the former die before independence and are therefore not available to be recov-
ered by the public later. If chicks are ringed at a sufficiently wide range of ages,
the entire survivorship curve in relation to chick age can be reconstructed, or an
index of overall chick survival can be calculated (Beintema 1995).
3.3.6Estimation of nest success from nest check data
In most studies of nest success, nests are first located after the breeding attempt
has begun. If that is the case then taking the proportion of nests found when
active that escape predation or other causes of failure as a measure of nest success
produces an overestimate (Green 1989). This is because nests first found at more
advanced stages of development are exposed to the risk of failure for a shorter
time than those found at an early stage. The Mayfield method (Mayfield 1961;
1975) enables the probability of a nest surviving from the beginning to the end
of a particular stage of breeding to be estimated from nest check data without
this bias. The principle is simple. The number of days after each nest was found
for which it was exposed to the risk of a failure that could have been detected by
the observer is tabulated together with its outcome (failed or not failed). The
number of exposure days is then summed for all nests and a daily failure proba-
bility calculated as the number of observed failures divided by the total exposure
days. The probability of a nest surviving the whole of the breeding stage is then
given by the daily nest survival rate (1 minus the failure rate) raised to the power
of the length in days of the stage. In practice nests are not usually checked every
day and in calculating exposure days it is assumed that a nest that failed did so
halfway through the period during which it could have failed. Suppose that a nest
contained young that were estimated to be 10 days old on the penultimate check
and that the last check, with signs of nest failure, was made 10 days later, 4 days
after expected fledging at 16 days old. The best estimate of the time of nest
failure would be midway between the 10th and 16th day of the nestling period.
Information on the stage of development of eggs and nestlings and signs recorded
at nest checks can often be used to define the beginning and end of the period
during which failure could have occurred and improve the accuracy of Mayfield
estimates.
It is important to divide the various stages of breeding appropriately before
carrying out Mayfield analysis. If two stages that really have substantially differ-
ent daily nest failure rates are combined and analyzed as if they were one stage,
the probability of nest survival over the whole of the combined periods will be
overestimated (Willis 1981). In some birds, such as waders (Charadrii), the daily
failure probability during egg-laying is much higher than during incubation
66 |Breeding biology