Trophic Niche in a Raptor Species: The Relationship between Diet Diversity, Habitat Diversity and Territory Quality

Abstract

Recent research reports that many populations of species showing a wide trophic niche (generalists) are made up of both generalist individuals and individuals with a narrow trophic niche (specialists), suggesting trophic specializations at an individual level. If true, foraging strategies should be associated with individual quality and fitness. Optimal foraging theory predicts that individuals will select the most favourable habitats for feeding. In addition, the “landscape heterogeneity hypothesis” predicts a higher number of species in more diverse landscapes. Thus, it can be predicted that individuals with a wider realized trophic niche should have foraging territories with greater habitat diversity, suggesting that foraging strategies, territory quality and habitat diversity are inter-correlated. This was tested for a population of common kestrels Falco tinnunculus. Diet diversity, territory occupancy (as a measure of territory quality) and habitat diversity of territories were measured over an 8-year period. Our results show that: 1) territory quality was quadratically correlated with habitat diversity, with the best territories being the least and most diverse; 2) diet diversity was not correlated with territory quality; and 3) diet diversity was negatively correlated with landscape heterogeneity. Our study suggests that niche generalist foraging strategies are based on an active search for different prey species within or between habitats rather than on the selection of territories with high habitat diversity.

Providing longer post-fledging periods increases offspring survival at the expense of future fecundity

“…The distance between the two farthest nest-boxes in our study area is 5 km [ 48 ]. Thus, the study of the dispersal between nest-boxes, as conducted in other studies [ 47 , 54 ], would not be very informative in our case. We are confident in the reliability of our survival estimation, as we did not find any individuals breeding in the surroundings of our study area or any dispersing chicks breeding in any of the nearby nest-box populations (Villalar de los Comuneros– 112 km from our study area; San Martín de Valderaduey– 155 km from our study area) that we also study [ 58 , 59 ].…”

“…Both fledgling and adult survival were estimated by banding individuals with PVC and metal ring codes and recapturing them in the subsequent years as breeders (using nest-traps when nestlings are 10–13 days old), using nest camera recorders or reading ring PVC codes with the aid of a telescope when it was not possible with either of the two first methods (see [ 47 ]).…”

Trophic niche divergence among colour morphs that exhibit alternative mating tactics

“…The divergent patterns emerging from these two sites add support for the importance of habitat heterogeneity in the generation and maintenance of trophic niche variation within a population [ 56 , 57 ]. However, the observed trophic patterns in U. ornatus are also contingent on behavioural asymmetries among the male morphs, as our mixing model results support that morphs also differ in their degree of diet specialization.…”

Landscape homogenization due to agricultural intensification disrupts the relationship between reproductive success and main prey abundance in an avian predator

“…The vole cycle highly influences the breeding density of kestrels [50], to account for this varying density, we varied the buffer scale accordingly between years: r (2011) = 650 m (132.7 ha); r (2012) = 694 m (151.3 ha), r (2013) = 843 m (223.3 ha). We used the Simpson’s Index to quantify Territory Land Cover Heterogeneity (see [55] for a similar method of characterising raptor territory composition), hereafter called ‘TLCH’, whereby lower values represent more homogeneous landscapes, and higher values more heterogeneous landscapes (Fig. 1b).…”