Colony- and age-specific seasonal dispersal of Herring Gulls <i>Larus argentatus</i> breeding in The Netherlands

The Herring Gull population in The Netherlands went through phases of exploitation, protection, persecution and again (partial) protection during the nineteenth and twentieth centuries. The numbers of breeding pairs peaked in the 1980s at approximately 90,000 pairs, at which point a colour-ringing campaign was organised to evaluate dispersal and distribution patterns. Herring Gulls were ringed as chicks, predominately near-fledglings, in 12 colonies in 1986 (1,247 individuals), 13 colonies in 1987 (1,354 individuals) and 14 colonies in 1988 (1,396 individuals). Between 1986 and 2009, of the 3,997 Herring Gull chicks colour-ringed, 3,124 individuals (78.2%) were seen and reported at least once, while 453 (11.3%) were recovered dead. In total, 86,247 ring-readings of living gulls were received and processed, originating from 1,358 locations by 868 observers. One-fifth (20.5%) of all sightings originated from the home-ranges (areas within a radius of 5 km around the ringing place). Only 691 sightings (0.8%) were reported at over 300 km from the natal colony (10.7% at 6-10 km, 8.9% at 11-25 km, 17.7% at 26-50 km, 22.9% at 51-100 km, 14.4% at 101-200 km and 4.1% at 201-300 km). Colony-specific differences in travelling distance, dispersal rate and direction of movements suggest a grouping of colonies in three areas: ( I) eastern Wadden Sea islands (Rottumeroog-Vlieland), with significantly higher dispersal rates and movements mostly towards the south-west to south-east; (2) Texel and the four colonies along the mainland coast (Callantsoog-Wassenaar), with shorter mean range and movements mostly to the south; (3) colonies in the Delta area (Europoort-Saeftinghe), with rather short range movements and dispersal in many directions. The maximum distance travelled did not vary much between adults, immatures and juveniles, but the timing of outward and return movements was different for each of these age categories. Adult birds reached their greatest mean distances on average 1 month earlier than immatures, which in turn arrived at this point 1 month earlier than juveniles. These age-specific differences were enhanced in the spring, when birds were moving towards the (natal) colonies, but when adults moved on average closer and 2 months ahead of immatures, which in turn moved earlier and closer to the natal home-range than juveniles. With reference to findings from other studies in other European countries, Herring Gulls breeding in The Netherlands occupies a mid-position between dispersive and sedentary tendencies