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. (Strict biological definition? 'capable of but not restricted to a particular function or mode of life'.)

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 at 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 decision is 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 make 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. Their annual obligatory migration routes between core breeding areas and overwintering areas are long-established. Within their wintering range, there can be movements during that non-breeding part of the year. Radar studies have shown movements can happen almost daily throughout a winter, in variable numbers. There is a continual readjustment, a refinement, as birds make choices.

These are movements, 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 themselves hard weather- 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 - mainly deep snow cover, freezing temperatures, chilling winds - puts pressure on a number of the birds here. At an individual level.

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 called like an alarming Redshank. The next clump over answered, then the next, and then the next.. Yesterday, one flighting Redshank crashed into a clump and put out twelve. They all moved to another nearby clump where, finding no room at the inn, they gathered on the sheltered side and huddled.

Take food. Those Redshanks, 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 of late. 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 individual 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 of snow cover, the ducks might move after a couple, but never usually all of them so long as some feeding remains available. Weighing up when to go is a balancing act.

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Back to those Redshank. Like quite a few wader species, once they have chosen an estuary in their first winter, they then stick with their choice in future winters. Makes sense. They know it well. For most of the winter, they will not carry big food reserves. Carrying additional fuel outside of migratory periods means slower day-to-day flighting from ever-present predators. Much better to be a mean, lean, flighting machine and hope any poor feeding period only lasts a short while, because they won't have the reserves to make a big distance movement without having an impact on their survival odds. If the extreme weather lasts is long lasting, 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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Right now in the belly of 'the Beast from the East', 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 cover. They escape early.  Haven't been in huge flocks here, birds are making the decision individually; twos, threes overflying aren't that exciting (but add up the local day total and the movement is clear).

The field-feeding thrushes, Fieldfare and Redwing, are escaping in a similar manner. Numbers break off from one large-scale gathering and wander. Take how garden birders might report 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, many are trying to avoid costly flight, but some 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 flying by. 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), not really a full-on escape movement between estuary complexes (we're all the Greater Thames around here) but others may chance finding an unfrozen estuary (remember, they may not have visited sites south of us before) by fleeing overland.

The county avifaunas often refer to the largest movements in the worst winters, and we birders sometimes head out hoping to see hundreds and thousands of bird racing past us, to experience the escape 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 viz mig during an obligate migration period.

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

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Escape movements. Yes, they correlate to the weather, and some may move early on sensing weather systems, but for many it comes down to food. Passerines may go early in comparison to larger non-passerines. Can't go as long being cut off from feeding. Juveniles may go early, being routinely pushed down pecking orders (eg think Woodpigeons).

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. Not simply a happy switch between sites, often this is escaping. 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 because it's what these bird species do, but fail to appreciate these individual birds are appearing because they're in trouble..

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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. Ohhh, time enough for a second breakfast, lucky ol' me (!)


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