locate individual nests and estimate their stage of development when found yield
distributions of first egg-laying dates through the season which are the most
widely available data on timing of breeding. However, it should be recognized
that these distributions may be biased if the effort made to search for nests, or the
ease with which nests could be detected varied through the season, for example,
because of the growth of concealing vegetation.
Alternative methods for studying the timing of breeding that do not involve
locating nests include finding the chicks of species with precocial young and
estimating their age from measurements so that dates of egg-laying or hatching
can be estimated. This method is subject to the possible biases due to variation
in effort and ease of chick location described above for nests. However, because
the chicks of some species are much easier to find than their nests, it can be
a useful method, though it only reveals the timing of those nests that hatched
young, which may not be typical in their timing of all nests. Beintema et al.
(1985) were able to document changes in the distribution of hatching dates
of wader chicks in the Netherlands over a 50-year period using the dates on
which chicks were ringed.
Data obtained when adult birds are captured can also be used to study the
timing of breeding. In some species, it may be possible to identify females which
have recently laid eggs by the stretched appearance of the cloacal aperture and
incubating parents by the presence of a naked, oedematous brood patch. However,
experience of birds at known breeding stages is usually required before assessment
of these characters is reliable and in some bird species a distinctive brood patch is
not developed during incubation. Assays of the level of vitellogenin-zinc in samples
of blood plasma can be used as an index of vitellogenin, an egg yolk precursor,
which is elevated during egg formation. Lougheed et al. (2002b) used this method
to estimate the time of breeding of a Marbled Murrelet population from samples
taken from adults captured at sea. All of these methods require that adults are
caught and examined or sampled across a wide range of dates throughout the
potential breeding season.
Examinations of mist net captured adult and juvenile Bullfinches were used by
Newton (1999b) to determine the timing of the end of the breeding season,
which is difficult to determine by finding nests because they become difficult to
locate when vegetation is dense in late summer. Adult bullfinches begin their
molt after their last breeding attempt and juveniles begin their body molt after
fledging. By backdating the onset of moult from observations of juvenile and
birds captured while in molt in late summer and autumn, it was possible to
estimate the relative numbers of young fledged from early and late broods.
This revealed that seasons with good productivity tended to be those in which
Timing of breeding| 77