Saturday, 25 February 2017

Reed Buntings- court out in the cold

One might be forgiven for thinking I'm a waders and wildfowl fanatic. I'm lucky enough to live on an estuary, but I'd have to admit my favourites always were, and still are, passerines. So, I really should write a bit on one here.

Reed Buntings.


We're now at that time of year when the males start to stake a claim a territory. They leave their roost before dawn to make for their chosen territory, where they then sing for an hour or so. After that they head off to loiter around the flock's feeding areas. As the season progresses, they stay longer, and longer. The wintering groups break up slowly.

Sing out loud and show off your home in case a potential mate comes by, then go strut about a bit in their company. Male aggression starts to build now as well. It's tough trying to persuade a potential mate you're the one that's worth it.

I was lucky enough to live on a lowland farm in East Sussex for several years, where Reed Bunting was a very common bird at the garden feeders at this time of the winter- counts of twenty, thirty, were not uncommon, a hundred or could be ringed each winter. In February, their timing was always the same, matching the text books. None for the first hour or so, a steady build (females predominating at first) to a mid-day peak, and finally all disappear an hour or so before sunset. Late-winter males, such as the one below, coming into breeding plumage, were usually ringed late morning.


So, late February I'm always listening out for Reed Buntings in the mornings here along the Medway, and an insistent male at Eastcourt Meadows the other morning prompted me to write this note.


Winter roosting

Sadly, numbers along the southern Medway are nowhere near those of my old East Sussex stronghold. And nowhere near where they were when I started birding just over thirty years ago. The texts always talk about the collapse in numbers in the seventies (intensive farming), but here an almost imperceptible decline has continued. There used to be a regular roost at Horrid Hill, the scrub reaching climax (tall and closing off light to the herbage growth below) plus the Country Park policy of cutting the grass edges right up to the clumps put paid to the dense herbage they prefer to roost in some years ago. Increased human footfall has probably not helped.When I watched the areas in the 1980s I could manage counts of 20+ in the late autumn; now, lucky to manage a handful on any day.

A smaller roost on a private part of Motney Hill is now the most best site for double figure counts along the inner basin. Of course, Motney does has the most preferred roosting habitat- reedbed edge interspersed with rank herbage.

There are a couple of smaller gatherings around Ham Green and Lower Halstow. Small numbers can be found at the foot of Raspberry Hill and there is certainly a roost somewhere on the private Barksore; birds flitting around the Funton Creek saltings head off that way at day's end.

Most UK roosts are small in number- 40 is seen as a large roost, most are less than half that. Easy for birders to miss.

Winter roosting actually becomes spring roosting, in that birds continue to make nightly returns even when a territory is held and nest being built; only once the first egg is laid will a bird sleep there.

Throw in big variances in defended territory size during firstly establishment, then nesting and finally fledging, add in a mix of bigamous males, and you start to have a real problem with judging population sizes.


Migrants?

The Migration Atlas is unequivical: "...ringing data have been used to show conclusively that almost all breeding individuals from the British breeding population winter within Britain, at which time they are joined by very small numbers of predominantly Scandinavian breeding birds..."

Some ringing chums have been quite excited this year, by trapping a Scandi bird here and there around the UK; the strength of the easterlies last autumn may the have helped displace some movers but this does not detract from the bigger picture.

Taking a look at the Reed Bunting account in the excellent free download of the Nord Pas-de-Calais migration book the ringing recovery map shows Scandinavian birds will, for the main part, be taking the coastal European flyway I have mentioned in several previous posts in relation to waders and wildfowl; passerines may not 'use' the mudflats, but the majority will be hugging that coastline as inferred in the screengrab below:


So, any autumnal fluctuation in numbers here in the Medway is most likely due to local dispersal, moving to suitable wintering roost catchment areas; males on average move only a couple of kilometres, females maybe about six km (ensuring a local gene flow). Having one or two broods a year, fledging just over two young per brood, you might well encounter two, three times the number seen earlier in the year. And that's it.

The Reed Bunting is often overlooked by birders nowadays, but is well worth recording in the notebook. For those who say 'why should we bother, we know enough already', here's one take on how much we really don't know.


Tetrads- distribution, not density

For overseas readers, tetrads are particularly 'British' in their use on our Ordnance Survey maps and are well explained here. They are used routinely for mapping distributions in our avifaunas. Recording absence or presence is easy enough. Possible/probable/definite breeding have often been mapped. Thanks to the statistical wizardry and the size of data collated during their national surveys, BTO can go for a form of robust abundance mapping. Some counties still want to try to come up with a figure, a numeric, and Kent is one that tries hard to come up with such a quotable total.

How useful are these figures?

