Estimating the Seasonal Distribution of Migrant Bird Species: Can Standard Ringing Data Be Used?

The true distribution of migrant species is rarely immediately apparent from the distribution of ring recoveries due to a heavy bias in regional recovery probabilities. For western Palearctic species, the recovery probability is especially low in Africa, but also varies within Europe. However, little work has been done to derive actual estimates of these recovery probabilities needed to infer the “true” underlying distribution. Here, we investigate the potential of using ringing data to estimate the seasonal distribution densities of migrant species. Using likelihoods based on a two point mixture distribution, the proportions of individuals wintering south of the Sahara are estimated using differences in recovery distributions among species in species groups where the location-specific probability of a ring recovery can be assumed to be essentially the same among species. We consider two such approaches. In the first, survival associated with a wintering area must be set constant across species. In the second, we assume the time series is long enough that a single binary response (recovered/not recovered) may be modeled independently of survival parameters. Under the first approach, we estimated the proportion of sub-Saharan migrants, together with 95% profile likelihood confidence intervals, for redstart as 0.84 [0.70,0.93], thrush nightingale 1.00 [0.49,1.00], garden warbler 0.95 [0.85;0.99], blackcap 0.60 [0.32;0.78], reed warbler 0.87 [0.72,0.95], and pied flycatcher 0.90 [0.76;0.97] using recovery data for birds ringed in Denmark and assuming that all robins winter north of Sahara. In the second approach, estimated proportions of sub-Saharan migrants were similar, but the confidence intervals were somewhat narrower. Although further work is required to examine the underlying assumptions, the models and analyses presented here provide a framework for making better use of existing ring recovery datasets to understand the “true” seasonal distribution patterns of European birds.