Food Dispersion and Foraging Energetics: A Mechanistic Synthesis for Field Studies of Avian Benthivores

Abstract

Much effort has focused on modeling and measuring the energy costs of free existence and the foraging strategies of animals. However, few studies have quantitatively linked these approaches to the patch structure of foods in the field. We developed an individual—based model that relates field measurements of the dispersion of benthic foods to search costs and foraging profitability of diving ducks. On Lake Mattamuskeet, North Carolina, Canvasback ducks (Aythya valisineria) eat only the belowground winter buds of the submerged plant Vallisneria americana. We measured and modeled the patch structure of winter buds at the level of potential foraging "loci," defined as contiguous circles 1 m in diameter. In the field and in the model, Canvasbacks make repeated vertical dives in such loci, foraging in the sediments by touch, before surface—swimming to another locus. We quantified first—order patchiness by fitting a negative binomial distribution to core samples taken at 50—m intervals along transects, to yield the frequencies of loci with different bud densities. Second—order patchiness was measured by taking cores at 1—m increments radiating from each sampling point, and regressing bud density at each sampling point on densities at these increments. No significant correlations were found, indicating that Canvasbacks could not predict food densities based on densities in nearby foraging loci. For the model, we generated food grids from the negative binomial distributions of core samples. Energy costs of diving were calculated by applying aerobic efficiencies (mechanical power output $\div$ aerobic power input) to biomechanical models. Unlike respirometry alone, this method accounts for effects on dive costs of varying water depth and dive duration. We used measurements of Canvasback intake rates at different bud densities to calculate profitability (energy intake minus expenditure) for each dive. Multivariate uncertainty analyses (Latin hypercube) indicated that profitability for Canvasbacks foraging on Vallisneria buds is determined mainly by food—item size and locomotor costs of descent. Bud metabolizable energy, water temperature, bud dispersion, and search and handling time coefficients of the functional response for intake rate have relatively minor influence. Individual—parameter perturbations indicated that to maintain the same foraging benefits, the total area of Vallisneria habitat would have to increase by 1.4—fold if dry mass per bud decreased from 0.10 to 0.03 g, and by 2.1—fold if water depth increased from 0.5 to 2 m. Our method allows study of interactions between patch structure and foraging energetics without detailed spatial mapping of foods, which is not feasible at appropriate scales for highly mobile benthivores. The model yields estimates of energy balance, contaminant intake, and amount and quality of foraging habitat required to sustain diving duck populations under varying environmental conditions. More accurate prediction of giving—up times and giving—up food densities will require better unde...

Effects of clam species dominance on nutrient and energy acquisition by spectacled eiders in the Bering Sea

“…In the equation, I = aX/(b + X), I is the number of clams consumed per second at the bottom, X is the number of clams m -2 , a is the handling time coefficient or the maximum rate at which clams can be consumed regardless of clam density, and b is the search time coefficient or the clam density at an intake rate of 0.5a (Lovvorn & Gillingham 1996).…”

“…Dive bouts alternate cyclically with preening and resting periods (Guillemette 1994). Because of the high energy cost of diving (de Leeuw 1996, Kaseloo 2002, intake rate should be maximized during each dive to minimize time underwater, with bouts of a series of dives simply being terminated upon satiation (Lovvorn & Gillingham 1996). Digestion appears to occur mainly during a resting period after the dive bout, when costs of floating on the water surface are far less than during dives (Kaseloo 2002).…”

“…Rate of food intake at a range of prey densities, known as the functional response, is critical to foraging profitability (energy gain minus cost) (Myers et al 1980, Wanink & Zwarts 1985, Draulans 1987, Piersma et al 1995. A type 2 functional response, in which intake rate increases with increasing prey density up to an asymptote where intake is limited by handling time, often typifies diving duck foraging on benthic foods (Giles 1990, Lovvorn & Gillingham 1996. Prey depth in the sediments has important effects on the functional responses of crabs and shorebirds eating clams (Wanink & Zwarts 1985, Lipcius & Hines 1986, Seitz et al 2001), but effects of prey depth have seldom been studied for diving ducks (Carbone 1995, Richman 2002.…”

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