British Reed Buntings are largely sedentary and these small numbers seen recently reflect the small population along the southern Medway; numbers are nowhere near the estimated "30-50 pairs per tetrad for reed-beds" used by the County's latest breeding atlas- based on my own mapping singing males and ringing dispersing young, within my four local tetrads that contain reedbed/wet ditch/rank herbage habitat mixes we only have about 10-20% of that guesstimate. Perhaps because, on the ground, there are tetrads completely covered by optimum habitat. A tetrad is some 400 hectares- the main Motney reedbed/ditch complex covers 25 hectares, and might have five pairs in a good year. So, yes, 30-50 pairs per reed-covered tetrad, but in reality just 16% of that particular tetrad is 'suitable habitat'.

At least Kent had stayed faithful to their previous atlas, published in their 1996 Report, when they dismissed the old CBC methods (which produced a figure of some 4,800 pairs) 'since the county's marshland areas support high densities in reedbeds and dykes'.

That really should be 'some' marshland areas. Take Chetney. A paper published in 80s had shown Reed Bunting numbers to be way below that.

The truth is that estimating a common bird population is exceptionally hard, and if you want them from historic comparisons you need robust models that remain useable atlas after atlas.

At a time when 'professional' BTO survey results show there to be a continuing decline in the south-east, for any species author to state an optimum guesstimate of 'numbers broadly stable' for the county has to stand up to scrutiny. At least the new county Atlas acknowledges the BTO's BBS decline in the south-east before ignoring it. The following graph is reproduced from the BTO/JNCC/RSPB Breeding Bird Survey population trends webpage:


The 'changes in relative abundance' map in the latest national Atlas also showed a decline around Kent over a twenty year period. It would have been useful to read how that 'broadly stable' stance was taken by the county.

To this observer, the calculation is too simple. In statistical language, it is both 'imprecise and inaccurate'. Some comment is made- slight losses in western and central Kent, with slight gains in the same area. Looking at the maps, the losses are linked to watercourses (the Medway and its tributaries and the ignored east Kent losses along the Stour). Local 'improvements' with loss of rank herbage? A continued drying out, as rainfall continues to decrease, a whole host of questions not covered.

The tetrad losses have almost been balanced out by tetrad gains. But the county's latest (six year) Atlas period was longer than the previous, and made use of the social media to appeal for extra observer effort to fill gaps in that extra year. The 1996 Atlas made some attempt to quantify the number of observations received (a 20% drop on the '66-'73 Atlas), so the parameters changed. What consideration was there for the old chestnut of 'observer bias'?

Their 1988-94 county survey stated 8,000 to 13,000 territories. The 2008-13, 7,000 to 13,000 territories. Pessimistically a drop of just 12.5%, at the most optimistic a zero percent change. At least the Reed Bunting entry in this latest county Atlas ends by stating 'we need more evidence of current breeding densities, in reed-beds and on farmland'.

That's why birders still have a part to play. From published counts, very few sites in Kent have achieved any notable counts in recent years. True picture? Or extremely few attempts to find them?

All probably why several reviewers commented on a lack of relative abundancy maps in the latest county Atlas. Population estimates can be horribly inaccurate and imprecise. With just over a decade to the start of field work for the next Atlas, perhaps enough time to reflect on goals at the local level.

Has Kent really have bucked the trend of the rest of the south-east? Are populations holding up better here? I very much doubt it. But  hopefully local birders will rise to the book's challenge for more local studies. Go and count the songsters this spring to help them and, perhaps, prove me wrong.

Just as this published guesstimation is likely imprecise and as inaccurate, I could be hopelessly, hopelessly wrong myself. (Wouldn't be the first time !) It's why many records from many sources will always be needed.


Song of the South(-east)

If you need to brighten up a dull day, there is a great game you can play with the song when starting to record your Reed Buntings. A short burst of song, a long gap, then another short burst is most likely an unpaired male (what you hear around now). But a longer burst, shorter gap, longer burst is most often a paired male. Examples can of course be found on xeno-canto, but the one I'd like to link to is the oddest, and one I'd love to hear; an extreme version of the long song (described by the recorder as a 'marathon song') here. Going at full throttle. Now thought to be the sign of a successful Lothario going for as many extra-pair couplings as possible. When it comes to sexual shenanigans, Reed Buntings are up there with Dunnocks in my book (but that's for another blog post).

The 1996 county Atlas acknowledged the suggested population numbers better reflected territories than actual numbers, and while pair estimation worked well for some species, for the many species where polygamy, polyandry and the like are a norm, it is not that easy to say pairs. Count your singing males.

Here along the south Medway, continuing losses in suitable habitat (e.g. away from reedbeds, lot more saltings now go under on spring tides, making unsuitable for nesting), at the same time as a growing decline in habitat quality (the general tidying up) have been major drivers in the local decline in Reed Bunting numbers between the last national atlases. Together with a continued drying out in the south-east through climate change, and growing development pressures, numbers will have declined further by the time of the next national atlas.

Prove me wrong people.

1 comment:

  1. Intreresting, I have been thinking about the Kent Atlas estimate for yellowhammer which seems optimistic but as more widely distributed probably harder to get to grips with.incidentally we regularly catch between 20 and 60 reed buntings in late summer when juvs doing pj moult. Not sure where they disperse to.

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