Subdivisions

The last post talked about Black-tailed Godwit sub-roosts. Time to strike while the iron is hot and add another element to the subject: satellite roosts.


Sub-roosts and satellite roosts- what's the difference?

A week or so ago the local Oystercatchers had been using their sub-roosts for longer, on the neap tides. They had not found themselves pushed off by higher waters and had managed to loaf quite well at some places that, this week, are just short-term pre-roost gatherings on high spring tides.

Bishop Ness (red circled) is one such spot.

The Google Earth image shows all of the Bishop complex at low tide, above South Yantlet Creek to the north of Nor/Friars. Bishop was once one body. Now the westernmost section is Darnet- Fort Darnet standing on Darnet Ness ('Ness' being a promontory). The large central section is Bishop Saltings (a major gullery) and finally, to the east, the rump of Bishop Ness.

As can be seen, not very much remains above water on a higher tide. And on the highest springs, Bishop Ness goes under so Oystercatchers from there, with a much larger number from Bishop Saltings, usually then choose to move north to the Kingsnorth seawall. Often when gathered on the wall, these Saltings birds and Ness birds still remain separate from each other. Check through them closely enough and you'll find you have mainly younger birds in the smaller group from Bishop Ness. This is a satellite roost.

The younger Oystercatchers are kept from the best roost sites. Experienced adults have earned the best spots. Bishop Ness is nowhere near as good a site as Bishop Saltings, so most of the young gather there.

Motney saltings had been used by the young Oystercatchers in spring and summer in the past, Again, not a prime site, going under on the highest tides (when birds switched to Friars saltings). Motney was used routinely in 2013, my first year back in North Kent. Sadly, the amount of human disturbance there increased in 2014 and, along with the Black-tailed Godwits, they gave up on using Motney that spring. The young Oystercatchers didn't bother to return in 2015, now preferring Friars and Bishop Ness. They did use Motney a few times in 2016, but they have given up on it as a preferred choice. One of the Medway satellite roost sites has been lost for now.

Put simply, an estuary can't rely on just one, or two, safe roost areas. It needs a choice of sites. A choice of main roosts, a choice of sub-roosts and for some species a choice of satellite roosts. Take away enough of any mix of these safe roosts and the birds will start to desert and decline, however good the feeding. Yes, it is possible in the worst case scenario they might try to aerial roost- stay in a holding flight pattern over their inundated safe spot for a half-hour, an hour during the highest point of the tide, but this is a real calorie burner and the birds aim for peak condition. Or waders could choose to go several miles to roost further down the estuary, or to another estuary. Would this affect condition? Oystercatchers may switch between the Thames and Medway for feeding/roosting for certain tides, but as yet they prefer not to do this for every tide.


Where to see the Oystercatcher roosts:

The following notes are made to help understand what might be going on- none are that close to shore, and Oystercatchers may rest up in numbers on any favoured flat, so it cannot be an exhaustive list of sub-roosts; hopefully knowing about them might allow birders to understand and enjoy the spectacle a little more. Breaking the southern shore into two;

a) The western basin, to the west of Ham Green-

1) RSPB Motney Hill saltings- now infrequently used as a satellite roost/ sub-roost.
2) Friars saltings- sometimes used as a satellite roost/ sub-roost.
3) Bishop saltings- often used as main roost, neap tides.
4) Bishop Ness- often used as a satellite roost on neap tides only, sometimes as a sub-roost.
5) Kingsnorth sea-wall- main roost on spring tides for southern shore birds. Also used in poor weather. The lack of disturbance here has come about because of the old Power Station behind the wall. The eventual site redevelopment may well change disturbance levels here.

b) The eastern basin, to the east of Ham Green-

6) Chetney Hill- a main roost; on springs birds may move a short distance up onto quiet sections of the adjacent private sea wall
7) Barksore- a back-up main roost, often first choice when Chetney Hill is disturbed (by poor weather or human disturbance). This might be on the seward side of the wall, or on the fields behind.
8) Greenborough- a satellite roost/ sub-roost.
9) Burntwick (note exact position cannot be viewed from the southern shore)- a main roost for Thames birds, more often chosen on night roosts than Deadman's.
10) Deadman's- a main roost, swollen by Thames birds especially on spring tides. Technically, the birds may often sub-roost on the flats immediately below the roost and walk up to the roost itself.O
11) Swale Ness- a satellite/ sub-roost.


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Ageing Oiks

Young Oystercatchers at this time of year are fairly easy to age; most guide books include illustrations of 'first summers'.The wing and tail feathers were not moulted after fledging and appear much worn and not as black as in older birds. Unlike adults, they retain the white collar through the summer and have a dark-tipped bill, paler eyes and much paler legs. They do not attain an adult plumage until their third summer. Second summer sees varying degrees of adult features coming through- brighter legs, traces of dark tips to the bills, all with enough variation to befuddle birds. When when we think they can't be told apart in their third summer, the birds themselves can do it- when it comes to choosing partners, the average age for first breeding is four.

For their first summer youngsters are often the furthest from home they will ever be. They then return a little closer each year after that which means (based on ringing evidence) collared youngsters we see here on the Medway during the coming weeks will (with luck) end up returning to nest back home in Norway. Older immatures here right now might be birds of Kent or Kentish birds (depending on which shore of the river they were born) waiting their chance. And some 'probable breeding pairs' you might find later in the spring dossing along the tideline will actually just be canoodling immatures- because, if really a nesting pair, one should really be sitting tight on their nest.

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The Motney forecast for spring '17? Not good- another cairn (after one there in 2015) has been built recently smack in the middle of the roost (see first photo) by some intrepid walkers/campers and will now, no doubt, lure a few more wanderers out onto the reserve saltings in the coming month. Still, those young Oiks have got other sites (for now).