because illegal hunters are not likely to report the kill of marked birds. Special
studies using surgically implanted radio transmitters would provide a means of
estimating harvest rate associated with illegal kill in local study areas, but this
would be a relatively expensive endeavor.
Finally, we note that the translation of estimates of harvest rate (defined with
respect to retrieved harvest) into estimates of kill rate (defined with respect to all
hunting-caused death, regardless of hunter retrieval) requires additional informa-
tion on the rate of retrieval of shot birds. Such information on “crippling rate”
can be obtained from hunter observation studies and hunter questionnaire surveys
(e.g. Martin and Carney 1977).
13.5 Assessing population size
13.5.1Overview
The subject of estimating avian abundance and density was discussed in Chapter 2.
The basic concept underlying all of the methods described in that chapter
involves the issue of detectability. All estimation methods entail collection of
some sort of count statistic, C. This may be the number of birds detected while
walking a line transect, the number caught in a mist net, the number counted
from an airplane, or the number shot and reported by hunters. This count can be
viewed as a random variable, the expectation of which can be written as a func-
tion of the quantity of interest (Nabundance) and detection probability
(probability that a member of Nappears in the count, C):E(C)N. If
detection probability associated with the count statistic can be estimated, then
this relationship can be used to estimate abundance as:
(13.10)All of the methods available to estimate animal abundance require estimation of
detection probability associated with the count statistic and then application of
equation (13.10) (Chapter 2; also see Lancia et al. 1994; Williams et al. 2002).
These approaches include such observation-based methods as distance sam-
pling, multiple observers, sighting probability modeling, and temporal removal
modeling, as well as capture-recapture and catch-effort approaches based on
marked animals, and change-in-ratio approaches for harvested species (Seber
1982; Lancia et al. 1994; Williams et al. 2002).
13.5.2Coping with uncertainty
Population size is seldom known with certainty, and this “partial observ-
ability” is an important source of uncertainty in the management process
NˆC/ˆ.
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