Monday, 9 April 2018

The lure of Bramblings

In 1951/52, attempts were made to count a winter roost of Bramblings in Switzerland. One stream into the roost was estimated to hold around 36 million birds. To quote from Newton's 'NN 'Finches', "..however, only about half the birds could be seen from the observation points, so the total number was probably at least 70 million.."

That is a lot of Bramblings. To a soft Kent birder, a three figure count would be impressive. This winter, I have been more than happy with a number going to roost somewhat under that. Of course, I knew, like the Swiss, I probably wasn't counting the whole flock from my viewpoint, but it was fun to dream it could have been millions. Or a hundred. I'd have settled for counting a hundred.

Berengrave Chalk Pit

Go back to the 1980s, a regular spot for Brambling in the run of hard winters back then. I still have fond memories of trudging through the snow to see them, standing under the ivy covered trunks watching birds sneaking in.

Since I moved back from East Sussex, I have been counting the Magpie roost at Berengrave. As mega-exciting as that sounds, I decided from the start to count everything that roosts (a series of links at the end of the blog will take you to past summaries for most of the more common species including Brambling). This year autumn passage on the estuary hinted at a small but slightly higher than usual passage and, sure enough, after a run of nigh-blank winters, I finally had some Brambling to look at. Not 70 million, not even 70, but it would do.

I must be one of the few birders who routinely takes a telescope into an overgrown quarry. Simple fact of the matter, the birds were always at the northern end, the only decent viewpoint at the southern.

Pre-roost assembly and roosting

The Magpie count period has always been a set timed count from 60 minutes pre-sunset to 30 after. The finches often start arriving before that so, on arrival, a scan of the distant tree line would often find a mix of species up on display. When I say mix, there tended to be a general clumping of one species over another. Chaffinch, in the largest numbers, were all through the higher trees but the Goldfinches and Bramblings would be in tens, twenties, both having favourite trees and both able to flight without taking any of the other flocks with them.

Unlike the Magpies, the finch show would be over by sunset. Usually roughly twenty minutes before when the various groups would drop into lower branches. Mainly without any fuss, only sometimes, especially later in the season, making a few circuits of the Pit before piling in. Another interesting difference was that during the coldest part of the year perching in the tree tops would still happen, but for much, much shorter periods. Any urge to advertise the roost to newbies was overridden by an urge to secure a good spot.

Arrival was almost always from the north, and mainly the north-east. This put the majority on a flightpath back to what I discovered to be their favourite local food source.

Bramblings along the Medway

Taking winter as December 1st to February 28th, sightings were clustered around the main early season food source before spreading to a later winter support system of gardens. The initial attraction was not, as the field guides would have it, beech mast, but brassica, in game cover crops. Looking at 'BWP', brassica is a source many switch to when the mast runs out. There was clearly enough good feeding on these local farms, with safe roosting nearby, to keep a few birds present all winter.

Sadly, I was hampered by my early December operation and did not get to search the eastern end of the shore as often as I would have liked; Brambling arriving at the other more easterly roost I did visit tended to fly from the north-west and north-east, so were also no doubt also taking advantage of local cover crops; another local farmer more to the east always has them in his cover crops when they are in the county. Another site for me to check up on in the next good winter.

The second coming

Many texts hint at late winter hard weather movements. My set counts might very well hint at such a thing, but I can't say for sure. I found myself asking another question instead.

Go back to that monster Swiss roost. The actual count was never the number of birds present. That had to be estimated. In no way do I think my counts were the true number present. Just a nice comparison of relative numbers throughout the winter. But because my site is a b*gg*r to view, and with birds dropping down much more quickly during the coldest periods, could it be I missed more birds in the cold than the mild? My primary reason for counting here is, after all, the Magpies, so I never kept my eyes glued to the finches' favoured pre-roost perches. Did more sneak in past me at this time? Was the drop as strong as suggested by the graph?

Which had me thinking on other aspects of Brambling wintering behaviour and movement where the texts tend to differ a little.

Unlike other finches, Brambling are not, in the main, diurnal migrants; many birds, some texts suggest the vast majority, actually move at night. (BWP makes an intriguing statement that most diurnal movement takes place overland, while more nocturnal movements are over large water bodies. Years later the Migration Atlas states '..Brambling movements are often nocturnal and may leave exhausted stragglers grounded at the coast during the day..') The best bet is that we often just pick up on early morning stragglers or daytime adjusting movements; why, on  a good autumn morning, we never have more than a handful in comparison to Chaffinches.

