Several comparisons of failures of real and artificial nests in the same areas have
indicated that predation rates and predator species differ. Daily predation rates
on artificial nests were usually, though not always, higher than those on real nests
(Major and Kendal 1996; Davison and Bollinger 2000; Weidinger 2001). King
et al. (1999) found that artificial nests suffered higher failure rates than real nests
even when differences in concealment by vegetation were allowed for by logistic
regression. Differences in failure rates might not be important if absolute rates
were of less interest than differences among habitats, years, or times of year.
Variation in failure rates of artificial nests might be correlated with variation in
the failure rates of real nests. However, among year and within season trends of
failure rates of artificial and real nests in the same areas are not always the same
(Weidinger 2001).
In view of these problems, it is unsafe to assume that the rates and causes of
failure of artificial nests reflect those of real nests unless studies that compare the
two show that this is the case. Finding and checking enough real nests to obtain
a reliable result in a comparative study of this kind might often require as much
effort as would be needed to conduct the study wholly on real nests. Hence,
studies of artificial nests are not recommended in most circumstances.
3.6 Measuring annual productivity
3.6.1Why measure annual productivity?
Annual productivity or reproductive success is the number of young fledged per
adult female (or adult bird or pair) per year regardless of the number of breeding
attempts. In studies of population processes, it provides a measure of mean
reproductive success, which can be used in simulation models of the population
to estimate the population growth rate. In studies of evolutionary ecology, pro-
ductivity can be assessed at the level of the individual as a component of fitness.
In species in which females never lay more than one clutch of eggs per year,
productivity per breeding female or pair is the same as the mean success of an
individual breeding attempt and productivity per adult or pair can be calculated
from this and information on the proportion of females that attempt to breed.
However, most bird species make more than one breeding attempt per year, even
if they only produce a replacement clutch after failure of the first clutch at the
egg stage, and many species are capable of rearing two or more broods of young
per year. Even so, it might seem that the success of individual breeding attempts
would be a good index of productivity. The extent to which this is true depends
upon how much variation among areas or years there is in the duration of
72 |Breeding biology