Date Published: May 21, 2019
Publisher: Public Library of Science
Author(s): José E. Martínez, Iñigo Zuberogoitia, José M. Escarabajal, Ginés J. Gómez, José F. Calvo, Antoni Margalida, Charles R. Brown.
We examined copulatory behaviour in the Bonelli´s Eagle (Aquila fasciata) at nesting sites in the eastern zone of the Baetic Cordillera, southern Spain, between 2010 and 2012. We observed the copulatory behaviour of 15 pairs during the pre-laying period. Bonelli´s Eagles commenced sexual activity ca. 69 days before egg-laying. Ninety-six percent of mounting attempts were successful. Bonelli´s Eagle pairs averaged 99.8 copulation attempts per clutch, with an average copulation frequency of 0.86 copulation attempts per day. Pairs displayed a daily bimodal pattern of copulation activity, with copulations occurring most frequently in the evening. We used our data to test three predictions with regard to the paternity assurance hypothesis. Prediction 1, that within-pair copulations increase with local breeding density, was rejected because our models showed no evidence for it. Prediction 2, that within-pair copulations increase during the female fertile period, was marginally supported. Finally, Prediction 3, that mate attendance increases during the female fertile period, was also rejected because mate-guarding did not increase as the fertile period approached. However, mate-guarding was positively correlated with within-pair copulation frequency. Moderate copulation rates compared to other raptors and the absence of mate-guarding suggest that, in the study area, Bonelli´s Eagles exhibit only partially adaptive behaviour to assure their paternity. A possible explanation could be related to the low number of extra-pair encounters observed (opportunities for which appear to be rare), although the gradual increase in within-pair copulations during the female fertile period is consistent with the sperm competition hypothesis. The results are discussed based on the signalling hypothesis, which proposes that raptors signal territory ownership to conspecifics, and possibly to other raptor species, by copulating frequently and conspicuously in the defended nesting area.
Copulatory behaviour and the frequency of mounting attempts is extremely variable in vertebrates . In most taxa, copulations occur several times before the female fertile period, during each breeding season [2, 3, 4], whereas in other taxa, such as birds of prey and dolphins, copulations may occur many times over long periods, both inside and outside of the female´s fertile period . However, to date, the factors determining this aspect of mating behaviour are not well understood [1, 5].
Our study shows that Bonelli´s Eagles copulate over a long period (69 days) and at a high rate (99.79 copulations attempts per clutch); considered a common pattern in raptors . Previous studies have proposed that the copulation frequency be considered as high whenever it occurs 20 times or more per breeding season, or whenever the daily number of copulations during the female´s fertile period is considerably more than two [2, 8]. However, despite the numerically high copulation rates observed in Bonelli’s Eagle, they are moderate in comparison with other raptors such as Northern Goshawk Accipiter gentilis, Montagu´s Harrier Circus pygargus, Black Kite Milvus migrans, Red Kite Milvus milvus, Osprey Pandion haliaetus and Bearded Vulture Gypaetus barbatus [4, 12, 29, 40, 44, 45], all of which exceed 200 mounts per breeding season [5, 6, 40, 45]. In fact, the copulation rates observed in our study are only higher than those described for Egyptian Vultures: 55 copulation attempts  and Griffon Vultures: 71 copulation attempts . However, all of these reported estimates of copulation rate should be considered with caution given that most studies have focused only on the nesting area without considering the number of copulations that take place during foraging movements or at roosts.