causes of failure. Cold searching, that is searching visually for nests in all poten-
tial nesting habitat in the study area, is a frequently used method for finding
nests. For some species of woodland raptors that re-use nests for several years or
build new nests near to old ones, it can be productive to search for old nests in
winter when deciduous trees have shed their leaves and nests are easier to see.
This gives a good indication of where there are likely to be active nests in the
spring. There is a danger that cold searching can give a biased sample of nests
for study because some habitats are easier to search than others. For example,
several bird species that nest on farmland in Britain build hidden nests in
hedgerows, the lines of shrubby vegetation on uncultivated field margins, but
also nest in field crops. Cold searching for nests by carefully looking in trees,
bushes, and ground vegetation is a practical, though time-consuming, method for
finding nests in hedgerows; but it is unproductive in field crops because, even
though there may be as many nests in fields as in hedgerows, the total area of the
fields is much larger and farmers dislike researchers walking through their crops.
Nests in hedgerows are probably at greater risk of being located by predators, but
nests in crops are at risk from farming operations.
For species that can easily be watched, potential bias of this kind can be
overcome by first finding a bird and then watching it back to its nest during visits
for nest building, incubation changeovers, or feeding nestlings. The use of a hide,
or using a car as a hide, makes this practical for many species. Some ground
nesting birds can be watched back to the nest by an observer who has climbed
a nearby tree and are especially unwary if the observer has a companion who leaves
the area. Providing that birds can be located and watched with similar ease in
different habitats, then watching back may well yield a less biased sample of
nests than cold searching. The efficiency with which nests can be found in this
way can be increased if the observer is aware of the significance of special clues
provided by behavior or signs. In many species of galliform birds, females that are
foraging during a break from incubation peck at food much more rapidly than
normal and they also produce unusually large droppings because they accumulate
fecal material during long incubation stints. Other signs that can draw the obser-
ver’s attention to a nest include anxiety calls, carrying of nestling droppings away
from the nest by passerines, carrying food or nest material and displays used by
male birds when leading their mate to a nest site. Changes in the height or density
of foliage during the breeding season can also affect the ease with which nests can
be found and may lead to undersampling of late nests. This can lead to serious bias
in estimates of nest success if success varies markedly with time of year. As with
habitat differences, it may be that finding nests by watching birds back to them
is less susceptible to the effects of vegetation changes than is cold searching.
Measuring the success of individual breeding attempts| 59