initial weight during incubation, mainly because of water loss (Ar and Rahn
1980). If eggs are weighed and their length and maximum breadth measured,
the ratio of weight to the product of length and the square of breadth can be
calculated, and from it the proportion of the incubation period elapsed. Simple
home-made charts can be used to estimate this in the field (Green 1984;
Galbraith and Green 1985). It can be difficult to measure small eggs safely and
accurately, so an alternative method is to place the egg in a transparent vessel
containing tepid water. Recently laid eggs lie on their side on the bottom of
the vessel with their blunt end slightly raised. As incubation proceeds, the blunt
end rises higher, though the pointed end remains on the bottom. The observer
estimates the angle that the long axis of the egg makes with the horizontal floor
of the vessel to quantify this change. As incubation advances the long axis of
the egg eventually rises to be oriented vertically. Next the egg floats to the surface
with the blunt end uppermost. The amount of shell that rises above the surface
then increases as incubation progresses (van Paassen et al. 1984). Eggs can also
be candled, that is placed on a strong light source so that the shadows cast by the
developing embryo and blood vessels can be seen through the shell. However,
this is difficult to do safely for small eggs and it may not be possible to see the
shadows at all in species with strongly pigmented or patterned eggshells. Medical
gloves can be worn to reduce the chance that handling eggs might leave scent
on them that could affect the behavior of egg predators or contaminate them
with pathogenic bacteria, but using gloves may make it more difficult to avoid
breaking small eggs. For nests first found at the nestling stage or precocial chicks
located away from the nest, age can be estimated from weights, measurements,
or descriptions of plumage development. The information to do this may be
found in the literature or from your own measurements and descriptions of
chicks whose age you know because you observed them at hatching.
3.3.3Precautions to take so that nests can be relocated for checking
It may seem obvious to record precisely how to relocate a nest once you have
found it, but nests of many species are surprisingly easy to lose. The conse-
quences are worse than just reducing your sample size; it can also bias estimates
of nest success. This can arise because a nest being tended by adult birds at
the time of a nest check is often easier to relocate than one that has failed. If you
are more likely to lose nests that have failed, then nest success calculations based
on the remainder will be too high. Researchers often mark nests with a visible
artificial marker, such as a stick or tag, to aid relocation. This has the potential
disadvantage that predators may also learn to use the markers to find nests
Measuring the success of individual breeding attempts| 61