Intake rates and the functional response in shorebirds (Charadriiformes) eating macro-invertebrates

Abstract

As field determinations take much effort, it would be useful to be able to predict easily the coefficients describing the functional response of free-living predators, the function relating food intake rate to the abundance of food organisms in the environment. As a means easily to parameterise an individual-based model of shorebird Charadriiformes populations, we attempted this for shorebirds eating macro-invertebrates. Intake rate is measured as the ash-free dry mass (AFDM) per second of active foraging ; i.e. excluding time spent on digestive pauses and other activities, such as preening. The present and previous studies show that the general shape of the functional response in shorebirds eating approximately the same size of prey across the full range of prey density is a decelerating rise to a plateau, thus approximating the Holling type II (' disc equation ') formulation. But field studies confirmed that the asymptote was not set by handling time, as assumed by the disc equation, because only about half the foraging time was spent in successfully or unsuccessfully attacking and handling prey, the rest being devoted to searching.A review of 30 functional responses showed that intake rate in free-living shorebirds varied independently of prey density over a wide range, with the asymptote being reached at very low prey densities (<150/m x2 ). Accordingly, most of the many studies of shorebird intake rate have probably been conducted at or near the asymptote of the functional response, suggesting that equations that predict intake rate should also predict the asymptote.A multivariate analysis of 468 ' spot ' estimates of intake rates from 26 shorebirds identified ten variables, representing prey and shorebird characteristics, that accounted for 81% of the variance in logarithm-transformed intake rate. But four-variables accounted for almost as much (77.3%), these being bird size, prey size, whether the bird was an oystercatcher Haematopus ostralegus eating mussels Mytilus edulis, or breeding. The four variable equation under-predicted, on average, the observed 30 estimates of the asymptote by 11.6 %, but this discrepancy was reduced to 0.2% when two suspect estimates from one early study in the 1960s were removed. The equation therefore predicted the observed asymptote very successfully in 93% of cases.We conclude that the asymptote can be reliably predicted from just four easily measured variables. Indeed, if the birds are not breeding and are not oystercatchers eating mussels, reliable predictions can be obtained using just two variables, bird and prey sizes. A multivariate analysis of 23 estimates of the half-asymptote constant suggested they were smaller when prey were small but greater when the birds were large, especially in oystercatchers. The resulting equation could be used to predict the half-asymptote constant, but its predictive power has yet to be tested.As well as predicting the asymptote of the functional response, the equations will enable research workers engaged in many areas of shore...

Ways to be different: foraging adaptations that facilitate higher intake rates in a northerly-wintering shorebird compared to a low-latitude conspecific

“…Furthermore, Macoma that are ≤8 mm long constitute a high proportion of the standing Macoma biomass in upper Cook Inlet (table 2 in Ruthrauff et al, 2013c). Because molluscivorous shorebirds reach digestive bottlenecks at relatively low prey densities (Goss-Custard et al, 2006;van Gils et al, 2005b), we predict that ptilocnemis rock sandpipers in upper Cook Inlet feed on Macoma of high quality (i.e. small sizes) such that net energy intake is maximized as a function of shell waste.…”

“…As a result of their relative ease of observation and diversity of foraging strategies, shorebirds (Charadriiformes) are common subjects of foraging studies (Colwell, 2010;Goss-Custard et al, 2006;van de Kam et al, 2004). During the nonbreeding season, shorebirds experience high energetic demands (Kersten and Piersma, 1987;Wiersma and Piersma, 1994), a natural history trait that also makes shorebirds ideal study subjects of the interplay between an organism's foraging ecology and its energetic requirements (Kvist and Lindström, 2003;van Gils et al, 2005a;Yang et al, 2013).…”

Intercolony variations in movement patterns and foraging behaviors among herring gulls (Larus argentatus) breeding in the eastern Wadden Sea

“…Prey depletion close to the breeding colony may be counteracted by a habitat switch (Schwemmer & Garthe, 2008) or a change in the diet (Goss‐Custard et al., 2006; O'Connor & Brown, 1977; Zwarts & Wanink, 1993). A previous study on the food choice of oystercatchers noted a shortage of suitable prey in the vicinity of Oland (Schwemmer et al., 2016).…”

Ecosystem services provided by waterbirds

“…of how pest control or seed dispersal is influenced by changing abundance of pests or seeds. For example, existing work already gives some idea of how ducks should respond to changing densities of weed seeds in ricefields (Arzel et al, 2007;Greer et al, 2009), or how shorebirds should respond to changing densities of invertebrate pests (Goss-Custard et al, 2006). These functional responses can also be applied within individual-based models that could be developed to allow prediction of when birds switch habitats (Stillman & Goss-Custard, 2010), e.g.…”