Thursday, 8 December 2016

Raving about 'ranting

Say to a birder 'what's a pelagic?' and you'll get a reply along the lines of 'a boat trip out to sea, to search for seabirds'.

Say to a birder 'what's the pelagic?' and quite a few will be hard-pressed to answer.

They might make a stab at ocean waters or similar (the ancient Greek word, pélagos, meant 'open sea') but technically the pelagic zone is something that occurs in oceans, in estuaries, even in lakes. It is the name for the distinct ecological region formed within any column of water that sits away from shore, and above the floor (the latter being the benthic zone, the ecological region at the bottom of a water body which includes sediment and sub-surface layers).

All of which came in useful when I was thinking about the number of Cormorants vs the paucity of Shags on the Medway. Uber-simplistically, the favoured diet of the former consists mainly of benthic fish species, whereas the latter tend to go for the more pelagic fish species.

Part of why there are just a handful of Shag records each winter as against a jettyload of Cormorants.

Some of you will want to look away now; from this point on the post is purely about Cormorants, in particular their feeding behaviour, their loafing areas and their roosts. And their numbers- what is a jettyload?

Feeding



Cormorants are such a successful feeder they only need spend an hour or two a day hunting and can spend the rest of the time lounging around. They tend to get feeding out the way early as well. Many birds leave the Medway to fish elsewhere; in first hour after dawn good counts can be obtained from Queenborough as birds depart out into the greater Thames. At the same time anywhere along the southern stretch an odd bird or three might well be heading inland to feed and several also make their way into the tidal river via Chatham.

Some feed in my stretch, the southern Medway estuary. The texts state they do not often hunt cooperatively, but as I started watching closely I thought they they did it quite often. It took me a little while to work out what was actually happening.

Watch Bartlett and South Yantlet Creeks on a dropping tide in the morning, and there is a good chance you will spot one of these 'feeding flocks'. Each usually around 10- 50 strong (but up to 60 recorded), they raft together and several will be diving together- fish have become concentrated in the creeks on the retreating tide and these birds follow them out.

But watch more closely, and the birds do not arrive as a flock; sometimes in handfuls certainly, but numbers usually build. Nor do they hunt cooperatively, by taking turns in a rolling formation with birds at the rear overflying to take a go at the front- it is simply a melee- nothing frantic, simply each to their own. They are individuals coming together for the food, nothing else.

Loafing or roosting?

Cormorants are diurnal. They migrate during the day, they feed during the day. They need a safe night roost. The 'roosts' we see during the day are loafing areas- haul out spots for the well fed. A place to get rid of the wet as well; Cormorant feathers soak up water to help overcome buoyancy issues, and need time to dry. These loafing sites are deserted at night for the safe roosts. (Logically, in winter most loafing sites will be under water for part of the tidal cycle.)



Loafing spots can be as simple as a marker tower (Pinup Reach off Copperhouse is a good example), discreet as a shallow sandbank (Bartlett, Bishop, Sharpness- tail feathers dry better on sand than wet mud), as obvious as a wreck (the caisson off of Motney) or as straightforward as the actual roost site.



There are two main roosts on the estuary, both viewable distantly from the southern shore. Technically, they are both north shore sites, but they are as easy to count from over here as over there- so it seemed a shame not to(!)

~ Bee Ness Jetty

In the ornithological literature you will find Oakham Jetty, Kingsnorth Jetty and Bees Ness Jetty all listed as roost sites. They refer to the same thing- Bee Ness Jetty. Anyone interested in a potted history of what was, at just over 2.5km, Britain's longest jetty can read about it here. The jetty was abandoned after the site closed in 1977 and so became a safe roost, especially after sizeable sections collapsed, or were removed for safety reasons.

The present roost (shown in red) is on a distant section out over Bee Ness itself.



~ Grain Tunnels



At first glance, this structure is often dismissed as a derelict pier, but actually sits above the twin cable tunnels linking Grain with Chetney (if anyone knows the exact purpose, I would love to know) and was never linked to the shore. The tunnels are 1.7 km in length, and 2.8 metres in diameter- workers do access them, at that size I certainly wouldn't like to(!) Aerial views courtesy of Google Earth also confirm the 'jetty' to have two large open spaces- all in all a perfect, safe remote roost site.



It seems this has been overlooked in literature, perhaps because this part of Grain is often overlooked by birders; most published counts for Medway have been simply for Bee Ness.

Birds from both roosts will disperse in all directions. Watch them often enough and you start to realise whilst more tend stay in the Medway when waters are low and feeding is easy, most will still head out into the Thames estuary to the north and east of Sheerness; Cormorants are known to travel routinely up to 15 kilometres to feed- so this sort of distance is just a short daily commute for the Medway's birds.

Numbers



Historic counts reflect the increase in overall numbers seen since Cormorants started breeding inland in the south-east of England during the 1980s, with the increase in early autumn numbers being down to dispersing young. Before that, peaks were always in winter, and were British birds, mainly Welsh, also Scottish and north of England. Wales might seem an odd source at first, but Cormorants are not routinely pelagic- they like to stay in sight of land, so overland dispersal is just as routine as coastal.

