For species that are secretive and conceal themselves in vegetation, another
possible solution to finding an unbiased sample of nests is to radio-tag adult
birds and later track them to their nests. Radio-tagging is expensive and time-
consuming, so choosing this method is likely to put severe limits on the number
of nests you can study. However, this approach can work well for difficult, secre-
tive species providing that a representative sample of adults was tagged and that
their nest site selection is unaffected by tagging. An example of this is the finding
of a sample of Corncrake Crex crexnests by radio-tracking females. Corncrake
nests are on the ground in dense vegetation and are difficult to find by cold
searching. Furthermore, females are likely to desert the nest if disturbed from it.
The beginning of incubation can be recognized by radio-tracking because the
female remains still in one place for long periods and this can be detected from
the pattern of fluctuation in the radio signal. The radio signal fluctuates when
the female is walking because of changes in the orientation of the transmitter
antenna. When a female was found to be inactive at the same location on several
occasions, her signal was then monitored continuously. Incubating corncrakes
leave the nest about once per hour for 10–15 min to feed. The start of a feed-
ing period was identified from the change in fluctuations in the radio signal
and the observer then went to the presumed nest location and searched for it
(Green et al. 1997).
Sometimes nests are easy to find, but some are more difficult to access than
others. Examples of this are nests on cliffs, in tall trees, and in deep holes. Since
accessibility to researchers may well be correlated with accessibility to predators,
it is important to ensure that nest checking methods are developed to the point
that the sample of nests that can be observed is not markedly biased towards
those that are easiest to get to. Specialized climbing techniques and devices such
as endoscopes for examining the contents of hole nests may be needed.
3.3.2Recording the stage of a breeding attempt when it is located
There are various advantages in having estimated the stage of a nesting attempt
when it is first found. Counting the eggs and feeling whether they are warm or
not can provide clues about whether incubation has started. The blunt end of
the egg can be checked for star-shaped cracking caused by the chick preparing
to hatch. These signs can sometimes be seen 2 days or more before hatching. By
learning how long it takes for a typical egg of your study species to proceed from
the first signs of cracking, through stages of progressive enlargement of a hole
in the shell to emergence you can estimate when the egg is likely to hatch relative
to the time of your observation. If there are no visible signs, the weight of the egg
relative to its size can provide useful information. Eggs lose about 15% of their
60 |Breeding biology