Sunday, 28 July 2013

Visible migration


Although autumn migration has been going on for some weeks now, today was noteworthy inasmuch that quite a few species were visually easy to see on migration.  Gulls were drifting south over the county all day, in the main Lesser Black-backed and Black-headed, with smaller numbers of other species.  Other birds on the move today included Swift, hirundines with Sand Martins in particular, and small numbers of wagtails.  At Pitsford Res this morning, birds south of the causeway included the Bar-headed Goose, a Little Egret and a Dunlin.  The first returning adult Great Black-backed Gull was by the dam this evening, and there were two Grey Wagtails at Hanging Houghton.

Many adult birds, particularly the larger birds, will now be in the process or just starting to moult their flight feathers.  Many of the gulls overhead are clearly moulting primaries and secondaries, the stark gaps in the trailing edges of the wings often denote this.  This process often leaves ducks and geese temporarily flightless as they often tend to drop their major flight feathers all in one go. However, most birds sequentially moult these larger feathers so that they can continue flying and even migrate whilst undergoing the process, and frequently both wings mirror each other to stay on an even keel! Just a few birds actually undergo primary and secondary wing moult whilst still feeding young (Sparrowhawk is probably the best local example).  Many juvenile birds at this time of the year undergo a body moult, where they begin to replace their first 'nest' feathers with more substantial attire.  In the main though, most juveniles will retain their flight feathers.  It is these processes that sometimes helps us to age the birds we are looking at.


Eleanor & Neil

Lesser Black-backed Gull
Pitsford Reservoir

If I hide behind this board
at Pitsford you won't be able
to see me!

Greylag Goose in flight
Pitsford Reservoir

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