The functional response in three species of herbivorous Anatidae: effects of sward height, body mass and bill size

Abstract

Summary1. The functional response, i.e. the quantity of food consumed per unit of time as a function of food availability, is a central process in foraging ecology. The application of this concept to foraging by mammalian herbivores has led to major insights into the process of resource acquisition, but it has so far been little used to understand foraging in avian herbivores. 2. In this study, we describe the functional responses of three grazing Anatidae, the wigeon Anas penelope L. (mean body mass in this study 620 g), the barnacle goose Branta leucopsis B. (2000 g), and the greylag goose Anser anser L. (3500 g). We measured instantaneous intake rates on eight grass heights from 1 cm to 12 cm, as well as pecking rates and peck sizes, and used the Spalinger-Hobbs model developed for mammalian herbivores to explore the mechanisms limiting intake in these three species. 3. Greylag geese increased their intake rate with increasing food availability (a Type II response), wigeon showed a weak quadratic (Type IV) response, and intake rate by barnacle geese did not vary significantly across the range of variability we offered the birds. 4. Intake rates differed markedly between individuals, especially in greylag geese, where body mass explained much of the variation. For individuals in all three species, peck sizes strongly influenced instantaneous intake rates, and the size of the bill (width in particular) appeared to be an important determinant of peck size. 5. Peck sizes increased with sward height (although only very weakly in wigeon), but so did cropping time for wigeon and barnacle geese, which led to a significant decline of intake rates on the tallest grass, at least in the small wigeon. For these very selective small herbivores, the time to crop a mouthful was therefore a significant limiting factor for the birds' instantaneous intake rate (in addition to peck size and swallowing time). This differs markedly from the situation in mammalian herbivores where bite size (through chewing time) is the principal controlling process in food concentrated patches, a result that we found in greylag geese. We discuss the differences in foraging between the three species in relation to their principal food resources, body mass and bill morphology.

Variation in Elevation and Sward Height Facilitate Coexistence of Goose Species through Allometric Responses in Wetlands

“…This result is in line with previous findings (de Boer and Prins 1989;Prins 1996;Durant et al 2003;Si et al 2011) and our prediction. Increasing vegetation height normally results in a decrease in forage quality (van der Wal et al 2000).…”

“…Foragers face a trade-off between maximizing forage quantity and quality. Foraging theory suggests that herbivores select their habitat based on differences in body size (Gunnarsson et al 2005;Hopcraft et al 2010;Clauss et al 2013); smaller bodied species generally forage on shorter swards that contain a higher nutrient content, whereas larger species exhibit a higher intake rate and are less sensitive to variation in forage quality, and therefore select taller swards (Durant et al 2003(Durant et al , 2004Heuermann et al 2011). Hence, body size plays a pivotal role in habitat selection in relation to forage quality and quantity, and the effect of these forage variables on differences in species abundance could be used to understand the differences in spatial distribution of the species and species coexistence.…”

A tree of geese: A phylogenomic perspective on the evolutionary history of True Geese

“…For instance, the Taiga Bean Goose has a long thin bill adapted for probing soft substrates of bogs and marshlands to feed on underground plant material, whereas the Pink footed Goose has a short bill suitable for grazing and seed stripping (Owen, 1976;Reed, 1976). Finally, the trait utility (i.e., evidence that morphological or physiological traits are indeed useful when used in the associated environment) of beak morphology in geese has been confirmed experimentally (Durant et al, 2003;Heuermann et al, 2011;Zhang et al, 2015). The Grey Geese can thus be regarded as an adaptive radiation in terms of beak morphology.…”

Gizzard vs. teeth, it's a tie: food-processing efficiency in herbivorous birds and mammals and implications for dinosaur feeding strategies

“…For example, studies with geese have shown that animals foraged preferentially on smaller food items (Amat 1995;Durant et al 2009), and that birds may crop food items partially so that the ingested piece is smaller than the whole plant part. Beak size and morphology are correlated with bite size in birds (Cope et al 1995a,b;Durant et al 2003; Van der Graaf et al 2006), but a potential effect on fecal particle size has not been quantified yet (Durant et al 2009). Moore (1999) demonstrated that the particle size of the ingested food can affect particle breakdown in the gizzard in geese, but the effect was apparent only when food particle size was already smaller than 2 mm.…”