Thayer’s Gull Larus thayeri along the coast of the SW. Netherlands
Between 19th March and 20th May 2019 an imm (4CY) Thayer’s Gull Larus thayeri was present in the Dutch Rhine- Meuse delta; the first record for The Netherlands. Since ca. 1990 Thayer’s Gull from the Canadian arctic has been found in the East Atlantic i.e. in Ireland, England, Norway, Danmark and NW Spain, all records in December– March.
Because Thayers ’ Gull resembles Herring Gull (L. argentatus i.c. smithonianus) some taxonomists believe it to be a geographical race of that species say Sutton and Parmelee (1978)
Although,they said, we have long entertained the belief that a fairly close taxonomic relationship exists between
thayeri and argentatus, substantiated in part by the studies of Smith (1966), we also recognize the possibility that
thayeri may, in view of recent findings by Earl Godfrey (pers. comm.), be conspecific with the Iceland Gull (L. glaucoides) .
Sutton and Parmelee (1978) carried out an interesting expereriment on the plumage development of young gulls kept in captivity during maturation see here:
As can be seen in pic.nr. 4 the upperparts of ad. Thayer’s Gull L.thayeri compared to Dutch Herring Gull L.a.argenteus are a fraction darker grey similar to Kumlien’s Gull L.c.kumlieni, see here:
SMITH, N. G. 1966. Evolution of some arctic gulls (Larus) : an experimental study of isolating mechanisms. Omithol. Monogr. No. 4.