rather than an anode cone eliminates distortion away from the image center and
has light amplification in the range 20,000–30,000 rather than 150 – 400 times.
Infrared telescopes, binoculars, and videos can be improved by using a searchlight
(e.g. 1 million candlepower) with an infrared filter. A searchlight did not effect the
foraging behavior of oystercatchers (Sitters 2000). The range of these illuminators
usually does not exceed 100 m. A bracket can be attached to the illuminator so that
it moves with the optical equipment.
10.5.4 Handling time
Handling time, the time spent catching and consuming prey, can be estimated
directly by starting a stopwatch when the individual concentrates on a prey item
to the exclusion of others and stopping it when the bird moves on to the next
activity. This is only accurate for relatively long handling times (e.g. over 5 s) or
if the behavior is videoed so that it can be replayed at a slower speed.
The method of Goss-Custard and Rothery (1976) can be used for measuring
handling times or pecking durations and is especially useful if handling times are
short (e.g. under 3 s). The time taken for a given number of paces (say 40) is mea-
sured along with the number of pecks made. The linear regression is calculated
with the numbers of pecks on the horizontal axis and the time taken for the pecks
and 40 paces on the vertical axis. Each additional peck increases the total time, so
the slope is the peck time. The intercept is the time for 40 paces and no pecks, and
so dividing the intercept by the number of paces (40) gives the time for one pace.
10.5.5 Intake rate and the functional response
The intake rate is the rate at which prey is acquired. It can be measured as: prey
items taken /foraging time or biomass intake/foraging time. The biomass intake
is obtained through multiplying by both prey size (10.3) and some component
of prey quality (see 10.4) such as energy content. Intake rate is measured either by
watching foraging birds (and stopping if the bird stops foraging) or by selecting
birds to watch regardless of their activity.
The intake rate depends upon the prey density; the functional response
describes the relationship between the intake rate (expressed as number of items
or biomass) and prey density. Following Holling (1959), the number of prey
eatenEduring time Tis related to the prey density Nby:
,where a is the searching efficiency and This the handling time (see 10.5.5) and
their values can be derived from using a curve fitting procedure (available in most
E
T
a N
1 a NTh246 |Diet and foraging behavior