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Biases in diet sampling methods in the Spanish Imperial Eagle Aquila adalberti

AuthorsSánchez, Roberto; Margalida, Antoni ; González, Luis Mariano; Oria, Javier
Issue Date2008
PublisherFinnish Ornithological Society
CitationOrnis Fennica 85(3): 82-89 (2008)
AbstractThis study quantifies for the first time the biases associated with different methods (direct observations, pellets, remains, and pellets plus remains referred to as pooled data) used to assess the diet of the Spanish Imperial Eagle Aquila adalberti. With respect to prey items delivered to the nest as a surrogate of the most objective method to assess the diet, pellets, remains and pooled data overestimated birds and underestimated mammals. With regards to the size of the prey, prey remains, pellets and pooled data underestimate larger prey (>601 g) but overestimated smaller and medium size preys (<300 g and 301–600 g). Concerning dietary breadth, prey remains provided the widest trophic spectrum. Our results suggest that the three methods used to quantify the diet in Spanish Imperial Eagles are significantly different and that the pooled data (pellets plus prey remains) is not a reliable alternative method for direct observations to reduce this bias. Biases in the diet can differ considerably among prey categories, and based on our data it is not possible to generalise about the most appropriate method for estimating raptor diets. Thus, the reported biases should be seriously considered in management and conservation strategies for threatened species based on indirect studies on raptor diet.
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