Ural howl

Recently treated myself to 'Bird Migration across the Himalayas'. Nice little read, focusing on how the high-altitude lakes in the mountains and on the Tibetan Plateau aid wetland non-passerine species to get over the highest mountain range on the Planet. Not huge amounts in it for passerine lovers, just one detailed chapter which concludes 'a sizeable majority of migrant passerines appear to avoid the Tibetan-Himalayan region but a large number appear nevertheless to pass through or over the mountain ranges..'

Which got me thinking about our own important but distant mountain obstacle- the Urals. (There, you thought I was going to say the North Downs, didn't you?)

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'Our' birds, on a large scale, breed mainly in one ecozone. The largest ecozone on the planet, the Palearctic, stretching from Western Europe to Kamchatka and the Bering Straights. Things like the Atlantic, the Pacific, desert chains and those Himalayas help to create hard-to-cross boundaries.

And within that Palearctic zone, one central mountain range, running north to south, creates another major boundary of sorts- yes, a little more porous, but keeping many species to each side of a biogeographic divide.

Now, those Siberian Accentors many twitched in 2016.. Yup, even though going to see one for me would have been poison, no law stops me thinking about 'em. And the latest issue of 'British Birds', with two articles related to that invasion, had got me thinking about that mountain obstacle of the Urals. Mainly because one theory pushed within had me spitting tea.

"(..refined by Kenneth Williamson into our modern understandings of migration and vagrancy) ..Siberian birds reaching Britain were, he argued, young birds on 'post juvenile dispersal', not a true, purposeful migration towards the wintering grounds as undertaken by adults but a random initial movement in all directions away from the breeding area.."

Why exactly did I spit at that? Ken Williamson himself stressed dispersal should never be confused with true migration, which are being muddled here. Let me explain.

Firstly, post-juvenile dispersal is not a good term nowadays. Newton, in NN 'Bird Migration' uses post-fledging dispersal, before going on to talk about migration.

Once fledged, no longer dependent on the parents, most young migrant passerines do leave natal area to explore in all directions of the compass for a few weeks. At the end of it they find somewhere to carry out a pre-programmed post-juvenile moult in preparedness for actual migration. If they make it back the following spring, it is often to this post-juvenile moult site. They chose it because it hit all the right buttons for their species.

So, within the first seven weeks of their short lives, rapidly grown a relatively weak juvenile plumage to get them clear of the nest whilst at the same time registering their natal area, to which some (mainly of one sex) will return close-by to breed. Then imprint a second site, their post-juvenile moult site, to which some will return straight to in spring in order to breed. Bearing in mind first month of life is pre-fledging stuck around the nest, they do not really have long to find somewhere; the post-fledging dispersal period is just about three weeks or so.

Function of dispersal is about gaining a head start for following years' breeding. Return and quick start. Part of why those near continent passerine spp. are not colonising. Post-fledging dispersal flights generally too short for crossing the Channel/southern North Sea. Because they are usually only airborne for an hour, possibly two, on any given dispersal night flight. Studies on post-fledging dispersal among Blackcaps have shown an average nightly flight distance of just a handful of kilometres.

This is not going to get any Siberian waif to us.

The post-juvenile moult is something virtually all young passerines go through. It comes about mainly from internal cues (endogenous mechanisms). Vast majority replace only some feathers, and is not a full moult. They usually does not have the time, nor the resources before having to migrate or suffer the first hardships of winter. The p-j moult is mainly only covering feathers, those that will work the hardest until the first full moult a year away. Feathers grown in the nest are weaker and looser than adult feathers. New body feathers will insulate more. New coverts on the wings will act like the protective tiles on the Space Shuttle. Why, if only some greater coverts are changed, they will be the ones on top covering the other greater coverts, those that suffer most wear from sunlight through to vegetation. Also why the old name for tertials, 'shield feathers', is a great descriptive term.

At the same  time endogenous cues start to set migration proper in motion. Migratory restlessness kicks in. The bird builds muscle, puts on weight, and effectively slows and stops any short-range random movements. Pre-programmed general directions kick in.The famous experiments with Robins and Blackcaps in tippex-lined buckets to have their overnight directional flutterings recorded, have shown that change from short-period random scratchings to purposeful one-direction all night flutterings; the general migratory directions are inbuilt.

Now, all those Siberian Accentors that turned up were in post-juvenile plumage. They were no longer moving randomly. A North Sea crossing would be bloomin' tough for any post-fledging disperser. A bird under the control of those migratory drives could cross it.

They also came in October. Migration time for adult and juvenile Sibe Accentors. The p-j moult is in the month before.

The initial random dispersal may well be influenced by weather conditions. How far do Siberian Accentors travel before the post-juvenile moult? Winds might have pushed them further on these short wandering flights. But they still shouldn't be dispersing as far as to Blighty. None of the birds here were in full juvenile plumage, and post-juvenile birds are no longer dispersing. They can, of course, still be blown a long way off course, wind-drift occurs more in youngsters, but many will now be attempting to readjust flightlines in line with their internal directional drives.

