aquila fasciata). One of the two adults currently tracked by satellite in the province of Badajoz (Ángel Sánchez).

Satellite tracking is giving us enormous insights to the movements of birds. One single tagged bird produces more information than hundreds or thousands of birds marked with the traditional metal rings (or bands in US). It is also much more informative than distance reading of colour rings, above all in species hard to get close to, like raptors. Although the results are more eyecatching in migratory birds (see Egyptian Vulture), they can also be revealing in the case of sedentary birds like the Bonelli's Eagle (Aquila fasciata). At the moment there are two satellite tracked adults in the Extremadura area called "La Siberia". Given that their movements are restricted to their nesting zone (Ángel Sánchez, pers. obs.), the detailed information has obviously not been made public. Applied to juvenile birds in dispersal, however, the system is less hermetic. We have found two studies of this dispersal, one from Portugal, which could be perfectly extrapolated to the Extremadura population,and the other from the Spanish Mediterranean arch.

In the first case four Bonelli's Eagles chicks were tagged in the nest and then tracked for three to seven months afterwards. The two born in the Duero moved on to the Rías Baixas en Galicia. One was shot dead and the signal was lost from the other, maybe due to a technical fault. The two eaglets born in the Tagus area (one on the border with the province of Cáceres) also moved north as juveniles, to Galicia and Asturias. After a few months the signals were lost; in one case death is suspected due to unknown causes; the other case is put down to a technical hitch.

Movements of four satellite-tracked young Bonelli's Eagles (Aquila fasciata) from Portugal [in yellow, the movements of a Golden Eagle (Aquila chrysaetos)].

The Mediterranean area study was conducted on 14 young birds from 9 nests between Barcelona and Murcia. During their first year these birds stuck pretty close to the nest; then they dispersed over much of the Iberian Peninsula before settling down for varying periods in one or other dispersal area. A total of 16 areas were identified in prey-rich farming areas with no breeding adults. None of these dispersal areas was in Extremadura, though two birds did make brief stopovers in the province of Cáceres (Sierra de Gata and Llanos de Trujillo).
Juvenile dispersal areas of 14 birds (top) and first-year movements of seven Bonelli's Eagle (Aquila fasciata) juveniles born in the Spanish Mediterranean Arch (bottom).

- Portugal: Seguimiento de aves via satelite. Aguias (eagles).

- Cadahía, L., López-López, P., Urios, V. & Negro, J. J. 2010. Satellite telemetry reveals individual variation in juvenile Bonelli's eagle dispersal areas. European Journal Wildlife Resesearch.
- Cadahía, L., Urios, V. & Negro, J. J. 2005. Survival and movements of satellite-tracked Bonelli's Eagles Hieraaetus fasciatus during their first winter. Ibis, 147:415-419. - Bibliography on Bonelli's Eagle in PDF format.