Estimating population size in Black-tailed Godwits Limosa limosa islandica by colour-marking

Capsule Adult population size of the Icelandic race of Black-tailed Godwit was estimated at 37 500 individuals. Aims To estimate the population size of the Icelandic race of Black-tailed Godwit and infer numbers wintering in areas where both the Icelandic and nominate races overlap. Methods Adult population size was estimated using resightings of individually colour-ringed birds in spring staging flocks in Iceland. Winter distribution was assessed by comparing January counts to the population estimate, assuming 0.5 chicks per breeding pair to estimate the number of first-year birds and the resulting winter population size. Results The adult population size of the Icelandic race of Black-tailed Godwit was estimated at about 37 500 individuals (range 35 565-41 999) between 1999 and 2002. This gives an estimated winter population of about 47 000 individuals. From counts in wintering areas and the calculated population estimate, we estimate that about 15 000 Icelandic Black-tailed Godwits winter in areas where the races overlap (mostly in Portugal, Spain and Morocco) but the numbers in each country remain unknown. Conclusion For species which can be fitted with conspicuous colour rings and for which a high proportion of individuals can be observed at particular times of year, mark-recapture methods can be very useful techniques to estimate the number of individuals of different races of birds wintering in the same areas.