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Old Thursday 16th November 2017, 18:40   #101
dantheman
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Originally Posted by Nutcracker View Post
True, but is that because they are strictly night migrants where they have the option to be, or because they are migrating too high up to identify? They'd only have to be 100 m or so above ground level to be effectively unidentifiable; if they're 3 km high (as many small passerines can be), no chance!!
Be interesting if someone has access to the data from that vismig from a tethered ballon experiment back in the past in the Sheffield area.
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Old Friday 17th November 2017, 08:47   #102
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Originally Posted by Nutcracker View Post
True, but is that because they are strictly night migrants where they have the option to be, or because they are migrating too high up to identify? They'd only have to be 100 m or so above ground level to be effectively unidentifiable; if they're 3 km high (as many small passerines can be), no chance!!
It's getting a little off topic but still I feel I'd like to answer.

You are right, how can you be sure they don't just fly higher than the others? Well, you can't on a certain day. Possibly one shouldn't be too dogmatic about the whole thing. Birds can be very flexible. Still there are species you would call (predominantly) nocturnal migrants. As I said before barrier crossing is different. This recent paper is adressing the very question:
Adamik et al. 2016

They analysed geolocator data of four predominantly nocturnal migrating passerine species and did find regular diurnal movements over barriers but not on regular migration north of the Mediteranean.
Surely not a proof against your hypothisis but a good indication...
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Old Friday 17th November 2017, 10:00   #103
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Finally ! had one over the Canley area of Coventry this morning
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Old Friday 17th November 2017, 10:45   #104
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This recent paper is adressing the very question:
Adamik et al. 2016
Thanks! Very interesting
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