Nutritional composition of the preferred prey of insectivorous birds: popularity reflects quality

Abstract

Food availability is emerging as a key determinant of avian occurrence and habitat use in a variety of systems, but insectivores have received less attention than other groups and the potential influence of nutritional quality has rarely been considered. Rather than a uniform food source, arthropods vary greatly in terms of nutritional composition, but does this variation translate into differential consumption? Building on previous work that demonstrated clear preference for some arthropod groups by 13 species of ground‐foraging insectivores, we compare the nutritional composition of these arthropod groups with other groups commonly encountered but seldom consumed in the same habitat types. Using samples of arthropods collected from a eucalypt woodland in southern Australia, we found the high frequency prey groups (Coleoptera, Lepidoptera, Orthoptera and Araneae) consistently contained higher fractions of crude protein and total fat than the low frequency groups (Diptera, Hymenoptera and Odonata). Even more clear‐cut differences were noted in terms of micronutrients; high frequency prey containing significantly greater concentrations of seven elements than low frequency prey and significantly greater amounts per individual arthropod for all eleven elements measured. These results indicate that the nutritional quality plays an important role in prey selection in insectivores and suggests that micronutrients may be more important determinants of prey choice than previously recognized. Integrating these findings with previous work suggesting food limitation may constrain distribution patterns of birds in fragmented landscapes, we contend that variation in nutritional quality helps explain observed patterns in insectivore diets and occurrence. In addition to explaining why smaller and more disturbed habitats are unable to support resident insectivore populations, this bottom‐up mechanism may underlie the disproportionate sensitivity of insectivores to land‐use intensification.

Why are woodland-dependent avian insectivore populations vulnerable to declines in highly-modified landscapes?

“…However, drawing inferences from measures of total invertebrate density may be misleading if they do not accurately represent prey availability (Razeng & Watson 2012). Prey vary in nutritional value (Robel et al 1995;Arnold et al 2010;Razeng & Watson 2015) and this may influence birds to preferentially target certain taxa (Razeng & Watson 2015). Additionally, birds employ specific strategies, such as foraging in a certain strata, using particular capture methods, or feeding at a certain time of day (Higgins & Peter 2002;Antos & Bennett 2006;Moorman et al 2007) and invertebrate sampling methods need to take these into consideration.…”

“…There is doubt over whether Formicidae are a preferred prey item, or whether they are eaten opportunistically at times of peak physiological demand or to supplement the diet when more-nutritious food is difficult to find (Poulin & Lefebvre 1996;Razeng & Watson 2015).…”

“…Birds show preferences for particular prey taxa (Naef-Daenzer et al 2000;Hagar et al 2007;Moorman et al 2007) which vary in nutritional value (Robel et al 1995;Razeng & Watson 2015). Additionally, birds may exhibit specific foraging behaviours that target certain prey taxa, such as foraging within particular strata (Antos & Bennett 2006;Moorman et al 2007).…”

“…Out of the eight prey taxa most commonly eaten by Australian woodland-dependent birds, Formicidae contains the lowest proportion of crude fat and relatively low levels of crude protein and moisture (Razeng & Watson 2015). Finally, we determined how abundances within individual prey groups, especially those that have a higher nutritional value, were associated with woodland extent.…”

Implications of vegetation clearing biases for the interpretation of landscape-scale species-area relationships

“…These attributes were selected given that they have been shown to affect the occurrence of Australian woodland birds, and included soil properties (Thomson et al, 2007;Watson, 2011;Stevens & Watson, 2013;Razeng & Watson, 2015), topographic heterogeneity Burgess & Maron, 2015), and the occurrence of Manorina honeyeaters (Kutt et al, 2015;Thomson et al, 2015).…”