Siskins on a high!

As each year passes, the newly invigorated sport of visible migration (vis-mig) watching teaches us more and more about our migratory birds. In some ways it gets a little bit more predictable. Thanks to the wonderful Trektellen migration website, it's possible to produce up-to-the-minute graphs and maps showing the timing and extent of the movements of our visible migrants through the country. So we know that late August and early September will produce Grey and White Wagtails, Swallows and martins, mid-September is dominated by surges of Meadow Pipits and Pink-footed Geese passing through Scotland and northern England, and Linnets and Pied Wagtails flood through in late September and early October. October always has the greatest variety, with the main cast of Redwings early on, Fieldfares later, finally followed by Wood Pigeons in November. But each year is also different, and there's always an air of expectation at the start of each new autumn. The main species described above generally follow a similar pattern each year, though occasionally weather conditions conspire to produce breath-taking numbers of birds such as the big Redwing invasion on 13th October 2009. However it is often the supporting cast that provides the variety and the surprises. Last year the excitement came from Lapland Buntings early on, Waxwings a little later, and finished with an extraordinary movement of 419,125 Wood Pigeons on 7th November. What will 2011 bring? As with last year's Lapland Bunting invasion, the signs were there in August of what was to come.

A spring vis-mig Siskin, Kinderdijk, Netherlands, 11th March 2011 (Adri Clement).

This autumn has already been exceptional for the number of Siskins on the move in Britain. 'Vis-miggers' from Scotland to Yorkshire have smashed their previous best records and, so far, the new British record goes to...Carnoustie (Angus): my site! On 7th September an incredible 2,129 Siskins passed southwest along the coast there — more than twice as many in one morning as I'd previously seen at this site in total over the previous six autumns! The same pulse of birds was evident throughout the northern half of eastern Britain, with 503 birds past Inverkeithing (Fife), c.400 past Hunmanby (N Yorks), a superb count of 1,041 over Barmston (E Yorks) by Andy Hanby and co., and finally 620 over Andy Roadhouse and co. at the Narrows, Spurn (E Yorks). The grand total of birds on the day was 4,372 at only nine sites, the largest day total for Britain since the Trektellen visible migration database first became widely used.

Siskins filling the Carnoustie skies, 7th September 2011 (Clive R McKay).

The first indication of something afoot came from Carnoustie on 30th August. I have started to visit this site in late August (earlier than normal for most vis-mig sites) ever since I saw unusually large numbers of White Wagtails passing through in 2007. Other highlights on warm and sunny August days have included Osprey, Honey Buzzard, good numbers of Grey Wagtails and Tree Sparrows and, in 2009, an unusually early passage of 3,114 Meadow Pipits — much earlier than the classic mid- to late-September movement. It seems unlikely that these early Meadow Pipits can be of British origin, and this has led to a ringing study to try and identify their origins. Please look out for colour-ringed Meadow Pipits this autumn as you scan through them looking for rarities!

This year's 30th August surprise was provided by 198 Siskins flying southwest. This may not sound like much but it represented the second highest Siskin count for the site in six years. More important still was its unusual timing: more than six weeks before the usual autumn peak; the previous highest count at Carnoustie was as late in the year as 9th November 2005, when 217 birds passed southwest. Carnoustie is a fair distance from suitable Siskin breeding habitat, so this year's birds in August couldn't have simply been slightly off course random wanderers. Indeed random out-of-season flocks are quite unusual at vis-mig sites, and you soon get to know your locals from your movers. Moreover, the birds had passed in several small flocks typically comprising 15–20 birds, suggesting a concerted passage. Nevertheless, at the time it was merely a good count for the time of year.

Siskin flypast, Carnoustie, 7th September 2011 (Clive R McKay).

