The Joys of Visible Migration! Part 2

Pink-feet and Pied Wags (the blurred blobs in the foreground!), Carnoustie coast, September 2009 (Photo: Clive McKay)

It's been a busy month, and as September hands over to October, autumn begins to feel as if it's really here. The leaves are turning yellow, and the summer's abundance of insects is beginning to wane. Meadow Pipits, the September visible-migration talisman, no longer dominate the skies above VisMig watchpoints across the country. Most of the Swallows and martins have already left, soon to be followed by Mipits, Pied and Grey Wagtails — all sneaking southwards, and some leaving the country — in their thousands! Linnets, Goldfinches, Tree Sparrows, Siskins and Crossbills are restlessly passing over from wood to wood in search of winter quarters.

But luckily, as the insect-eaters leave for warmer climes, the grazers and the berry-pickers start arriving for the winter. Pink-feet and early Whoopers are forging southwards from Iceland, and the first rushes of winter thrushes have hit the country. October is the month we've all been waiting for, but more of that next time.

Late September VisMig diary

September was dominated by high pressure, great for the birds but not for the vismiggers. Watchers in England waited for the big Mipit rush, but it never came. Without any blocking weather systems the birds trickled away day by day on a broad front, without any major concentrations at the usual spots. So it goes! But towards the month end, the alba wags really got going, and the BigVis weekend gave a national picture of what's on the move on a very average couple of days.

26th & 27th September, the BigVis weekend, Carnoustie coast, Angus

The first national BigVis co-ordinated count on the weekend of 26th/27th September logged over 44,200 birds on the Saturday, and 28,400 on the Sunday (when quite a few sites in the uplands were fogged off). Not bad for a quiet migration weekend suffering from the high pressure blues.

Knocked off the top spot by House Martins — one of the 11,367 Mipits logged passing Carnoustie so far this autumn. I wonder where this beauty is now — Dorset, Biscay, Iberia? (Photo: Clive McKay)

The top three most numerous species over the weekend were 22,497 House Martins (the majority at Christchurch Harbour - see www.chog.org.uk), 17,559 Meadow Pipits and 8,509 Pink-footed Geese. Alba wagtails were surprisingly numerous, with 2,513 birds — quite impressive for a species that isn't the first to spring to mind when you think of British migrants.

Total number of visible migrants recorded at VisMig sites in Britain 26–27 September 2009. If you do VisMig counts and you're not on the map — please get in touch via the Trektellen website. (Map: www.trektellen.org)

I visited the Carnoustie coast on both days, and my highlights were 84 Little Gulls coming south over the land (!), 4,270 Pink-feet, my first 5 Whoopers of the autumn and over 650 alba Wagtails (probably mostly Pieds). Carnoustie lies at the southern end of a long leading line of coast which stretches over 70km from Fraserburgh in the north, to Dundee in the south. Crucially, the coast line here runs in a SSW direction, meaning that there is always land — and hence a source of birds — to the north! Any southbound bird from this segment of northeast Scotland is likely to "bump up" against this coastline, and since the line of the coast isn't too different from southerly, then the birds might as well follow it. So no need for westerly winds to push birds onto the coast — although it does help.

The Pied Wags over the BigVis weekend were a good example of VisMig in action. In the first hour after dawn, the wagtails chizzicked through at a great rate — over 300 per hour, with none stopping off. But as the morning progressed the urgency waned, and small parties would drop in to feed for a few minutes before moving on. Had I only looked at birds "on the deck" I would never have seen more than half a dozen at a time, but looking skywards showed what a massive number were actually passing through.

Post-dawn flocks of Pied Wags pass south at Carnoustie — can you see them? (Photo: Clive McKay)

A bit closer (Photo: Clive McKay)

Pied Wagtails, near Carnoustie 26th September 2009. Taking a time-out before heading on — to the lovely short grass greens of Carnoustie championship golf course perhaps? (Photo: Clive McKay)

If we look at the migration pattern of alba wagtails over the last two months, there is a strong suggestion that birds are following a SSW route through the country, with east-coast sites such as Spurn missing out on the bulk of the birds.

Average hourly passage rate of alba wagtails at VisMig sites, 15 Aug–15 Oct 2009. (Map: www.trektellen.org)

Interestingly, this pattern is strongly mirrored in the recovery and sightings pattern of alba wagtails ringed at Slapton Ley NNR over the past 7 years by Dennis Elphick and others. Please check out your wagtails for colour rings! If you see one, report it to dennis.elphick@tiscali.co.uk or via the BTO website at www.bto.org.

