Food hoarding of an avian predator: sex- and age-related differences under fluctuating food conditions

Age and sex differences in numerical responses, dietary shifts, and total responses of a generalist predator to population dynamics of main prey

“…Differences between age classes were especially distinct in years of vole scarcity, but sex differences were present both in years of low and high vole abundance. The age differences in the food store content of pygmy owls likely arise from lower hunting experience of yearlings compared to adults (Wunderle 1991), also supporting the hypothesis that they rely more on stored food (Masoero et al 2018). In high vole abundance years, voles likely maximize Table 3 Estimated prey weight (kg) consumed by pygmy owls during the food-storing season, the percentage of biomass consumed by the age and sex classes, and the estimated number of the five most important prey groups consumed in the study area during Estimates and standard errors (slope), and χ 2 and P values for the GLMs for the numbers of prey animals consumed in relation with the vole abundance index are provided.…”

“…An increase in the main prey (voles) population induces higher numbers of breeding pairs (Solheim 1984b), and of prey items per food store in winter (Solheim 1984a;Suhonen et al 1993;Terraube et al 2017). Recently, we found that pygmy owls showed age-and sex-related variations in the total amount of food stored in relation to fluctuating vole abundance (Masoero et al 2018). However, we did not find any study investigating sex or age differences in numerical responses and dietary shifts in relation to vole abundance.…”

“…Agricultural fields are interspersed all over the study area. The data were collected from 2003 to 2017 in 305 sites (hereafter 'forest sites'), each provided with two nest-boxes for pygmy owls (for further details on the study system, see Terraube et al 2017, Masoero et al 2018. To prevent other species of owls from entering the boxes, the diameter of the entrance hole of nest-boxes was 45 mm, which corresponds to a cavity excavated by the three-toed woodpecker Picoides tridactylus (Solheim 1984a).…”

“…Captured owls were ringed with an aluminium leg ring for individual identification, their wing and tail lengths were measured, and they were weighted, sexed, and aged. Pygmy owls are sexually dimorphic, with females larger than males, and sex was therefore determined on the basis of wing length, tail length, and body mass (as in Masoero et al 2018). The age was estimated according to wing moult (Lagerström and Syrjänen 1990), and individuals were divided in two classes: individuals at their hatching year (hereafter 'yearlings') and older individuals (hereafter 'adults').…”

“…The bank vole and the Microtus vole abundance indices were used for the respective groups, whereas the pooled vole abundance index was used in the models for the other three groups of prey (birds, shrews, mice). For this part of analyses, only the abundance index in the autumn of the current year was used, since the content of the food stores closely reflects prey availability in the environment when the food is hoarded (Masoero et al 2018). The nest-box identity, nested in the forest site (i.e., where each box pair was set), was included as random effect to account for spatial and temporal pseudo-replication (two nest-boxes available for each forest site and most boxes were used multiple times during the study).…”