However, news from Keith Clarkson of 230 south at Hunmanby Gap in North Yorkshire on 3rd September suggested that this was not a one-off local event. A clearing weather system on Sunday 4th September had me back down at Carnoustie, even predicting the possibility of a Siskin movement to fellow vis-miggers Ali Shuttleworth and Chris Smout in Fife on the southeast Scotland coastal flight-path. Unlike most of my predictions, this one came true, with an impressive 843 Siskins southwest at Carnoustie, 691 of these in one hour between 07:15 and 08:15 alone. This was a truly exceptional movement — three times higher than my previous site best, and a month earlier than usual. What was going on? Whatever it was, it was happening on a wide front, with 1,457 migrants passing over 11 sites across the country as far south as Spurn on that day. When well-watched sites like Spurn start chipping in with notably early and large counts, you know that this phenomenon is real, as they have years of past counts for comparison. Up to and including 11th September, Spurn has had 2,006 Siskins "over", almost twice as many as the 1,238 seen in total over the previous five Septembers!

When another migration window appeared on 7th September, it was all hands to the pumps again, with expectations rising. Rising expectations are a dangerous thing in the field of vis-migging: so often a great day is followed by a disappointing one. There are good reasons for this. During periods of less-than-ideal weather for migration, diurnal migrants spend their time feeding, and build up modest fat reserves in readiness for a migration flight when the weather improves. This often occurs following the passing of a cold front. With rising pressure, gentle winds and clearing skies, the migration springs into life. But the fat reserves are quickly used up in a day or two, so the migration comes to a halt just as quickly, despite the weather conditions seeming identical to those of the day before (see Newton, 2008. Migration Ecology of Birds pp. 90–91). The rule of thumb for a good day is to get out on the first day of the weather improvement. And 7th September appeared to have the right credentials. Would it disappoint? No, quite the opposite — it was the highlight of my birding year so far, with flock after flock of Siskins passing Carnoustie throughout the morning. They're great vis-mig birds. Identification is usually straightforward as the Coal Tit-like calls draw attention to the flocks as they buzz past. They fly in fairly tight, fast flocks, very sprightly and bouncy, almost like a flock of manic Blue Tits, and they usually appear quite pale below. Their body shape is also distinctive, being very short tailed and with the head looking quite pointed due to the spiky bill. Flock sizes are great for counting. usually about 20–30 birds, occasionally as many as 60, and there's usually time to write them down in the notebook before the next flock appears on the horizon.

Head on attack: Siskins at Carnoustie, 7th September 2011 (Clive R McKay).

On a good day you can clock up a few hundred birds without even knowing it and it's always a surprise when you add up your notebook totals at the end of the morning; and so it proved on the 7th when Ali Shuttleworth texted me to see how I was getting on. He'd had his best Siskin morning at Inverkeithing with 500+ birds before he had to head off to work, and I replied that I thought I'd had a similar number. During a brief lull in activity I quickly totted up the figures in the notebook to find I'd had 800+ birds in the first hour and 600+ in the second! Crikey. I texted Ali again with the corrected estimate of 1400+ birds and, now back at work, he was able to look at the Trektellen website and replied that the British record count was 1,115 — a spring count by Gary Hibberd and Robert Sturgess at Hunstanton Cliffs in north Norfolk. I'd beaten it in only two hours, and the birds were still piling through! Thanks to Ali's communications I was as happy as a sand lark, and probably glowing on the spot. There's something strangely monumental about a large movement like this, even if it is of a common bird. The constant procession of birds passing overhead keeps you completely focussed, and the knowledge that you may never see an event like this again provides what I consider to be the ultimate in birding experiences. And there's also the knowledge that what you're seeing is probably telling you something about a whole population of birds, rather than just a few off-course, wind-blown individuals. A similar pattern was repeated down the eastern side of the country, as described above, with 4,700+ birds seen during the day — not bad a for a weekday, when fewer sites are covered.

As ever, the next day proved to be "the day after the Lord Mayor's parade", as Keith Clarkson calls it, and numbers were generally lower with "only" 800 past Carnoustie (my second highest count here!); the graph of numbers across the country has gradually tailed off since. So we've had a large early migration, but there's every chance that this will be followed by another large passage at the more usual time of year.

