to be reliably identified, but each does so only at a proportion of nests, which
differs between the two predators. At the remainder of nests, no signs or egg
remains are left by either predator. The researcher, who has no information about
the proportion of depredated nests at which each predator leaves signs, con-
cludes erroneously that the predator that is less likely to leave signs is a less
frequent predator, whereas in fact the two predators are equally important.
Recording of signs is therefore useful in defining a list of predators, but caution
is needed in assessing their relative importance.
The signs found at failed nests include intact, cold eggs, holed eggs, shell
fragments, egg contents, nestlings dead in the shell, nestling body parts and
intact, dead nestlings. Signs may also be found at some distance from the nest.
There may also be hairs, tracks, feces or scent left by mammalian predators
or feathers, down or feces left by avian predators. Hoof prints and crushed
nest contents are usually found at ground nests trampled by cattle, other domes-
tic livestock or deer. Desertion of the nest or the death of a parent or parents
is indicated by a check of a previously incubated nest at which cold eggs are
found and at which incubation does not resume. Close examination of remains
of eggshells and egg contents may permit the identification of a predator from
the type of damage to the shell and the spacing between pairs of toothmarks
(Green et al. 1987). Shells opened by birds tend to have smaller holes with neater
edges than those opened by mammals, which often crush large areas of shell.
However, in many bird species there are no visible signs left at most of the
nests that fail and both bird and mammal predators are capable of taking
eggs long distances from nests. If knowledge of the stage of development of the
breeding attempt and the absence of signs of hatching or fledging indicate that
the breeding attempt cannot have ended successfully since the previous check,
then it is usually reasonable to assume that an empty nest has failed because of
predation.
3.4.2Wax or plasticine eggs in the nests of wild birds
Model eggs that resemble those of the study species can be made by filling
blown eggshells of similar size with wax or by moulding egg models from wax,
modeling clay or plasticine and painting them. The model eggs can be added
to clutches of real eggs in natural nests and may retain impressions of the
bill or teeth of predators that will aid in identification of the causes of failure
if they are compared with skulls, published measurements, or eggs given to
captive animals. There are several reasons to be cautious about this method. The
behavior of the parent birds may be affected by the addition of the model eggs.
Predators may be attracted by the smell of plasticine or paint or repelled if they
68 |Breeding biology