functional response (see 10.5.6) with the daily energy requirements to ascertain
when the daily intake is insufficient, or by field measures of the prey density at
which feeding ceases. This model provided a good description of the number of
Black-tailed Godwits Limosa limosausing different areas (Gill et al. 2001).
10.5.8 Prey availability
A major problem in relating foraging behavior to prey density is that not all prey
are available. Thus, prey may seem abundant to the observer, but is largely inac-
cessible to the bird. Availability is always difficult to quantify and sometimes
impossible. The usual first step is to analyze the prey species, size classes, and
locations in which birds feed and restrict the study to these prey. For long-billed
shorebirds, the depths at which prey are taken can be assessed by comparing the
probing depth to bill length.
Prey depth of slow moving species can be quantified by digging out soil/mud
cores pushing out the contents (or having cores that open along their length) and
then quickly slicing through at measured depths. The depth at which bivalves
occur has been assessed by gluing thin threads of known length to bivalves and
measuring the length above the surface after they have reburied themselves
(Zwarts and Wanink 1996). Furthermore, by exposing these prey to predation
(easiest in captivity), it is possible to relate predation risk to depth.
The depth from which immobile prey can be extracted can be determined with-
in artificial feeding sites by experiments on captive or free living birds. Mark prey
individuals at each depth with a different mark and allow birds to feed and record
those taken or left. Thus Robinson (1997) marked seeds with felt tip pens and placed
the seeds in different depths within trays of soil placed in the wild. He watched to see
which species fed there. After birds had fed he sieved the soil and recorded the seeds
left and could thus determine the proportion taken from different depths.
10.5.9 Exclosures
It is often useful to compare changes in food abundance within exclosures from
which birds are excluded and control areas. The main issues are:
- Excluding the birds without also reducing the use of nearby control patches.
This is a risk especially if the materials flap in the wind or otherwise scare
birds from a wide area. For timid ground-feeding species, four corner posts
with bird-high wires around and across the posts is often sufficient. Netting
may be necessary for more confident species. - Whether prey will move into the exclosures so that depletion will
be underestimated. It is often unrealistic to consider using exclosures,
248 |Diet and foraging behavior