statistical packages) to fit this equation. However, this approach should be used
with caution. If a food category includes a range of items of different profitability,
the bird may become more selective within a category as the density of that
category increases. If this heterogeneity is not recognized, foraging parameters
estimated in this way will be biased.
Daily energy expenditure can usually be assumed to balance consumption.
Bennet and Harvey (1987) showed that across bird species the active metabolic
rate (AMR) in KJ per day can be estimated from
Ln (AMR)0.61 Ln (body mass in kg)1.18.10.5.6 Interference
Interference is the short-term reduction in intake rate resulting from the pres-
ence of others, including the effect of disturbing the prey. This is usually assessed
by marking out an area for which bird density varies markedly over the study
period but the prey availability and density do not. Count bird density and
intake rate. The standard method is to plot log 10 intake rate against log 10 bird
density (Yates et al. 2000) with the slope indicating the extent of interference.
Fighting and kleptoparasitism (food stealing) often contribute to interference
and their rates are usually determined by focal animal sampling. Susceptibility to
interference is likely to vary between individuals, with juveniles suffering most.
Interference cannot be assessed by combining data across sites as better quality
sites tend to have both higher intake rates and more birds, resulting in a positive
correlation between intake rates and density.
10.5.7 Depletion
Depletion is the removal of food items that would otherwise be available to others.
It can be studied using exclosures (see 10.5.11), by estimating the total intake of
all individuals or by relating the decrease in prey population to the initial prey
density. The maximum number of individuals Pthat can be sustained in a site,
assuming no replenishment or growth of food items, can be calculated from:
where fis the area with density of available prey jat the start of the study, Mis the
highest recorded prey density, a is the searching efficiency, This the prey handling
time and dcis the threshold prey density at which feeding is no longer possible
(Sutherland and Anderson 1993). The values of a andThcan be determined from
the functional response. The value of dccan be estimated from combining the
PTh
M
dc(jdc)fj1/a (^)
M
dc
fjloge(j/dc)
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