Kent has rather more overseas ringing reports than neighbouring Essex and Sussex, which makes sense and hints towards the possible small but regular arrival of the subspecies sinensis from increasing numbers breeding on the near continent in the 1980s being the source of the new colonies.

Put simply, sinensis favours fresher water and breeds inland in trees, whilst carbo favours saltwater and the coast. I never really bother to assign to race here. They are usually way too far out to play with assessing gular patch angles (which in any event overlap slightly) and, besides, work on brood DNA in the new inland colonies has shown carbo to have got themselves caught up in small numbers. They just go down as Cormorant.

You can, however, record fairly easily the slow drop in numbers of young as they continue to disperse, and are replaced by more adults arriving to winter. Or seemingly winter. The term in 'BWP' is individual nomadism. Birds can and will continue to change sites during the winter. Some years see more birds than others, which is down to prey availability. Cormorants are fickle.

There has been a small collapse in numbers during the present decade, mainly down to small collapses out along that final kilometre of the Bee Ness Jetty. The safe roost capacity has been shrinking. It will be interesting to see what happens in the next few years. Historically, a jettyload was up to around 350 birds, 400 tops. During the past four years my roost counts (dusk/dawn) have never really moved much above the 200 mark, and the favoured stretch has been packed. (Grain has remained steady at around the 50 to 80 mark.)

If the crumbling continues, could they switch elsewhere? Probably not that easily. One or two alaready loaf on Kingsnorth Jetty, but that is usually atop the old lamp posts- Kingsnorth has been all too regularly disturbed by security patrols in the past. The patrols could be tolerated by gulls, which would rise and return when the danger was gone, so the handrails beyond Oakham island were claimed as a regular roost for Common Gulls when the power station was in operation.

Counting

Being diurnal, roosting Cormorants tend to congregate in number as dusk begins, and only a few stragglers leave returning until the final fifteen minutes before sunset. They do seem to start to leave at the first traces of dawn, and can be missed- a morning count following an evening add-up have tends to be down by about 10-20%.

Counts of the roost during other times of the day can be misleading. Counts in discreet areas of the estuary can seem not as useful at first glance- but do confirm feeding areas and can be hugely entertaining. In early November this year, on an uncovering tide just after dawn, I watched a number of Cormorants leaving the Bee Ness Jetty roost and ending up on the edge of Bartlett Spit as it began to uncover.



My notebook read as follows:

0 @ 06:50
14 @ 06:55- flew in together to land on mudflat edge.
19 @ 07:05- a further 5 flight in
32 @ 07:10- further arrivals take group to 32. Most are near to each other in loose gathering, one c. 50 metres away.
32 @ 07:23- first five enter water, rest stay on mudflat. Fishing commences.
1 @ 07:27- female Marsh Harrier crosses estuary from Bayford, keeps some distance from Cormorants over Bishop Ooze. Panicking waders cause all Cormorants (bar the individual) to take to air and drop into South Yantlet Creek out of sight behind Nor/Friars Saltings.
47 @ 07:30- 40, then a 3 and a 4, drift back out (clearly more had been loafing out of sight), none are feeding.
0 @ 07:31- all back into South Yantlet Creek as Marsh Harrier reappears, with a Buzzard for company.
16 @ 07:33- 14 and 2 back out, fishing.
34 @ 07:39- 27 swim back and haul out, another 5 remain in creek, 3 individuals, 2 close together, starting to dive. 2 more fly in onto flat.
34 @ 07:41- female Marsh Harrier passes back along South Yantlet Creek- 1 Cormorant takes flight from water and is harried for a short distance before MH gives up.
43 @ 07:42- 9 more swim in from behind Friars, to haul out some 60 metres west of main group.
42 @ 07:54- 1 from main group takes off and heads inland.
42 @ 08:02- all but two enter water, many starting to dive.
56 @ 08:04- the Marsh Harrier returns and 39  birds leave water for mudflat. At same time numbers on water are boosted by more birds arriving from behind Friars- 17 now swimming together.
56 @08:05- Marsh Harrier flights at group, all bar 12 enter the water and start to head behind Friars.
0 @ 08:07- remainder enter water and disappear behind Friars- waders also panic and flight. Marsh Harrier now seen to have claimed fish, and drops onto flat to eat. The Cormorants do not return.


All good stuff, but if I had simply counted and moved on I could have had any figure between 0 and 56 down as present:


Of course, each total would have been right for that exact minute, but it wouldn't have told the story.

Why, if you can spare the time, always worth staying and watching a spot for as long as you possibly can. I know most of us like to use the term 'birder' nowadays, but being a 'birdwatcher' can still be fun.

At the moment I have one burning Cormorant-related question. When you scan Bee Ness, it appears that birds are sometimes in pairs. Years ago it was thought that behaviour might be pairing up prior to returning to the breeding grounds, but that was dismissed in more recent texts- display and pair formation happens back at the colonies. So what is this apparent pairing all about? Answers on an electronic postcard, please...

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