characteristic behavior, such as alarm-calling and distraction displays, in
the presence of the researcher. If marked parents are seen persistently not to be
showing this behavior at a time when their young could not yet have become
independent then it can be concluded that their chicks have died. The disap-
pearance of marked parents from the study area can alert the researcher to search
further afield for them and their brood. In some species, recently fledged juvenile
birds gather at feeding or roosting areas. Searches for individually color marked
birds in such areas, together with searches in subsequent years when they are
adult can yield mark-resighting estimates of the proportion of marked chicks
that survive to independence.
The attachment of small radio tags to chicks allows their survival and fate
to be monitored (see Chapter 6). However, this method is difficult to apply to
large numbers of chicks and tags and attachment methods may reduce survival.
Adverse effects of tags can be checked by comparing the survival or growth of
tagged and untagged chicks from the same brood, but a large study will be
needed before it can be inferred that any effect is negligible. Another problem is
that small tags are usually low powered and consequently have small detection
distances. Tags that fall into water after becoming detached, are carried a long
way or buried by predators, or are placed on chicks that move outside the study
area, are frequently lost to follow up. This makes the estimation of survival rates
and the unbiased assessment of causes of death problematic. Radio-tracking of
parent birds can often be combined with counts of their young to measure chick
survival in species where the parent and young are usually close together. For
species in which young roost at night on the ground with their mother, the roost
site of a radio-tagged mother can be noted at night and the researcher can locate
it on the following day to look for the droppings of parent and chicks. This allows
the death of all chicks to be detected from the absence of the smaller chick drop-
pings (Green et al. 1997). The droppings can also be collected to determine diet
(Chapter 10). Broods of Grey Partridge Perdix perdixchicks tracked and sampled
in this way whose reconstructed diet contained a high proportion of preferred
prey insects showed higher survival to independence than broods that fed mainly
on less preferred prey (Potts and Aebischer 1995).
The intensive methods described above are necessary when precise estimates
of early chick survival are required. However, estimates of age-specific survival
of chicks can be obtained from recoveries by the public of chicks marked with
numbered metal rings. This method requires that the age of each chick is esti-
mated when it is ringed by taking a standard measurement or record of plumage
development. The method relies on the fact that the proportion of ringed chicks
recovered after independence is lower for young than old chicks because more of
Measuring the success of individual breeding attempts| 65