Those late winter second pushes into the U.K. from the continent; how much evidence actually exists? Why couldn't late winter peaks be birds adjusting from, say, already within the U.K.? Again, the Migration Atlas " conditions may bring new arrivals from the Continent in mid-winter, often in substantial numbers.." referencing the Birds of Norfolk, but intra-seasonal ringing controls that back this up are lacking somewhat, plus, on reading that referenced species account the mid-winter flight directions are mentioned, but none suggest an 'in-off'; movements south through that county could just as easily be birds already in the U.K.

The probability is good, but there is still much to confirm.

Spring departure and some real experiments rather than my guessing games

A direct comparison of Chaffinch vs. Brambling shows a typical Chaffinch curve, albeit steeper than usual thanks to higher numbers. A continued rise into December as the migrants find the better roosts/feeding areas, a decrease after New Year as winter bites, then a drop from March onwards as the locals move out to set up territories and the winterers start to feel migratory restlessness setting in; Chaffinch move a few weeks before Brambling. Makes sense, Brambling being the more northerly breeder, no need to move off quite as quickly; those breeding in, say, northern Finland, can't really get started there until warm enough in mid-, late- May. Their diet becomes more insectivorous, why risk getting back there and finding everything still frozen if there's still food available further south? Why some texts hint at their peak departures from here being mid-April, a fortnight or so after the Chaffinch peaks.

Having a trawl around the internet, I found a couple of interesting papers on work on migratory cues for Bramblings.

In one experiment, birds were kept under artificial light set to short (winter) daylengths. These lights were switched abruptly to 14.5 hours. It was like turning the dial up to eleven. Within three days, the birds had developed migratory restlessness. Zugunruhe. Fat deposits grew. So, effectively by mid- to late-April, migratory condition in Bramblings should be well advanced. Doesn't mean they all move at once, the texts say they stay in some number throughout the month.So why do we see less and less? Migration Atlas hints at an answer. Large flocks may have short-hopped to the continent, or they may have dispersed. Diet again. Stalling an early arrival on snowy breeding grounds again.

A second, more technical experiment, supported the hypothesis that "changes in the amplitude and level of the daily melatonin cycle are involved in regulating migratory restlessness, by either allowing or inhibiting nocturnal activity.." "Nocturnal activity"- now that made me sit up. Despite those more recent texts hinting diurnal migration might well be as strong as nocturnal, this species has true nocturnal urges. Puts me in the camp standing by the theory we only ever see a fraction of the true numbers of Brambling in our viz mig efforts. All the guesses that there's probably a lot more Brambling mixed in a high-flying Chaffinch flock than we can pick out? Nah, there's probably just a similar percentage to those we pick out in the lower flocks; a handful in the hundreds. The majority arrived earlier. (Be interesting to see if I eat these words in the coming years as more birders take up 'noc mig'..)

So, we might not be able to see them go. Do we know when they really do leave?

Borderline differences

Newton, in New Nats, stated British winterers move early March to April. What have our county avifaunas said?

Norfolk (Taylor): "..April records are now not at all unusual and large flocks can occur.. can still be present in good numbers into early April.. most tend to have slipped away by the middle of April.. coastal passage in the spring is much less noticeable than in the autumn.."

Suffolk (Piotrowski): "..pre-emigrating flocks.. ..feeding flocks are frequently reported until early May.."

Essex (Wood): "..the status of those birds occurring in the county from March onwards is unclear. Many are probably wintering birds, but it is likely that from the end of March there is an increasing number of returning migrants involved..

London (Self): "..several influxes during the middle of winter.."

Surrey (Wheatley): "..some evidence of passage March and larger arrivals in January.."

Kent (Taylor) "..small, but often noticeable spring passage, usually indicated by the presence of temporary flocks, during March-April.."

Sussex (James): "..a small spring passage is evident from mid-February onwards.."
(Thomas): "..spring passage is much less noticeable than in autumn.."

Where specific, not exactly any consensus among the local counties then.

Nationally, the Migration Atlas goes for "..large wintering flocks have typically dispersed or returned to the continent by mid-April.." so, they may or may not have left. It also asks if late April/early May birds on the coast are late leavers or passage birds heading up from south-west Europe.

We really still don't know as much as we like to think we do.

Among the county avifaunas, as always, for me, Wood stands out (fnarr). It makes plain we don't really know, and hints at what might be going on. Why when anyone asks me what's the best text on birds for Kent, I always say 'Birds of Essex'. I've always stated outstanding for esturine birds, but it also really works for a lot more species as well.

Not much left for me to do now this season. Just the one Magpie count left, in second half of April, but I'm already geared up for a nil return on Brambling, if only because my allotment backs onto the northern edge of their assembly area, and this past week I've not been hearing any in the late afternoons- or will they prove me wrong? Who would guess forecasting negative data for a common species could be so much fun?


Links to other Berengrave roost posts:
- Introduction
- Finches
- Thrushes
- Non-passerines
- Other passerines

No comments:

Post a Comment