Total number of Siskins counted on visible migration, Great Britain, 1st August–15th September 2011 (Trektellen).

Should we have known?

Looking back at Siskin records earlier in the year, it is interesting to note that there were records at Spurn as early as 8th July. Only one or two birds were seen there on five dates in that month, but these birds must have come from somewhere fairly distant as Spurn isn't close to Siskin breeding areas. You have to be a pretty determined little Siskin to get to a far-flung outpost like Spurn in July! The only other year since 2006 with Siskins at Spurn in July was 2008, which before this year was the only other year with a significant early-September movement. So maybe in future we will be more aware of these early signs. In 2008, the early-September movement carried on through to the third week, followed by a lull and then a resurgence during the normal peak period in October and early November. If this pattern is repeated in 2011, then there are still a lot more Siskins to come.

Total number of Siskins recorded on visible migration, Great Britain (blue bars) 2011 vs. total in all previous years (red bars), based on counts submitted to the Trektellen database, mostly 2006–2010. (Trektellen).

What could be behind this explosion of birds? Clearly there are lots of birds involved, suggesting that Siskins have had a good breeding season — in Scotland at least. A possible hint at this comes from the field observations of Mick Marquiss, who has studied Crossbills and other forest birds in the plantations and pine woods of northeast Scotland for many years. This year he noticed that the usual fall of Spruce seeds (the main food of breeding Siskins) had been delayed by the wet weather. This is good news for Siskins, as it means that their food source remains available in the cones on the trees for longer. In a normal bright and sunny year, the cones open and the seeds are shed en masse over a short period of time, marking the end of the Siskin breeding season. So perhaps this summer's cloudy skies had a silver lining for the Siskins. But why should the birds also be migrating so early in such large numbers? We don't know the answer to this one. It's possible that the large population size has simply initiated an early irruption of birds, as has been shown for Crossbills (which often irrupt before their food source has dwindled — see Newton, 2008. The Migration Ecology of Birds pp. 546–555). On the other hand, such a large-scale movement suggests to me that other factors may be involved. The usual migration season for Siskins is mid-October to early November, soon after the normal seed-shedding period of birch, which is one of their main foods from July to October (Newton, 1972. Finches p. 42). One possibility, then, is that this autumn's crop of birch seeds has been shed unusually early, forcing our Siskins southwards in search of new birch woodlands (and niger feeders!). Watch out for an earlier-than-usual influx into your gardens this autumn, and please report any sightings on BirdTrack. If the birch story holds any weight, we should also expect larger-than-usual numbers of Lesser Redpolls on the move too, as birch is also their number one autumn food source.

A bird in the hand: a fine adult male Siskin, Thetford (Norfolk), 28th February 2009 (Jeff Kew).

Down in deepest Norfolk, Jeff and Allison Kew and Jo Lashwood spend a lot of time ringing the Siskins of Thetford Forest, many of which come to feed in their garden. Jeff is very interested in Siskin migration, and has formed a Siskin Yahoo group for fellow Siskin fans. Every Saturday through the summer the Thetford team have ringed as many Siskins as they can, but as the summer progressed, the proportion of retrapped birds (those that they had already ringed) increased, suggesting that no new birds were coming in to the feeders. Jeff had been tracking the recent migration through the country on Trektellen, and wondering whether it would reach Norfolk. And sure enough, the pattern of the catch on 10th September suddenly changed. Three times as many "new" birds were caught as compared to retraps, and several birds were very light (below 11 g) indicating fresh arrivals following migration — the northerners had arrived! Interestingly all the new birds ringed that day were juveniles, perhaps suggesting that the bulk of the migration to date has been made up of young birds.

Juvenile Siskins on the move: Falsterbo, Sweden, 5th October 2010 (Jeff Kew).