Friday, 2 March 2018

Get the facultative movement outta here

“It is well for us to recollect that even in our own law-abiding, not to say virtuous cases, the only barrier between us and anarchy is the last nine meals we've had”
A.H.Lewis, 1896

The arrival of the Beast


Obligate movements. Obliged to do it; inbuilt. The urge, for some, to migrate, at the correct season, at the correct time.

Facultative movements. There is a choice. Stay or flee.



As I plot this blog out, it's 06:00 and the Queenborough weather site is showing minus one, with a minus ten windchill. North-easterly wind, averaging 35 m.p.h., gusting to 46 m.p.h. The 'Beast from the East' continues to hit. Do I go out tat dawn, or stay in? My choice. I wanna bird, but I also want to stay warm. If this blog gets posted before nine o'clock, you'll know what I chose. That's based more on the weather, because I've got a kitchen full of food to keep me going and I've already had my porridge.

But we're all only nine meals from anarchy. Birds? Some are nine meals away. Some, just a couple. Individuals of different species take decisions to move at slightly different times.



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Someone told me the Lapwings are migrating away at the moment. Well, strictly, no, they're not. The annual obligatory migration route is established. Within that range, there can be movements during the non-breeding part of the year. Radar studies have shown movements can happen almost daily throughout a winter, in varying numbers.There is a continual readjustment, a refinement, as birds make choices.

It is a movement, not a migration.

In the past I've heard it said this is a cold weather movement. That would be nice if it was, because temperature is measurable, and we could soon say what isotherm triggers the movement, just as we have an understanding of how temperature correlates with migration, but cold isn't the main drive.

All-too often you hear hard weather movement. Nice term, but what does it mean? Define hard weather. I've yet to hear someone call a drought 'hard weather', but in that circumstance, waterbirds often make a choice to move, often northwards. Our influxes of, say, Glossy Ibises, have yet to be called hard weather movements. Or autumn pressure systems bringing 'Yanks' to Europe being hard weather- but if that's not hard weather, I don't know what is. You could say 'weather-related' I guess, but many scientific texts avoid the phrase.

They tend to call it an escape movement.



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The hard weather we're having on the Medway at the moment is mainly deep snow cover, freezing temperatures, chilling winds, puts pressure on a number of the birds here.

Take sheltering. There is a limited amount of decent shelter from the chilling winds.Watch a clump of cord grass and you might not immediately realise how many birds are in there; scanning a birdless edge of saltings a couple of days ago, a cord grass clump callled like an alarming Redshank. The next clump over answered, and the next, then the next.. Yesterday one flighting Redshank crashed into a clump and put out twelve. They moved to a nearby clump where, finding no room at the inn, they gathered on the sheltered side and huddled.

Take food. Their preferred meals haven't migrated, but they're moving. Downwards. the colder the mud surface, the deeper the critters burrow. And the surface has been cold. Two nights ago, snow arrived during the low tide cycle. Usually snow on mud melts, but it was cold enough for the snow to form a thin sheet of ice over much of the estuary. The birds could not get at their food. Some could. Often the edges of a creek or tideway do not ice up; gravity plays a part in keeping water flowing, so the mud remains, well, muddy,and there is some feeding available. Squabbles ensue. Some birds chose to move. They chose to try to escape starvation.


Yesterday, the only birds sitting out on the more open flats were the Brent Geese and the Shelduck. The bigger waterbirds. They can play hunger games for a longer period than their smaller cousins, the ducks. These geese may not move for three or four days, the ducks might move after a couple, but never usually all of them while some feeding remains available. Weighing up when to go is a balancing act.



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Take Redshank. Like quite a few wader species, once they have chosen an estuary in their first winter, stick with it in future winters. They know it well. For most of the winter, they will not carry big food reserves. Carrying additional fuel means slower day-to-day flighting from ever-present predators. Better to be lean. And hope any poor feeding only lasts a short while, because they won't have the reserves to move a big distance without having an impact on their survival odds. If the extreme weather lasts too long, many will die. They run the risk. The strongest, the fittest, those that can hang on to a feeding spot, survive.

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Birders are reporting the 'usual suspects'.

Plovers on the move. Plovers rely more on eyesight to locate prey. The species that feed more in fields than mud flats, Lapwing and Golden Plover, have real problems with snow. They escape early.  Haven't been in huge flocks, birds make the decision individually; twos, threes overflying aren't that exciting but add up the day total and the movement is seen.

Similarly, the field-feeding thrushes, Fieldfare and Redwing, are escaping in a similar manner. Numbers break off from one gathering and wander. Garden birders are reporting one or two in the back yard one morning becoming a half-dozen later in the day, then a dozen, then two.. Snow cover in south-east England is patchy, more are trying to avoid fleeing, but birds in the snow need to move. The total involved is fluid.

Transfer that thought process to small calidrid waders and you begin to understand why inland patch workers are getting one or two Dunlin or an odd Knot. Some are choosing to make a short switch to another estuary (perhaps the Thames and Swale for Medway birds; our sister estuaries are that little bit different, with fewer sheltered waters that are freezing out from the shore), others chancing finding a new estuary by fleeing inland.

The county avifaunas often refer to the largest movements in the worst winters, and we birders sometimes hope to see hundreds and thousands of birders racing past us, to experience the movement. More likely we'll only pick up on a few dozen, or perhaps break three figures for a couple of species. Escape movements are nowhere near as predictable as obligate migration.



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Some are waiting to see wildfowl on the move. Probably won't happen, because most will still choose to move at night. We'll see changes in day-to-day local totals. But these might be very short movements, flights to nearest open areas (these birds are now starting to respond to their obligate drives and wanting to move toward breeding areas) rather than huge jumps. Looking at a satellite image for yesterday, it was clear that most of the low countries were free of snow. We've only got so much thanks to the North Sea providing moisture. Is there any reason for any duck there to escape? It might be bloomin' cold, but if they can feed..



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Escape movements. Yes, they correlate to the weather, but they come down to food. We humans are nine meals from anarchy. Some birds are one or two from death. Calling it a hard weather movement helps hide the life-or-death decisions being taken. At the moment, all too often we birders will get excited by the rare garden bird, the patch tick, the hard weather movement day total and fail to appreciate they've appeared because they're having troubles..

Hang on, it's 07:30 already. Time to stop. Think I'll leave going out for an hour or so, tide's not until 10:00. Ooo, I can have a second breakfast, lucky ol' me (!)

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