Distribution of alba wagtails ringed or controlled at Slapton Ley NNT, Devon. (Map: Dennis Elphick & Devon Biodiversity Records Centre)

30th September, Glen Isla, Angus — Pinks and Whoopers

A day at home with me and the kids all under the weather with a flu-like virus didn't augur well for VisMig on 30th. But a quick scan north towards the Cairngorms at 7:40 revealed a flock of 70 Pinks coming over south from the mountains through rather murky clouds. Pinks coming from the local roost at Lintrathen reservoir would never use this flight-line, and they usually pass over in very large flocks within 30 minutes of dawn. These birds were likely to be true migrants, and this proved to be the case as flocks continued to trickle over southeast throughout the morning, eventually totalling 990 birds. This is exactly the opposite direction to that taken by (perhaps the same?) birds heading northwest for Iceland in the spring. There can be little doubt that the Pink-footed Google map stored in their heads pre-dates the web version by a good few thousand years!

At the end of the morning the number of moving Pinks started to tail off, but a wavering white line appearing out of the misty clouds revealed an early autumn flock of 28 Whooper Swans. Perhaps they had set off from the same location as the Pinks? A second larger flock later on brought the total to 63 birds. You just can't beat Whooper Swans on the move. So big, so white, and those enigmatic bugling calls — Iceland all the way. Both flocks were comprised entirely of adults — family groups tend to arrive a little later. A great morning, although no sign of the Sandhill Crane that was also on the move that day!

What joy! 28 Whoopers >SSE, Glenisla 30th September, 2009, part of a total of 63 that morning. Falsterbo has only EVER had 5 Whooper day counts greater than 63 birds, whilst migration watchers at Gibraltar and the Bosphorus will NEVER see a sight like this! OK, they get lots of other things, but VisMig is all about celebrating and savouring what you've got on your side of the fence! Follow satellite-tagged migrating Whoopers at the Wildfowl and Wetlands Trust website. (Photo: Clive McKay)

The same flock of 28 Whoopers disappearing >SSE into low cloud As they did so they started bugling loudly — presumably to keep in touch with each other. Next stop Aberlady Bay? (Photo: Clive McKay)

Thanks to the satellite tagging studies of the Wildfowl and Wetlands Trust we can follow the routes taken by individual Whoopers. Baldur (ring U5B) is one of only 2 out of 42 tagged birds to have left Iceland by 15th Oct this year. He probably left southeast Iceland on the evening of 6th October, and had reached the Cromarty Firth by 11:00 am on 8th October — having spent at least one night at sea. He rested for a few days before moving on to Montrose Basin some time on 10th/11th Oct, and finally arrived in Northumberland (Budle Bay?) on 12th October, where he is still present. His flight-line (in grey) passes not far from that of the birds seen over Glenisla on 30th September (indicated by the red arrow). (Photo: Wildfowl and Wetlands Trust)

Back to basics — VisMig principles

The Snooker table

When I'm trying to visualise visible migration on a nationwide scale, I imagine the country as a huge snooker table shaped like the British Isles. The balls are scattered evenly across the table — and these represent the early autumn flocks of pipits, larks, wagtails and finches getting ready to migrate. Of course there would be millions of these, so let's not limit ourselves to 21 snooker balls — maybe a few thousand marbles scattered across the baize of our "green and pleasant land" gives a better impression. The cushions of the table are our coastlines, barriers that the marbles are reluctant to cross. Now put some obstacles on the table to represent our hill ranges — the Cairngorms, the Southern Uplands, the Cumbrian and Cambrian Hills, the Pennines, Cotswolds, etc., and we're ready for the migration season to begin!

All that is required now is superhuman strength to lift up the top end of the table slowly to see the marbles start their southward migrations. The marbles will have to roll around the edges of the hill ranges, or perhaps get funnelled into north–south valleys cutting into the hills — here we can expect concentrations of migrants. Marbles will be concentrated along the east and west sides of the Pennines, with bulges along the flanks causing blockages and concentrations.

In visible migration terms, these are clearly the potentially good VisMig watchpoints. The further south down the Pennines you go, potentially the more migrants there are to be seen, and likewise across the southern hill ranges such as the Cotswolds. Eventually the migrants will end up at the bottom end of the country — on our south coast. This is essentially what has been happening over the last month, as our southbound British breeders have left Scotland, Wales and northern England, and now find themselves concentrated along the coast from Kent to Cornwall. Depending on the species, some will end up wintering in these areas, others will cross to the continent. The barrier of the south coast runs perpendicular to the direction of movement of most birds, and so is not so much a leading line, but a hurdle. Perhaps for this reason, visible migration movements on the south coast can be either east or west — depending on the wind direction. Surprisingly, the birds appear to prefer to fly INTO a headwind — moving east in an easterly wind, and west into westerly! This is a universal VisMig rule, and the subject of much debate! More about this later.

But hills don't necessarily act only as barriers. On days with strong cross winds migrants can find slower wind speeds in the lee of the hills, and can follow the contours of the land to make their journey as efficient as possible. This may explain the good numbers of migrants recorded at watchpoints along the east sides of the Pennies — where the birds can get some respite from the prevailing southwesterlies.

