The Pied Wagtail roost near my home is still going strong, although slightly less strong than earlier as there are now c200 Pied Wagtails roosting there.
At the weekend I made a couple of early morning visits to watch the birds leaving the roost which they did over a period of just over half an hour each day. There is no mucking about as the birds leave the roost – they head upwards and outwards at pace in small groups from singles (not many) to c20 birds. The groups looked as though they knew where they were going too. Only a few singletons did not call as they left the roost.
Do they know where they are going when they leave? How are the small groups formed? Are they mates? Are they the same groups each morning? What makes some birds leave first and some last? Are the early leavers the same ones each morning?
Little Trotty Wagtail – a poem by John Clare.
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Light intensity could be a factor in timing re triggering departure of the first bird(s). If so, there could/should be a neat correlation with your local civil twilight times for clear mornings.
In turn, if that’s true, generally clouded mornings should result in a much later flight (least ten minutes later) compared to what might be expected for clear mornings.
As for the late departing individuals, perhaps they are the ones that are roosting in greater shade compared to their early rising friends.
[Armstrong did some work on Wren roosting and light intensity — a strong relationship if I remember but the reference is not to hand and needs checking.]
But rereading your previous blog, you describe a roof-top roost? No variation in shade there?
So what I suggested above, re the late departures, can’t apply. So it’s wrong platform, wrong station for that one.
Mark says that the birds eventually fly down into the local trees and bushes to spend the night, so your thesis is relevant.
A similar roost near us is the same. Hundreds of Pied wagtails congregate on the top of the local hospital roof, all neatly spaced as described by Mark. They then plunge all at once into nearby bushes. They chatter away for a while and then fall silent. Oddly the roosting area is well lit all night so I don’t know what stimulates their departure.
Thanks Richard. I should read more carefully.
100’s of P W’s, what a wild sight and sound that must be.
Internal clocks may be the thing that triggers departure in your well-lit roost site? Or, perhaps natural light still applies; it has a wider spectrum than artificial light. Most birds can see UV, so it’s possible that things are happening around that wavelength. (he says)
The subject needs a tame behavioural ecologist.
They might have tiny brains compared with ours but why shouldn’t they know exactly what they’re doing each day.
Birds are far more developed than humans.
They have learnt how to live with only the feathers on their backs.
They have no negative impact on this Earth.
They don’t need the materialistic life that we have developed.
They only need to know when they’re going to leave the roost, which group they’re going to join, who their mates are, what they need to do to survive the day, and how to wag their tails.
Oh to be a Little trotty wagtail.
I am sure that they know where they are going but do they know that they know? ‘Metacognition’ was supposed to be the ultimate defining difference between humans and other animals but, like every earlier suggestion, it has been shown not to be unique to humans. Apparently a level of metacognition has been shown in crows, so why not pied wagtails? Or maybe we should just stick with John Clare.
Alan – it’s always a good idea to stick with John Clare.
White Wagtail was one of the classic species used nearly fifty years ago to test the thesis that communal roosts serve as centres for information exchange. The idea is that birds that have fed well on the previous day will leave the roost in the morning and purposefully head back to their good site, while those that hadn’t found such a good area will follow them and hope for a better day. This explains Mark’s observation of small groups leaving the roost ‘at pace’ in the morning.
I’m not an expert and I can see how this applies for species like Starlings, that usually feed communally in winter, but I’m not sure how it works for (insectivorous) wagtails that often defend feeding territories and expel conspecifics.
beheco.oxfordjournals.org/content/7/1/118.full.pdf
https://en.wikipedia.org/wiki/Communal_roosting#The_Information_Center_Hypothesis_.28ICH.29
These might shed light on the information exchange subject. Looks a lot more interesting than the possible effects of increasing light intensity. But it’s brain bending reading. Let Clare rule.