Tuesday, 2 January 2018

The fifteen minute rule

Joined by a birder for a very short stay at the end of Horrid. You know the sort. Say they're a 'regular'. Come here two or three times each winter. They were in the mood to tell me all what was out there. Made my mood a mite mischievous, especially when he hinted at the estuary always being a bit, well, "dull".

"Not much to see here though. Twenty-three Greasies. That's about par for the course for me here."

"Forty-seven." I replied

Pause. Counting noises. "Well I can manage twenty-four. Just"

I smiled. "You counted that stretch in under three minutes. I usually take fifteen, because the distance between the two points is fifteen fields of view in my 'scope, you really need a minute for each.Also lets you get your eye in for any distant birds"

Stunned silence. So, I fill it.

There's more chance of the bird being under than on the surface at the mo'. They're feeding on shoals moving up with the tide. Now the average Great Crested Grebe dive length is under for thirty seconds or less; but they might stay on the surface for much less than that.  So, watching each field of view for a minute means you should get them on the surface before you pan left and repeat (though you will still miss the odd one that swims submerged between fields).

"Seems like hard work to me."

"It is. The bad news is if you're checking for Great Northern Divers, they can routinely be under for up to a minute as a rule. Red-breasted Mergansers? Bit of a bummer, up to thirty seconds the norm, but they can chase prey for up to about the two minute mark. And they can move between fields a lot. Only found a couple out there so far, and only seen a couple of times. Goldeneyes are easier though, they're usually only about twenty seconds a dive."

"Haven't got time for all that." was the short version of their reply.

"Didn't think I had, either, when I started birding here all the time. Problem is, all these sorts of species are mainly seen here on the water. You can't really get by with seawatching tactics. These birds made an effort to come into the estuary, now they're here, they're settled. Little need to fly anywhere."

The regular birder went his way. I was sure he muttered twenty-four as he went. Nice chap. I made a mental note to explain how the true number was even higher than forty-seven, because some loafing birds had already drifted away into sheltered creeks such as South Yantlet. I'm really sure he'll appreciate more useless tips from me.

-------

Well, the conversation didn't exactly go like that, much more civilised, wouldn't have made a good read. But the important point was taking a count as Gospel. If someone doesn't like identifying distant birds, then counts will be low, and that is often the case for occasional visitor counts you see from around the estuary. That knocks quite a few off the total, then missing the feeding birds adds to the difference. Birds might dive for half-a-minute but that doesn't mean they're up on the surface for half-a-minute; small grebes might only be up for less than ten seconds before diving again; why some birders can never turn them up. While at the same time avid readers of social media start to think those lower totals are really what is there. As with the spin in the opening preamble, don't take everything you read as Gospel. Count for yourself.

'Birds of the Western Palearctic' has sets of average dive times for most species, often differing based on depth of water being utilised; the following table is my own rough and ready calculator based on those tables, the depths the birds frequent here on the Medway over the high tide and my own timings. The list has all the commoner diving birds, but that doesn't mean they're all here- one I haven't seen here for about three decades. The main thing is it might inspire some to think about such things.


One of the best B.W.P. entries is for Great Northern Diver. Studies have shown they can spend excessively long times under- up to eight minutes. But these longer periods were not thought to be due to feeding behaviour, more likely an evasive/escape behaviour when feeling pressured. To date this winter no reports from the old Docks basins, but they have been reported via Birdtrack, so only a matter of time and a good blow I reckon, then it'll be interesting to record how long dives are there, and how often the longer ones actually see bird(s) coming up with prey. Just feeding or evading birders? Something to brighten up a dull day. The more you read, the more you want to check things out.

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