Forton Services on the M1 (Photo: David Jones)

Motorways, Service Stations and Migration Watchpoints

Just like humans on a long car journey, our visible migrants need to stop off and rest, refuel and roost. The ability to choose good locations for these important activities must be one of the great advantages of diurnal migration. Birds of the year are travelling through country that they have never seen before. Migrating by day enables them to spot good feeding habitats and potential roost sites, and perhaps by joining flocks to benefit from local knowledge, or simply from safety in numbers. For some species these gathering areas are discrete and easily identified, like a motorway service station, e.g. the lochs and wetlands used by Pink-footed Geese arriving from Iceland — Strathbeg, Montrose Basin, Loch Leven, West Water, Martin Mere. These are great sites for witnessing the annual autumn immigration and spring emigrations of these charismatic species. But, like counting people milling around in a motorway service station, they're not necessarily the best places for quantifying how many folk are arriving and leaving, and what directions they're travelling in. For our human travellers, the best way to do this would be to count the cars using the motorway itself — they only pass once, and you can see which direction they're going in. It wouldn't take long to get a handle on the migrations of holiday makers on a Bank Holiday weekend if you counted the number of cars and their direction of travel on the motorways leading to and from our main cities. How much more difficult the task would be if you were restricted to the service stations — especially if you couldn't see the entrances and exits! This analogy holds good for bird migration too. One of the main strengths of visible migration studies is that you're counting birds actually in the act of migrating, and those counted today are different from yesterday's or tomorrow's. With "grounded" migrants, it is much harder to know how many individual birds are involved — counts of 5,000 Pink-feet at Montrose for seven days might represent 5,000 birds sitting on the mud for a long rest, or a total turn-over of 35,000 birds! So VisMig counts have a special value in this context — you just have to find the right places on the migration motorways to log them as they fly past.

VisMig Weather

What are the best conditions for visible migration? Generally speaking, rain or fog completely stops movement (though it may continue between showers, especially over the sea); murky days are usually poor, the birds seeming to prefer crystal-clear horizons. A light cross wind or a head wind will produce more birds than a tail wind, but strong winds (greater than force 4–5) will inhibit many species — except for seabirds, for which strong winds are usually a prerequisite. The direction of the cross wind is crucially important, especially at coastal sites where the wind must come off the land towards the sea to push the birds up against the coast (or vice versa for seawatching). In broader terms, many of the best movements occur on the first day or two of a sudden improvement of weather — usually associated with a sudden rise in atmospheric pressure. But a few days into a high pressure system the air goes "flabby" and movement tails off.

Prevailing weather will affect the drive of the birds to migrate — and hence their ability and/or will to cross the barriers. Observers on the south coast in the autumn don't spend much time looking skywards in a light northerly wind, as this is the perfect springboard for birds to launch off onto the continent, so they will probably be high and out of sight. More over, perfect conditions mean there is no need to follow the leading lines that produce concentrations of migrants. However, it's easy to imagine how prevailing winds will tend to push the stream of migrants one way or another. For example our prevailing westerly or southwesterly winds generally push migrants up against the east coast, producing the best VisMig days at east-coast sites such as Spurn and Carnoustie. But even a light easterly will push southbound birds inland — perhaps only by a few kilometres, but enough to take away the concentrating effect of the coastline, and to produce a poor day's vismigging.

It has been interesting over the years to follow the tracks of Roy Dennis's satellite-tagged Ospreys and Honey Buzzards (see www.roydennis.org) as they have journeyed from Scotland through Britain to Europe and Africa. Some birds move down the west side of the country (or even out to sea), whilst others follow the backbone due south. On such long flights it's easy to imagine how the prevailing wind could alter their course by a few degrees, which could mean the difference between Cornwall and Kent by the time they reach the south coast.

Mostly the birds are free to sit out bad weather. But an extended stormy period will cause a massive backlog of birds waiting to "go", and may force them to move in less than favourable conditions thereafter. It is these backlogs that vismiggers are always keeping a weather-eye on, as the first day of decent weather will surely release a rush of birds. And of course, ideally these days should be at a weekend! So an Indian summer of high pressure such as this September's is the last thing we want — there are no backlogs, no contrary winds and the birds are free to choose when and where they move — crossing coastlines and hill ranges at will.

October — Beds set on fire by Viking invaders

But the big month is here, and the pent-up energies of some Scandinavian Redwings has already been released — with almost 33,000 passing over Stephen Blain, Mark Thomas and others positioned on The Pinnacles watchpoint near Sandy on the 13th. What a day! A great headline (thanks Mark — and for pictures from the day see Mark Thomas's blogspot —  bucktonbirder.blogspot.com). And a great example of how migration spectaculars can be seen almost anywhere in the country if you choose the right place and the right time. More on this next time.

Getting involved

This weekend (17th–18th October is the second BigVis weekend of the year, and we're hoping for more favourable weather than we had on the September weekend. There are lots of winter thrushes about, Woodlarks have been seen passing over several sites, and there are lots of Crossbills on the move. So why not log your site on Trekellen, and find out more at the UK VisMig Yahoo group?

Next time

Choosing the right spot, Viking invaders, and tilting the snooker table to the west.

The Viking berry-pickers are on their way — 88 Redwings (Photo: Clive McKay)