Migration and breeding biology of Arctic terns in Greenland

46
the number of inviable eggs in the clutch and the binomial denominator
was the total clutch size. Covariates fi tted to the model were calendar date
and whether the eggs were in a fi rst or relay clutch. Model selection in
both cases was conducted using likelihood ratio tests. Both these models
were implemented in SAS.
Incubation and fl edging periods and their ranges were taken from the literature
(Cramp 1985). SDs were estimated by generating random normal
distributions from the published mean and iteratively varied SD values until
the maximum and minimum values matched the published range in values.
The mean replacement period and its SD were estimated from the intervals
between clutch removal and relaying in the experimental study plot.
The start date (average date upon which individuals laid their fi rst clutch)
and the stop date (the average date after which individuals would not
initiate further relay clutches) and their SD were estimated by iteratively
varying these values in a simulation model of the laying season (see below
for details). The resulting estimated frequency distribution of nest
initiation dates were compared with those observed in the experimental
study plot. The values that minimised the sum of the squared deviations
between observed and expected values were those used in further simulation
models (Green 1988, Ratcliffe et al. 2005).
Simulation model
A simulation models that allowed for re-nesting (Beintema and Müskens
1987; Green 1988; Green et al. 1997; Ratcliffe et al. 2005) was used to estimate
productivity and number of nesting attempts made by Arctic terns,
and the numbers of eggs that were harvested by humans, under a range
of harvest management options. Options involved variations in the date
harvesting started, the number of days the harvest remained open and
the amount of harvesting effort (spread equally over the open period) in
terms of man hours. We also only allowed one of the islands in the archipelago,
Basisø, to be open to harvesting, although the model has the fl exibility
to include variation in the area open for harvest.
Breeding pairs were randomly allocated to a management zone by generating
a random probability: if this was less than the proportion of pairs occupying
Basisø then the pair was at risk of harvesting, otherwise they bred
on another island where only natural mortality occurred. Breeding pairs
were also randomly allocated incubation, fl edging and replacement periods
and a start and stop date, all of which were drawn randomly from a
normal distribution with the mean and standard deviation appropriate to
each parameter. Clutch size was two unless a randomly generated probability
exceeded the estimated proportion of two egg clutches, in which
case it was one. The laying period was calculated as the clutch size minus
1 day, on the basis of one egg being laid per day and incubation started the
day the last egg was laid.
During each day of the laying and incubation periods, the clutch was
subjected to a likelihood of natural failure by testing whether a random
probability exceeded the date-specifi c daily nest survival rate until the incubation
period elapsed or the nest failed. On those days when harvesting
occurred, nests on Basisø were further subjected to a likelihood of being
harvested in the same way. The daily likelihood of harvest-induced failure
was calculated by multiplying the effort on that day by the probability of