Monday, 13 November 2017

Scoters on the Medway

Horrid Hill, Nov 2017

Scoters. Not rare birds at a national level by any means, but birds that brighten up a Medway day. Scouring the annual county bird reports, very few sightings have been deemed worthy of inclusion. Those that have made print indicate a smattering of birds on migration plus the odd immature in the summer. Most are winter appearances, mainly single birds for one or two days.

Of course, the reporters don't help matters when they just report as 'Grain', as so often happens; which river are they actually on? As discussed in my last blog, birds can and do enter Medway airspace.

A similar tale for the Velvet Scoter; just the odd record, indicative of wayward migration:

Just one single bird single day record in the five years back for me, so just think of Velvets as the close cousin that behaves in a much similar way, but never, ever needs to move as far from home.

Best we concentrate on Common. They have never been seen in the estuary in number, and have been nowhere near annual. Low observer coverage and limited shoreline access have probably hidden the true picture to date. Though still not in any number, they are most certainly annual.

While Scoters will feed on the most commonly available favoured shellfish, they have a preference for sandy substrates. The Medway does have a sandy element, but is much muddier than found in their usual Greater Thames hotspots. In such favoured wintering areas, numbers can vary wildly year-to-year, thought to be a response to prey availability. Such yo-yo-ing is harder to detect in the data now peak years are so much lower.

Migratory volumes may also be down, but migratory timings are still fairly similar Common Scoter Channel numbers build through July into August (immatures?), second peak mid-September- early October (females/immatures?) and a third short peak early November (more males?) Spring migration peak is late-March/early-April.

Half Acre, December 2016
To understand what we see, we need to forget the political boundaries. Wood's 'Birds of Essex' gave a status of 'common passage migrant and winter visitor with a small non-breeding population in summer. Common? Yes- on the shallow waters over and around offshore sandbanks in the mouth of the Thames. Land-based observers have always faced a difficulty picking up a true reflection of numbers present. A screengrab from the excellent shows the vast offshore mudflats (green) not always shown on a land-based OS map.

Wood makes particular reference to flocks off of Maplin Sands, and, just north, off of Dengie; 7,000 off Maplin in February 1976. Those days are gone. Present nos in the UK are estimated to be much lower around the Thames. "Although small flocks occur widely around the coast, the majority of Common Scoters wintering in British waters do so in just a few large congregations. Aerial survey work in recent years has revolutionised our knowledge of the true numbers of birds present, although estimates for particular areas do tend to be relatively imprecise. The present estimate is highly dependent on the numbers assigned to Liverpool Bay (55,000), Carmarthen Bay (20,000) and Cardigan Bay (12,000), with further counts of over 1,000 off Norfolk, the Forth, Tay, North-east Scotland and the Moray Firth. Large numbers also occur on the Solway but mostly in spring and autumn, presumably en route to/from areas farther south."

Year to year numbers do fluctuate (any cold weather displacement of even a small part of the 80,000 wintering off the Dutch coast will quickly paint a different picture). A screengrab from wheresthepath shows these banks, plus smaller ones a little further east. And the way the wind farms have been built away from them. The windfarm bird concerns have always been more about the pelagic species, such as Red-throated Diver; 'seaducks' are not as much at home out on deep water, being happier feeding at depths of up to around ten metres. Bird factoids often confirm that Scoters will dive up to 30 metres, but in reality most feeding takes place below ten- fifteen metres, much at one to three metres. Such areas are not often found for long periods in the Medway away from the shipping channels.

Defining the coastal zone. Looking at the map there may be a lot of open water, but the shipping channels are there, and studies have shown larger flocks (roosting, which break into smaller feeding flocks) tend to flush at the greatest distances, usually around the one kilometre mark. Nor are Scoter pelagic, avoiding the deepest waters (why spend time away from the easiest feeding?) explaining why the studies for the wind farms were more concerned with the likes of Red-throated Divers.

Back to my opening speck shot here, those four Common Scoter off of Horrid. Most birders would 'meh' at both the pic and the views. You can't blame birders for sticking to honeypot reserves. All part of why they are under-recorded. They're rare in these parts, as are birders.

For any Scoter to get here, they have to be drawn from those shrinking numbers still traveling this far south and west and not stop short for either their summer moult (immatures) or for their winter quarters (mainly immatures and females- a male is always a red-letter day).

They have to be willing to put up with the increased traffic in the shipping lanes next to their old favoured coastal zones, and they then have to have some obscure reason to then turn south into the Medway. In clear weather they'll avoid doing that a lot of the time, for sure. Why they more often arrive arrive on muckier weather, and why it sometimes takes them a few days to find their way out- feed up and leave when good conditions return.

Why always better to consider the Medway as 'inland' for such species. Why Medway estuary Scoter record patterns are a little more like inland sites such as Leybourne lakes.

Overland passage of waders and wildfowl from the main estuary (yellow) is seen often enough, but passage out via the river itself is less well observed. Leybourne lakes (orange circle), south of the Medway Gap through the North Downs, do get less than annual records. As with many seabirds, flights over water into headwinds are easiest observed, tailwinds see height and some missed, overland flight is even higher. And, thanks to radar studies in the Baltic, we know a lot of Scoter movement takes place at night.

(An aside, the Essex publications all speak of passage up the Thames. Some turn back, others disappear around the green line of Lower Hope Reach. Overland movement is probably a much easy subject to study there. Some birders have been working on it from both sides of the estuary for several years now, and will, hopefully, write up one day.)

The Shade, November 2013

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