Diet specialisation and foraging efficiency under fluctuating vole abundance: a comparison between generalist and specialist avian predators

Abstract

Specialist species, using a narrow range of resources, are predicted to be more effi cient when foraging on their preferred food than generalist species consuming a wider range of foods. We tested whether the foraging effi ciency of the pallid harrier Circus macrourus , a vole specialist, and of sympatric Montagu ' s harriers C. pygargus , a closely related generalist, diff ered in relation to inter-annual variations in vole abundance over fi ve years (including two peak, one intermediate and two low vole abundance years). We show that the hunting parameters of pallid harriers strongly varied with vole abundance (higher encounter rates, capture rates and proportion of successful strikes in high than intermediate and low vole abundance years, respectively), whereas Montagu ' s harriers showed stable capture rates and hunting success (proportion of strikes that were successful), irrespective of vole abundance. Encounter rates and capture rates were higher for pallid than for Montagu' s harriers when voles were abundant, but lower when voles were scarce. Th e hunting success of pallid harriers was also lower than that of Montagu ´ s harriers when voles were scarce, and when they had to target alternative preys, in particular birds. Overall, estimated biomass intake rate was 40% higher for pallid harriers than for Montagu' s harriers when voles were abundant, but 50% lower when voles were scarce. Our results indicate that specialists predators, like pallid harriers, which evolve specifi c adaptations or breeding strategies, do better when their preferred prey is abundant, but may face a cost of specialisation, being not effi cient enough when their preferred prey is scarce. Th ese results have broader implications for understanding why specialist predators are, in general, more vulnerable than generalists, and for predicting how specialists can cope with rapid environmental changes aff ecting the abundance or predictability of their preferred resources.

Does Seasonal Decline in Breeding Performance Differ for an African Raptor across an Urbanization Gradient?

“…Dietary requirements may be an important factor in determining the success of an urban raptor, specifically the degree of specialization in their diet choice. Highly specialized raptors may have to spend more energy and time seeking preferred prey, whereas generalists may have more flexibility in coping with fluctuating food availability (see Terraube et al, 2011 for a natural setting).…”

Diet specialization in a generalist population: the case of breeding great tits Parus major in the Mediterranean area

“…If vertical cultural transmission of the foraging niche develops a role in shaping feeding preferences of the offspring through the transmission of these specialized feeding preferences, parents raising larger broods may also be favouring (or limiting) the survival of their progeny, consciously or not. Operating at the pair level, and probably depending on variable year-to-year environmental conditions (Cherel et al 2009;Terraube et al 2011), this pattern links individual foraging niches to population densities (Svanbäck and Persson 2004). Regardless of the evident effect of prey availability on foraging patterns, parents may also have a limited capacity to exploit alternative prey types to their preferred prey, and may preferably exploit certain prey types within the whole available niche.…”

Assessing Diet of the Rufous-Winged Philentoma (Philentoma pyrhoptera) in Lowland Tropical Forest using Next-Generation Sequencing

“…Optimal foraging theory also predicts that predators are selective when faced with abundant prey, but become less picky when prey gets sparse (Emlen 1966). However, foraging opportunism may also occur in certain bird species, leading to dietary generalization and allowing niche separation through wider diet preferences (Sherry et al 2016), which were useful when food sources are limited (Terraube et al 2011).…”