breeding continued into late summer, so the duration of the breeding season
was apparently an important determinant of annual productivity. This method
could probably be applied to many other species, though the details of the timing
and rate of moult in relation to breeding would first have to be investigated.
3.8 Measurements of eggs and chicks
Measurements of egg size and weight have already been mentioned because they
permit the estimation of the stage of incubation. However, estimates of egg
volume from measurements with calipers of egg length and width are valuable
in their own right because female birds may lay larger eggs when food resources
or foraging conditions are good and this can influence the survival of chicks. An
index of egg volume can be used (the product of length and the square of maxi-
mum width) or the actual volume or fresh weight of the egg could be estimated
approximately using coefficients for a species with similar egg shape from Table 1
of Hoyt (1979). More precise estimates of volume that take account of differ-
ences in shape among eggs of the same species can be made using photogram-
metry (Paganelli et al. 1974). This method could also be used for small eggs
that are difficult to measure safely with calipers. Alternatively, the fresh weight of
small eggs could be determined with a portable electronic balance providing that
weighing was done before significant water loss had occurred.
The thickness of bird eggshells is an important correlate of breeding success
in species affected by organochlorine pesticide contamination (Newton 1979).
An index of shell thickness can be calculated from the weight of the eggshells
of blown eggs, usually in museum collections, after correction for the dimen-
sions of the egg and the pieces of shell removed when the egg was blown (Nygård
1999; Green 2000). Alternatively, the thickness of blown eggshells can be mea-
sured near the equator of the egg using a specially modified micrometer in which
a round-ended needle attached to the moving jaw of the micrometer is passed
through the blow-hole in the side of the egg and the thickness of the shell at the
other side of the egg is measured between the tip of the needle and the micrometer
anvil (Green 1998). It is also possible to measure the thickness or an index of
calcium content per unit shell area for living eggs non-destructively using
portable X-ray or beta particle backscatter apparatus (Fox et al. 1975; Forberg
and Odsjö 1983, 1984).
Measurements of chicks are valuable in estimating age, development, and
condition. Accurate weights are required, together with measures of body
dimensions such as the combined length of the head and bill, tarsus length,
maximum chord wing length. For chicks of known age, the weight or some
78 |Breeding biology