How the importance of survival estimates in estimating Whinchat population dynamics depends on the scale of migratory connectivity and site fidelity

Accurate monitoring of whinchat population dynamics requires accurate
estimates of breeding season survival and productivity, non-breeding survival
and site fidelity (dispersal, immigration and emmigration). But monitoring of
non-breeding survival between breeding seasons is confounded by the scale of
site fidelity resulting in low estimates, and this will vary dependent on breeding
success. Only one study (in progress) has measured true survival of whinchats
on the wintering grounds (in Nigeria) where site fidelity is probably very high.
Results from an ongoing geolocator tagging study also show only very large
scale connectivity (at the scale of thousands of kilometres) between breeding
and wintering populations. This means that annual survival rates measured at
any point on the wintering ground probably average true breeding and migration
survival for a large part of the breeding range, giving a representative true
survival rate to use in calculating population dynamics. More importantly, if we
have a measure of true average annual survival then we can calculate the
proportion of adults that are dispersing and also the scale at which they disperse
for breeding populations. Between winter survival rates for whinchats are the
same for first year birds and adults suggesting that the ubiquitous lower survival
rate of juveniles must arise between fledging and arrival at their wintering
territory: therefore survival estimates for this period should be investigated. If
survival immediately post-fledging or just before migration is variable then this
will greatly affect local population dynamics, but once migration starts – the
multiple routes and large scale connectivity - will mean populations over a large
area will be affected to the same degree. If survival during first migration has
declined substantially anywhere then many populations in the Palearctic will be
affected. Average annual true survival for whinchats greater than about 4
months old, across much of Eastern Europe is greater than 50%: therefore it is it
likely that local productivity or survival pre-migration determines an individual population’s dynamics, with the overall trajectory for the population being
determined by the additive effect of first migration survival. However, further
estimates of whinchat true annual survival are needed from other areas of Africa
to determine if overwinter survival is always high: if not then this variation would
negatively affect all Palearctic whinchat populations because of large scale
connectivity, in the same way that first migration survival may do.