2013: the worst reproductive rates for the Catalan population of Bonelli’s Eagle

As in previous years, the Biology Conservation Group of the University of Barcelona has monitored the Catalan population of the Bonelli’s Eagle during the breeding period (from January to July) of 2013, which allowed us to estimated the main reproductive rates for this endangered species. At the same time, more Bonelli’s Egle chicks have been marked with a conventional metallic ring and an alphanumeric coded band (readable from long distances). The resighting of marked or dead individuals is a basic source of information in order to estimate the survival of this endangered species. In fact, we have already obtained information from individuals marked in 2008 (when this work began) by our research group.

The results show that territorial eagles in Catalonia had an average productivity (average chicks flying per pair included in the study) of 0.71 chicks during the 2013 season (n = 52). This value is the lowest ever recorded in Catalonia since the 80s, when the monitoring began, and is quite close to values obtained in the North and Northwest of the Iberian Peninsula, where populations of Bonelli’s Eagle are very endangered or almost extinct. Actually, during the last 10 years there have been 7 years with productivity values below 1 chick per pair, which can be considered very low, being the average productivity during 1990-2013 on 1.02 chicks per pair.

The decline in reproductive parameters observed in recent years suggests that the Catalan population of Bonelli’s Eagle is experiencing a worsening of environmental conditions. While in previous years a large part of the low productivity could be explained by the high adult mortality (making that many territories were occupied by younger individuals with less experience), the higher survival rates detected in 2012 and 2013 suggest that there are also environmental factors affecting the reproduction of this species.

Photo: Carles Barés (Grup de Suport de Muntanya del Cos d'Agents Rurals)

First, in recent years there seems to be a general decrease in the abundance of rabbits and partridges, the most profitable prey for the eagles, due habitat changes in the territories of the eagles.

Moreover, in the particular case of 2013, the continued rain and low temperatures during incubation and rearing of chicks could accentuate the negative trend already observed in previous years. In this sense, there were several nests where chicks reached 15 days old, but after the rain occurred in April, one of the chicks (in case of nests with two chicks) or both chicks died. Another remarkable case took place in the Garraf Natural Park, where a couple stopped taking care of one the few days old chicks for hours and during continued days, an unusual behaviour, and the female finally ate the chick that was still alive, probably as a result of the intense trophic stress suffered. In the Ebro valley we also detected a chick death from trichomoniasis, a parasite commonly found in pigeons that also seriously affects the eagles

Breeding failure is more frequent during the last years due to disturbances caused by recreational activities (hiking, climbing, cycling races, etc..) and arrangement and promotion of hiking near nests. In this sense, it is remarkable the breeding failure during the incubation period of a new pair nesting in a cliff where several climbing ways and via ferratas were installed, and the breeding failure of another pair breeding on a sea cliff where there were many boats and divers around.

Finally, concerning the survival of territorial individuals, It is noteworthy that 2013 was the second consecutive year with a survival rate is above the threshold of 92.5%, which would allow the Catalan population of Bonelli’s Eagle being self-sustaining.