predators in the same way that visits to check nests may. However, camera
systems that use a long cable or microwave link to relay image data from the
camera to the video recorder may be used. This may allow the equipment that
needs frequent visits to be located far enough away from the nest for disturbance
to be avoided.
At present, camera systems that are most likely to allow the collection of
unbiased information on the causes of nest failure are costly. Hence, it may not
be feasible for a modestly funded research project to collect information at a suf-
ficiently large sample of nests using these systems alone. Using cheaper systems
runs the risk that the equipment and maintenance visits may affect predation
rates and predator species. A possible compromise in some cases is to use a few
cameras to validate the identification of predators based on signs left at or near
nests. These signs could then be used to identify predators at large samples of
nests by normal nest checking. Of course, if many nests fail without any visible
signs being left, then this compromise will be unsatisfactory.
3.4.4Temperature loggers
A sensor that responds to temperature can be placed in the nest and connected
by a fine cable to a data logger, which records the temperature and time at pre-
determined intervals. The sensor probe can be very small (a few cubic millimetres)
and the logger can have a capacity of thousands of records and can be a few cubic
centimetres in volume. A small battery with an operating life of weeks or months
can power the logger. By fixing the sensor probe into the nest cup and burying
or otherwise hiding the connecting wires and data logger, the system can be made
difficult to detect. If the logger is deployed when the nest is found, it can be
recovered after the end of the breeding attempt and the temperature record can
be examined to identify the time of nest failure. The sensor probe is warm when
parent birds are incubating or when live nestlings are in the nest, but cools rapid-
ly when the nest is unincubated or the nestlings have been removed or have died,
so the time of failure can be discerned and any preceding absences of parent from
the nest caused by disturbance may also be detected. The time of day of nest
failure does not identify its cause unambiguously, but it provides useful clues.
Jackson and Green (2000) used temperature loggers in the nests of waders
(Charadrii) to show that there were characteristic signs left at nests that failed at
night that were not present at nests that failed by day. When combined with
other evidence, this indicated that predation by the nocturnally active Hedgehog
Erinaceus europaeus, rather than by avian predators active by day, was the most
frequent cause of nest failure.
70 |Breeding biology