Post-juvenile dispersal for flights after the post-juvenile moult is a misnomer. It is not that post-juvenile dispersal is bringing them to us, it is something 'going wrong' with/during their migration. 

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Endogenous vs exonegous migratory controls. Wind, one of the easiest influences for us 'severe migrationists' to look to for cues, is an outside influence (exogenous). Others often help develop the migratory disposition; day-length and temperature play parts. But endogenous systems are big drivers. How else do birds wintering near the equator know to start back? Day length is similar, temperature similar, angle of sunlight similar... yet off they set. And again, experiments, taking birds into closed-off labs show such cues for movement, moult, kick in even if they can't pick up on the external clues- they have built-in circannual rhythms.

Of course we birders want to know what is going on. Knowing takes time, and we're still not there completely yet. Theories come along, theories go. They fail rigorous testing. Sure, reversed migration is out of favour, big time. But what if the endogneous drive is, for whatever reason, faulty? Might there be a 'favoured' remaining direction based on a combination of prevalent endogenous/exongenous drives? There are a lot of things in play. Some causative, some correlative. Whatever plonks a lost bird over the North Sea means a coastal landing for tired waifs and strays. We just need to keep refining our understanding- and keep refining the terminologies.

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Let's look at a 'North Kent' species for a mo'. Post juvenile dispersal in Reed Warblers. An example based on a paper by eastern Europeans, but if you take your Kent Bird Reports off the shelves and look at the ringing evidence for the timing of short-distance random direction flights between, say, Dungeness and Icklesham in East Sussex to the west, it translates well.

Nocturnal, occurring up to two months after fledging, involving flights of less than 75 minutes in the last two hours of night (the time when birds can start to see 'see'). In their paper, a lot of ne/sw movements detected, because, locally, that was where the suitable habitat lay. The birds did not carry  the fuel reserves for longer-distance movements. Some individuals shown to take several movements in differing non-migratory directions. And why youngsters respond well to all tape lures birders play- a clue for a possible good spot. Why adults largely ignore, they've already found their sweet spot and won't be looking to change.

Post-juvenile dispersal is not what gets a bird from Siberia to Blighty.

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If not convinced, just a quick thought on broods. Why do first broods do better than second broods? Studies on Tits have shown second broods end up in fragmented habitats, a secondary choice. Think of a species that throws in a third brood; they might well have very little time to disperse.

Sibe Accs leave late September to late October (in southern part of range). Post-juvenile dispersal starts earlier, but these birds will develop a migratory condition- build muscle, put on fat; p-j dispersal is over, endogenous and exogenous migratory systems are in control. Again, the post-fledging dispersal may have moved some birds a shortish distance, usually in all directions, and it might be strong winds influence the number that move in one direction in a particular year, but by the time these birds were reaching us, they were no longer dispersing.

Whilst the article gushed over their being 'Sibes' (and yes, that's in the name), it should be noted the species breeds in the Western Palearctic. Not huge numbers, but some make it over the Urals each year.These birds cross that mountainous boundary to reach their Chinese/Korean wintering grounds, but the geographical definition of Siberia has the western boundary as the Urals; hence not all Siberian Accentors breed in Siberia. Now, those Urals aren't the Himalayas by any standards, but how easy is it to carry out post-fledging dispersal over them? Like other divides, say like our English Channel, only a really small percentage of post-fledging dispersers will make it over easily, before true migratory urges take hold and enable more to cross a divide.

Although some of the references available are a little sketchy, another 'sibe', the Yellow-browed Warbler has, in recent decades, expanded breeding range and numbers on the Western Palearctic side of the Urals, which could help explain the increase in recent autumn numbers here. It could be the constant easterly airflows experienced in 2016 caused problems for Accentors this side of that divide. We just don't know until such time ringing, or similar, tracks individuals.

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Of course, this is a local blog, for local birds. Let's apply these theories to Nightingale dispersal and Chattenden Woods.

Their habitat is now fragmented. How easy is it for post-fledging dispersers to find a decent spot in that month following breaking the bonds to the natal area? The argument for the Nightingales over near the north shore is that their breeding grounds can be lost to development if suitable replacement habitat is provided within their dispersal area. The talk has been of over the Thames. Some might find it, only if they can readily cross that ever-widening estuary in time to set about their post-juvenile moult. Of course, it is much more likely that Essex dispersers will benefit. The Hoo birds will lose out, unless efforts at other reserves such as Northward Hill succeed in creating the correct habitat. A key feature is closed scrub, allowing favoured feeding on bare ground beneath bushes. And scrub remains a habitat grossly undervalued. As the closed low scrub is out-competed by maturing woodland numbers decrease (part of why the peak at Chattenden has been passed).

Here on the south shore our scattered numbers are dropping fast. Half-a-dozen breeding pairs lost their scrub to housing clearance last winter. Their last youngsters would have been struggling to find suitable habitat in the area. Scrub tidying by Council and private landowners knocked out another three or four pairs, and necessary improvement to reed-bed water levels drove back other birds in the couple of years before that. At other sites, pathway improvements are fragmenting scrub habitat and I'm putting money on their numbers falling.

Habitat. Build it, they will come. If they can find locally it in that fortnight after leaving the nest that is.