The search for the north-west Kent passage

Just how good is the Medway for spring passage? Interesting question, but judging by the perennial debate in some of the forums the question the county readership would probably want looked at first is- 'why is it that spring passage in Kent never seems as good as elsewhere?'

Last year blogged a rough outline, this time I'll just take on a few of the passerines. The African-Palearctic nocturnal migrants to be precise.

The first thing a birder has to understand is just how different their autumn and spring migration behaviour is, because our views of how spring should be are heavily influenced by the spectacle of autumn passage.

The first autumn

A typical nocturnal passage migrant passerine arrives in the world with inherited (endogenous) programming. It does not have all the information needed, and still has to learn quickly. Only about one in three will survive to return the following spring. (These odds are, in fact, better than for most sedentary species- one of the reasons why so many species choose to migrate.)

The weak juvenile feathers are enough to get it out the nest, and for the next few weeks, abandoned by parents, the youngster will search the local area, getting to know the good areas. This might be a few kilometres, it might be the whole of Kent, but soon a post-juvenile moult kicks in to replace the poor body feathers and, at the same time, the internal programming calls for an increased feeding regime, a laying down of fat and a building of flight muscles. By the time the post-juvenile moult ends, the bird has an overriding urge to travel. It has learnt the fixed position of the pole star, and the direction of night sky revolution. These factors will be what gets a bird back roughly to the natal area in the spring when those weeks spent wandering will give a youngster a choice of suitable sites to aim for.

Back at the start of the autumn, the first nocturnal flights during the completion of the post-juvenile moult are short; they take off not long after sunset, but land within a few hours. The jumps get longer slowly until the main thrust over north Africa, then slow again, finally terminated by the internal clock- the journey to their furthest point south can actually take three to five months.

The first spring

The young bird is now much better prepared for the journey. It has perfected reading the star-map, and polished up on polarisation of ultra-violet light and using a magnetic compass. Some scientists now theorise nocturnal calls are not for co-ordinating with others but more of a type of echo-location to find leading-lines; all these skills are now finely tuned.

It had suffered wind, rain and fog, andnow  has better coping mechanisms for such adverse weather, often putting down earlier rather than trying to push on through bad conditions.

Yet many species will again be relying on their internal pre-programming to take a slightly different route back ('loop migration').  Some take a large detour, with others it can be slight. It explains why 'our' UK breeders tend to come in to the west of the country in the spring- better for dealing with drift from prevalent winds, avoiding being blown out over the North Sea. After all, it knows roughly where the pre-chosen target area is, relative to any position- it can home in. The route home for many northerly breeding UK migrants is from the south-west up via the Midlands, no need to bother with Kent.

There is a correlation between arrival and temperature. The ten degree isotherm is often seen as a rough average mean- get the temperatures up at this level and the movement is on. Of course, this isotherm had arrived first in the UK in the south-west for millenia, slightly later in the south-east. The returners are aiming for the south-west for a reason. Why places like Portland start getting numbers before us.

The return journey only takes about a third of the time of the outbound journey, with migratory jumps adapted in length more for good weather than for the physical rigours of the journey - unlike in the autumn, they do not slow as they near their destinations and the longest jumps can even be those that get them home, their internal urges only beginning to switch off, somewhat abruptly, when they near their target point.

This explains why we do not have many passerine species from the near continent trying to breed in Kent. Those breeding in northern France and the Low countries are switching off when they get close to the Channel and really won't have an urge to cross the water. Birds breeding higher up in northern Europe can get drifted over the Channel, but even when they do, they want to keep moving.

Any birds coming in over Kent have a better control on fat reserves and will be able to choose to fly further inland for the optimal habitats rather than just flop down on the coast.

The second autumn

Birds know the good staging areas from the first outbound trip around the loop, and are less likely to get caught up in falls. They have often just had a full moult after completing breeding, so have an extremely strong set of flight feathers to carry them south- no wonder so few adults get caught up in autumn falls; they leave early and they leave fast. Why the vast majority of autumn migrants we see are youngsters.

The second spring

As with the second autumn, the return half of the loop migration is now known, having been completed once, and the bird has a good knowledge of the route to their breeding site- older birds are less likely to go off course, especially as they will also have a working plan for good refueling spots. Really no reason to drop into Kent in good weather, unless they breed here.

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Kent birders and rose-coloured binoculars

So, the bad news for Kent birders is just because there are a lot of migrants piling into the south-west, and some have already reached spots further north in the UK, we really shouldn't be expecting similar numbers of birds here.

Fog or rain over the south coast of Kent won't produce a big fall; quite a few of the birds will have known to put down before piling into the rough stuff, either over mainland Europe or, if over water, simply turning around mid-flight; reverse migration adjustments are common in spring. The best chance of any half-decent fall is if the hurdle starts right on the coast itself.

Coming in over the coast in good weather, a spring migrant will have the chance to keep flying on until it reaches preferred habitat- why waste time at Dungeness?

Spring. The season that consistently under-delivers for Kent birders. Expectation over reality. I could go on for other types of migrants, but far too much to cover this year.

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South Medway movements

The ideal, absolute ideal for nocturnal passerines is wet weather, or fog, starting over the Hoo peninsula northwards. A barrier.  If that arrives just before dawn, birds can stop short along the south Medway. On such a forecast get out for dawn, and pray the rain arrives (it often doesn't; north-west Kent is one of the driest areas in the UK). Search the marshes, hard, and you might find a few nocturnal migrants have dropped in. Only a few, because numbers trying to pass overhead will have been low.

Check during the first couple of hours after dawn for any travelers looking for the more suitable feeding areas; the eastern half of the south Medway is much more productive than the more urban west.

There are a couple of better periods for the higher volume of movers- we tend to fixate on earliest arrival dates, but if you check the dates for multiple/highest counts for several nocturnal passerines is around the middle of the second half of April, then followed about a week to a fortnight or so later by a second pulse (the females and first summers generally arrive after the adult males). These peaks may be slightly hidden by not-so-local breeders but, as already explained, there shouldn't really aren't going to be that many stopping off. Grit your teeth and hope for that 'patch gold'.

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That's a very rough outline for these long-distance nocturnal passerines. Sorry to have had such a downer on prospects for this, and, sadly, every subsequent spring. Only going to get worse. Read the blogs for some sites in the south-west and they too are beginning to bemoan the lack of passage even though they are in a much busier part of the country). Common species won't be any more common, and rares will be, well, rare.

Is all this 'missing out' really such a bad thing? Not if you just go out birding with really low expectations because you'll find you come back happier much more often, trust me.