Wednesday, 27 December 2017

When the red, red Robin comes mig, mig, migging along.. alo-oong

All those county bird reports us foot soldiers like to read. They have a nice, short statement on each species that you tend to take as Gospel, something akin to this for the humble Robin in most reports:

"Common and widespread resident, passage migrant and winter visitor."

And I used to believe that. Then along come the textbooks and the statement gets repeated:

"..large numbers of the nominate race from northeastern Europe occur here on migration and in winter.." ('Birds in England', Brown and Grice)

Trouble is, when you start to read more analytical texts, evidence for winter visitor in any number becomes a touch wobbly.

"Ring-recoveries support the idea that a few (nominate race) rubecula winter in Britain and Ireland.."(Migration Atlas, Wernham et al)

Of course, I always believed the 'big numbers overwinter'. I used to go do talks on garden birds, on migration, for a past employer, to local WIs and the like, and spend my time spinning charming little tales that 'their' Robin they see in the back garden each winter probably had a Polish accent. Argh! Now believe this to be well off the mark.

I'll try to explain, and as I keep saying, I'm no scientist, but.. Open minds everybody.

We'll stick to Kent. The BTO's wonderful website has a cumulative ringing recovery report for every county, well worth a visit. Kent is oft visited by me, and today I (statisticians close your eyes, this will hurt) just grabbed a sample of their sample of encounters with known foreign Robins here in the county. The list is a bit of a pain, containing as it does birds encountered in other countries less than five times in each direction, but you can see trends in both those ringed abroad and those ringed here and encountered overseas:

A lot of autumn movement there.

Now, lets overlay (well, underlay) the main migration periods, based on the excellent Migration Atlas and 'Birds of the Western Palearctic:

Suddenly the spring records show up as being after the peak passage period through the UK. (Question, is the peak through southern Britain a fraction earlier? Probably.) Just a handful can be felt to be at their overwintering areas (and I had to cheat to get some from the continental wintering grounds on the screengrab in the first pic, by leaving out a few from my unscientific selection of records).

The vast majority of the passage Robins passing through the south-east of England are not heading for Blighty, they're making for Iberia and the like.

Another small consideration. The concept that not all coastal Robins are continentals. Altering the chart further, by adding in moult finishing periods. Adult Robins, depending on when breeding finishes, up to end September (dark blue), their late broods finishing as late as mid September (light blue). Most longer distance dispersal of 'resident' UK Robins happens at this end of the moult period. Adults have territories to defend, first, second broods have had to find somewhere to defend and now the third broods, and the (small!) number of overwintering continentals now add to the disputes. Most Robins might not go far, less than five kilometres for a winter territory, but a number now wander a little further afield. Why if a ringer looks back through their records, they might find a lot of local county Robins were ringed in October.

We want these Robins we see in the migration period to be migrants. Our mindset is skewed towards it. We go to Horrid Hill and find a few extra and think 'Polish'. More likely got a Medway Towns accent; our impoverished gardens can only hold so many, and these youngsters have been kicked out. Ah well, a man can but dream.. But those Robins on Horrid have mainly come from points north of Twydall.

In that same BTO report is a tally of Robin encounters, based on UK (counties) and overseas (by country):

Now, for all those s-e birders who get excited by falls at the likes of Spurn, and say 'why don't we get the same here?', I'll point out, once again, Spurn might as well be another country, we're so different. We encounter very few Robins from other counties in a line from the Wash to the Bristol Channel (just three from east Yorkshire). We don't register as being on that North Sea route. Migrants arriving north of the Wash have little reason to filter down via Kent. They can go due south or south-southwest as they leave. Those that reorient on a night flight might well end up back on the continent than Blighty. Those Scandinavians we've had (see left hand column) will have, statistically speaking, more probably have used the main route through southern Scandinavia, Denmark, Low Countries. Passage migrants through Kent are often using a different flyway. (Trust me, just checked that BTO site, in comparison E Yorks has Robin just one exchange with Poland, none with Russia, none with the Baltic States.) The ring might say 'Norway' but you can't claim a direct flight.

Why do some counties report ringing encounters as 'origin' and destination'? They're not. Unless ringing as pulli in nest, you can't claim an origin. And we're seeing you can't claim UK as a destination for vast majority of foreign ringed birds encountered here. These are the sort of well-intended errors that lead to such urban myths in the first place.

All why I'm hoping to live long enough to see the European Migration Atlas published. Bring it on.

We get a number of Robins coming in from the edge of the Western Palearctic to overwinter. Really not enough encounters yet to confirm origins yet, alone trends such as any cold winter movements. Nowadays, if you tell me you have 'your' Robin comes to your garden just for a few months each winter, and that it has an eastern European accent I'll say it actually speaks fluent estuary English. That it's a new bird each year, a youngster, and that if it disappears by February, it's not made it out of the gene pool. Roughly, three out of four youngsters don't make it to breeding age. But if it disappears after that, it's gone to look then rather than starting to settle in a decent territory because your garden isn't good enough. And your neighbour's artificial grass and paved front garden aren't helping the town Robins any either..

Now that's a bit more like it.

Oh, and let's knock 'in the field' claims of Conti Robins into shape. Gurus like the great Chris Mead spent time poring over skins at British Museum, ringed tons in the field, and came to the conclusion the plumage differences are slight and on a spectrum; even if you've just seen a couple of dozen closely in the past half-hour, you'll as likely not be able to correctly claim a nominate race Robin. So, best treat majority of in field claims as unproven, along with a lot of in-hand as well. They *might* be, but more likely not. We see what we want to see.

There's a lot more evidence reflected in ringing studies; things like studies of passage migrants holding temporary feeding territies whilst refueling, and the like. But I'll save them for chats in the field when I need to bore someone..


PS for my BTO chums: that Kent summary, check those two Norwegian rings 956611 and AS956611- they're the same bird. You're welcome.

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