Friday, 15 September 2017

Horrid autumn- 15/9/17

6:30-8:55
Viz mig: Teal 3, Buzzard, Kestrel, Sandwich Tern 3, Common Tern 2, Swallow 18, House Martin 2, Sand Martin 2, Meadow Pipit 31, Kingfisher 2, Goldcrest 1, Pied Wagtail 2, Grey Wagtail 2, Linnet 2, Chaffinch 8, Goldfinch 27
Point: just 2 Chiffchaff

It takes a while for the flick to switch. The nocturnal summer migrants are beginning to disappear off the radar. Sure, you go coastal and the feel from talking with other birders is that they're still 'here'; but numbers are low, these are now late stragglers. A big trigger for migratory onset is daylength, and not just the flight proper. Second, third broods often feel the need to carry out their post-juvenile moult quickly, taking sometimes just about half the time of a first brood. So whereas you might think they could leave eight weeks or thereabouts after their kinfolk from another egg-yolk, they might only be two or three weeks behind them leaving. Of course, this has costs; weaker feathers and the suchlike. Better to be brood one stock. Later broods are insurance policies.

Same works for adults- those that have a full post-breeding moult before migrating might shorten their own moult periods. For a great many species, peak migration periods are short, perhaps just a few days. That may seem hard to digest, but we can actually see several peaks within the same species passing through over a few weeks as various populations do their thing; northerlies set off earlier than southerlies, but might actually go slower so their passages run into each other a little. Greenland Wheatears set off much earlier than our southern softies, but usually pass just after their main passage here in the south-east.

Look closely enough at these sorts of graphs and you'll find if birds arrive earlier, they tend to leave a little earlier, similarly later if late. The control that stops the whole clock going askew is the kick-off to the spring migration down in the wintering grounds. There, they have no idea of conditions oop narf, they just know when to start off from the subtlest changes in photoperiods a short way off the equator.

All why I smile wryly when birders moan about sites that underperform to expectations in autumn. They've had some good days in past years that tend to stick in the memory, but speak to any old 'un who slogged that site, there were plenty of quiet days back then when 'nothing' happened. Expectation over reality. We're spoilt by the internet and easy access to honeypots.

Enjoy the ride. The mainly long-distance nocturnals may be slowing down and there might be an obvious gap now until those shorter-distance diurnals show, but there's still more than enough to entertain. There should be a push of long-distance diurnals any day now; where's them Swallows?

House Sparrow: doing well just along the shoreline,
three figure counts possible in the Park. Will they
wander out the Hill? Will they heck! A single figure total
for the whole of the autumn to date.

Return of the wildfowlers

Grey Plovers, flighting past the Point.

A part of the Redshank roost, Rainham Saltings.

No comments:

Post a Comment