DOI: 10.5061/dryad.nd4g8
title: Data from: Field measurements give biased estimates of functional response parameters, but help explain foraging distributions

description: 1. Mechanistic insights and predictive understanding of the spatial
 distributions of foragers are typically derived by fitting either field
 measurements on intake rates and food abundance, or observations from
 controlled experiments, to functional response models. It has remained
 unclear, however, whether and why one approach should be favoured above
 the other, as direct comparative studies are rare. 2. The field
 measurements required to parameterize either single or multi-species
 functional response models are relatively easy to obtain, except at sites
 with low food densities and at places with high food densities, as the
 former will be avoided and the second will be rare. Also, in foragers
 facing a digestive bottleneck, intake rates (calculated over total time)
 will be constant over a wide range of food densities. In addition,
 interference effects may further depress intake rates. All of this hinders
 the appropriate estimation of parameters such as the ‘instantaneous area
 of discovery’ and the handling time, using a type II functional response
 model also known as ‘Holling's disc equation’. 3. Here we compare
 field- and controlled experimental measurements of intake rate as a
 function of food abundance in female bar-tailed godwits Limosa lapponica
 feeding on lugworms Arenicola marina. 4. We show that a fit of the type II
 functional response model to field measurements predicts lower intake
 rates (about 2.5 times), longer handling times (about 4 times) and lower
 ‘instantaneous areas of discovery’ (about 30 to 70 times), compared with
 measurements from controlled experimental conditions. 5. In agreement with
 the assumptions of Holling's disc equation, under controlled
 experimental settings both the instantaneous area of discovery and
 handling time remained constant with an increase in food density. The
 field data, however, would lead us to conclude that although handling time
 remains constant, the instantaneous area of discovery decreased with
 increasing prey densities. This will result into highly underestimated
 sensory capacities when using field data. 6. Our results demonstrate that
 the elucidation of the fundamental mechanisms behind prey detection and
 prey processing capacities of a species necessitates measurements of
 functional response functions under the whole range of prey densities on
 solitary feeding individuals, which is only possible under controlled
 conditions. Field measurements yield ‘consistency tests’ of the
 distributional patterns in a specific ecological context.
description: Field_data_IThis file provides the field data of observed female bar-tailed godwits foraging on lugworms. Data includes prey size, handling time, energy (g AFDM), total foraging time, time spent vigilance, preening time, plot nr, searching time (Ts), intake rate per second and prey density.Field_data_IISimilar file as Field_data_I, but now only for individual foraging birds.handlingThe data file from the experimental set up.Ivlev_densityData file containing the Ivlev index, per grouped food densitylong_term_intakeFile containing detailed (per sec) cummulative intake from 3 individual birdsSearch_timeDatafile containing search time (Ts), with a similar set up as the handling data file