Context: Demographic parameters in wildlife populations are
typically estimated by monitoring a limited number of individuals in
observable sites and assuming that these are representative of the whole
population. If individuals permanently disperse to unobservable
breeding sites, recruitment and immature survival are expected to be
negatively biased and breeding-site fidelity cannot be measured.
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| Photo: Leighton De Barros |
Aims: To
develop a method to obtain unbiased estimates of survival, recruitment
and breeding dispersal when individuals can move to, or recruit in,
unobservable sites.
Methods: We used the flexibility of
multi-event capture–recapture models to estimate dispersal and
recruitment to unobservable sites, merging observations made at two
sites within the same breeding locations. We illustrated the model with
data on little penguin (
Eudyptula minor) breeding in artificial
as well as in natural nests. Natural nests are unknown or inaccessible
and birds in these sites remain unobservable. Encounters at beaches
surrounding the colony suggested that marked animals can permanently
move to unobservable nests. We built the multi-event model considering
two possible states of the individuals (alive breeding in a nest box and
alive in a natural nest) and three types of observations (encountered
at a nest only, encountered at the beach only and encountered at both
places). This model ensured that the breeding dispersal to unobservable
places became estimable.
Key results: Results indicate that
the estimated survival was 8% higher than when recaptures at artificial
nests were analysed alone. Also, fidelity to artificial nests was 12%
lower than to natural nests. This might reflect the greater availability
of natural sites or, alternatively, a heterogeneity between these two
types of nest.
Conclusions: We obtained an estimate of
local survival of little penguins breeding at Penguin Island that
incorporates the permanent migration to unobservable sites and found an
asymmetric dispersion towards natural nests.
Implication: Our
conclusions suggest a need for more careful treatment of data derived
from artificial sites alone, as demographic parameters might be
underestimated if animals prefer natural breeding sites or if they are
in greater proportion compared with artificial ones. The analytical
approach presented can be applied to many biological systems, when
animals might move into inaccessible or unobservable breeding sites.
