Wednesday 1 February 2023

Are the Brents playing it by ear?

Strong overnight SW wind eased during the morning. Several showers but not too heavy.

South Shore (MD)
I went down relatively early to see if anything had been blown in. If there was, I missed it.
Rock Pipts, managed to hear two today. One on Red Nab and one near the waterfall.
Kittiwake 21 - 2 adult and a 1st winter on No.1 outflow. 3 adult and 17 x 1st winter in the harbour. This is a short clip of the birds on their pipe.

Redshank 235 gathering around No.2 outflow. This is some of them with Wigeon behind.

North shore - low water 15:15 (MD)
Writing last night's post got me thinking. It made perfect sense for the Brent to arrive in the skear corner, just as the tide had started coming in yesterday. But how would they know when the tide had turned when they were feeding over 400 metres away? The only solutions that I could come up with was, that it was just pure chance yesterday (and on Sunday), or perhaps they were using their ears. With these SW winds the waves change between the tide going out, when the water is moving in the opposite direction to the wind, and when it's coming in and moving in the same direction as the wind. In the latter, the waves break nearer the shore and the general sound of the sea is different. I am conscious of this phenomenon, but doubt if I could use it to confidently identify when the tide has turned. But when your whole life depends on the times and height of the tides it must be advantageous to be so well attuned (it would only need the group leader to be this aware).
So I went down again today to see what happened. Would the Brent again arrive for a drink and a bath at low water?
It was easy crossing the rocks today there was no one else around and two large groups of Bent were feeding some distance from me. Even so, looking back from the shore, both flocks had taken flight, nothing had spooked them, they just decided to move to a different feeding area. 

There is plenty of weed all around these rocks, so they can land and feed almost anywhere. There is no need to fly out to the waterline. On my way across the groundwater run off draining into the skear corner I tasted the water. It wasn't as salty as seawater, but too salty for my taste. I'm sure the water further up shore where it drains through the rocks would be less saline, so the Brent shouldn't need to fly here to drink.
I strolled around the skear till I could see the tide was just starting to come in. The wind had caused it to be a little early, it was 15:08. No sign of any Brent on the water. Then they arrived in two flocks, and settled directly in the run off channel in the skear corner.
There were 
Pale-bellied Brent 64
Dark-bellied Brent 1 at least
Although they hadn't landed in the water, they were at exactly the same place as yesterday, the tide had gone out further today. A few had a drink and they all has a preen. These are just some of them.
They stayed like this for about five minutes (enough to confirm that the tide was indeed coming in?) then flew back to the rocks to continue feeding, again in two separate flocks plus two birds together.
I now believe that they are deliberately arriving at low water, but perhaps the bathing and drinking is just a byproduct. They must have an internal clock that is roughly tuned to the cycle of the tides (it would be impossible to be accurately tuned, as the cycle varies and that variation is particularly acute during neap tides - it isn't just about the lunar cycle). Perhaps they did use their ears to identify a change in the tide, but turning up as they did would confirm it, and allow them to reset their "clock". It would also show them that today's tide went out further, which means we are moving back towards spring tides. All pure conjecture of course, and we would need several days of observation with no human disturbance to be more confident. But, I'll bear it in mind and note if future observations do or do not support this premise (MD)
As I was leaving, it was still quiet and both flocks had converged on the weed on the inshore rocks, where they had to feed last year as the outer rocks were weed free.

On the skear:
Knot c750
Redshank 81 in the stream where the Brent will later rest.
Eider 2 male
Male Eider resting on the skear pebbles