Migration and breeding biology of Arctic terns in Greenland

Figure 1. Daily survival rates in
relation to age and hatch order for
a breeding attempt initiated at the
average day of fi rst clutch initiation
(day 34). Age is expressed in
days to or from hatching (where
day zero is the hatching date).
The black line represents the survival
rate of eggs and the A-chick,
and the grey line the survival rate
of the B-chick.
48
Results
Model input
The average clutch size at Kitsissunnguit in 2002-2006 equalled 1.84
(± 0.012, n = 1752) expressed as a 0.921 (95% CI 0.850-0.960) probability of a
two egg clutch in fi rst nesting attempt. Clutch size (1.6 eggs, ± 0.13, n = 16)
in replacement nests was signifi cantly lower (χ2 = 16.8, p = 0.0002, df = 2)
than non-manipulated nests (2.04 ± 0.026, n = 367) in 2006 and expressed
as a probability of 0.654 (95 % CI 0.483-0.792) in the model. Daily nest survival
(egg harvest excluded) varied over the course of the breeding season,
and daily chick survival varied with chick age (slope 0.067 ± 0.021,
p = 0.0013) and with hatching order (Fig. 1). Egg inviablity probability was
estimated to 0.035 (95 % CI 0.020-0.058).
Daily survival rate
1.02
1.00
0.98
0.96
0.94
0.92
0.90
0.88
The incubation period was set to 22 days and the period from hatching to
fl edging to 27 days (Cramp 1985, Hatch 2002) whereas the replacement
period (period between a nest is harvested and a replacement nest produced)
was set to 9.5 days (± 0.23, n=16).
The population estimates for Kitsissunnguit based on transects varied between
15,354 and 21,760 pairs (Egevang and Frederiksen 2010) with an
average of 18,465 (± 1,023) pairs used in the simulation model. The land
area (Basisø) exposed to harvesting equals 38.0 % (3.21 vs. total of 8.43
km 2 ) of the total land area whereas 80.4 % (14,845 ± 1,136) of the total Kitsissunnguit
population numbers (average 2002-2006) is found at Basisø.
The line transect also produced estimates of nest encounter probability,
which can be used as an expression of harvest effi ciency as search for nests
in the two situations resemble each other. In 2004, 555 minutes of effective
searching over 6,360 meters of line resulted in 415 found eggs originating
from 234 nests. These results were used to calculate a probability that an
individual nest was encountered person hour -1 : 234 [nests found] / (9.25
[hours search time] × 18,465 [nests, total] ) = 0.001370
Model output
0.86
–30 –25 –20 –15 –10 –5 0 5 10 15 20 25 30
Nest age (days from hatching)
Model output consisted of predicted harvest size (number of eggs harvested)
and productivity (number of chicks fl edging per nest) under various
harvest scenarios (Figs 2 and 3, A and B). An Harvest-Impact Ratio (HIR)
is presented in Figure 2 C and Figure 3 C, and was calculated as:
(HIR = Nh/(P0-Ph)),