We recently published the first study that empirically quantifies the reaction of the threatened Spanish imperial eagles Aquila adalberti towards a variety of human activities around the nest, and an evaluation of its influence on breeding success, with the aim of recommending buffer zones in order to enhance the Spanish imperial eagle's reproductive success (González et al., 2006).
A reply of Ferrer et al. (2007) agrees with the interest and utility of our study, although they argued that we ‘underestimated that eagles can habituate to non-lethal human activities’, suggesting that ‘it seems counter-productive to impose restrictions on landowners and may even be detrimental if the presence of eagles diminished owners' income’. Ferrer et al. speculate that a change in selective pressure (based on reinforcement of nest defence behaviour with increasing intruder frequency, Ferrer, García & Cadenas, 1990) linked with the long reproductive life of the species would facilitate a decrease in the distance at which eagles would flush. This is not necessarily so. It could facilitate a decrease in the distance of attacks towards the intruders (as they state), but not necessarily a decrease in the distance from the intruders at which eagles would jump from the nest. Additionally, it is not clear whether the change in selective pressure indeed has the consequences predicted by Ferrer et al. (1990). For example, no further increase in aggressive behaviour of Spanish imperial eagles towards nest visitors has been observed, as Ferrer et al. (1990) expected. The small sample size in which their study was based may imply that those results were more individual-dependent than applicable to the whole of the species. These authors argue that it would be more helpful to get the eagles used to human intrusions, assuming that if human presence were to be excluded from the territories, eagles would become increasingly sensitive to human presence. However, even if our results showed that some habituation does exist, this habituation was not enough to prevent a decrease in reproductive success, and so we disagree with the fact that it would be better to get eagles used to humans (the risk that it could create a decrease in productivity before the predicted ‘total’ habituation is not worth taking, in our opinion).
Secondly, these authors minimize the importance of the effect of human activities on the eagles' hatching rate because, according to them, the relation is only ‘marginally significant’. According to their re-analyses, the probability of the correlation between hatching rate and number of days with intrusions is P=0.052 with r=−0.627 (being rounded off in González et al. (2006) as r=−0.63, P<0.05, n=10). The discrepancy may arise because of different ways of estimating the significance of r in different statistical software. Following the Sokal & Rohlf (1981) tables, with n=10, P is significant (0.05) when r>0.576. Furthermore, Siegel & Castellan (1988) indicate that when testing for the significance of Spearman's correlations, a special technique has to be used for a small sample size. According to that ouvrage, with n=10, P is significant (0.05) when r>0.564. Thus, we reassert that there exists a significant trend for the hatching success to decrease with increasing intrusions in the eagle's territories. And, as they are right to point out, there is also a significant (r=−0.677, P=0.02) and negative relationship between the hatching rate and intrusions per day, which also reaffirms our results. Ferrer et al. also argue that the significance disappears when excluding one of the territories, and so they conclude that the relationship is not important enough to support a proposal of a buffer around the nest. As Ferrer et al. know, it is difficult enough to have large sample sizes with these species (in terms of number of territories monitored). Even if significance decreased when excluding one of the nests, that could simply mean that the remaining sample size would not be large enough to detect significant results at 0.05 (a β test would be needed). Nevertheless, we have re-analysed the correlation eliminating the M-18 territory as suggested, and contrary to their arguments, the correlation is still significant (P<0.05) between the hatching rate and both variables (r=−0.6129 and r=−0.6129).
Finally, Ferrer et al. (2007) argue that an increase in fecundity is less important than a decrease in survival (which is obvious and logical for a species with this life history). They argue that if users are upset with this reduction in access, fecundity will increase but survival will decrease because users will harass adults. Our experience with land owners is that this would not necessarily be the case. Owners will accept this reduction if other compensations are made (which is feasible). We believe that the argument that it may be more beneficial for the eagles to get used to humans (and thus not to implement buffer zones) is not solidly argued, and may be in fact be detrimental to the eagles. The argument that it may be detrimental to eliminate activities from certain zones, and that this may ‘annoy’ some potential users of the zone, may be valid. But our study is a proposal, and it is up to the competent authorities (five Autonomous Communities in Spain) to make sure when implementing it (if they decide to implement it) that it is made with the agreement of land owners and other stakeholders, and so all potential views are taken into account and (for example) compensations are proposed. Obviously, it is important that we manage to incorporate simultaneously humans' use of natural resources, as well as the preservation of the wildlife that inhabits those natural habitats. However, with the incorporation of buffer zones, a conservation tool applied in most raptor species (see Richardson & Miller, 1997) and included in the Recovery Plans for the Spanish imperial eagle officially approved by the Autonomous Communities of Madrid, Castilla y León, Extremadura and Castilla-La Mancha, that accommodate 75.2% of the total breeding population, we are not talking about ‘human free areas’, but about temporary reductions in access. Thus, until new scientific data are available to show that buffer zones are not effective, we advocate that this measure should be adopted for the conservation management of this species.