Mortality of Transatlantic Passerines in the UK (1 Viewer)

The American warblers, vireos and other passerines that reach the UK in autumn are typically first-year birds and the received opinion is that they don't survive very long. I'm wondering why not. I can understand why the larger birds like cuckoos, nighthawks and Mourning Doves peg out. The cuckoos apparently can't metabolise the toxins in our caterpillars; the others may use up so much of their fat reserves on the crossing that they reach the point of no return - they can't eat enough to replenish the loss before they die. I'm sure that's true of some of the smaller birds too, but I wonder whether the problem is overstated.

As I understand it, the metabolism of warbler-sized birds is so fast that they would die within a day or so. Some do disappear almost immediately, yet more are twitchable for several days - even up to a week or so - which would suggest that at least they don't suffer an insurmountable problem on that score. Yet why are there no spring records of birds apparently returning from further south like there are with, say, Richard's Pipits? Such spring records as I can think of are all on the NW coastal areas of Britain, which suggest birds newly arrived from America rather than ones migrating on this side of the Atlantic. Any thoughts?

There are occasional spring passerine records - what was that singing American warbler somewhere in Scotland many eras ago? More generally though, given that the total number of American passerines turning up in a typical autumn is not much more than a mere handful, it could simply be that returning spring birds are never found. If we assume they head south from the UK onto the continent, there is no reason that a northward migration should necessarily bring them back across into the British Isles ...and in that case, ten or twenty scattered passerines (not concentrated into any headland, etc) wandering north through the massive European land mass are not going to be found in all likelyhood

Good theory Jos. I've often wondered about returning passerines and thought that it might be possible. After all - Northern Wheatears migrate from Africa to Canada. The one doubt I have is that it doesn't happen with Eastern Warblers, why don't we see returning Yellow-broweds. There are hundreds of those in Autumn in Western Europe yet none in spring. Do they all end up in the sea?

Darrell

Other ideas (not exclusive):

many American migrants winter in rain forests, while migrating south of Europe they get to Sahara and savanna.

There are less Amercian migrants than Siberian migrants, so even fewer in spring.

Darrell Clegg said:

The one doubt I have is that it doesn't happen with Eastern Warblers, why don't we see returning Yellow-broweds. There are hundreds of those in Autumn in Western Europe yet none in spring. Do they all end up in the sea?

Darrell

Click to expand...

Theory Doc has an answer for that one too

I don't see a big problem in the idea that all these eastern vagrants, i.e. the Yellow-browed Warblers etc, head south, then reorientate and head north-east from whereever they ended up wintering, i.e. not returning to Western Europe at all.

There would appear to be theories at the bottom of your jargon Jos.

I would agree with you that low numbers & large land masses are the most likely reason for low records, together with the alternative migration routes to and from sites.

Seems the most logical reason.

But think of Greenland Wheatears - they make it back and forth across the Atlantic ok. Not quite as far as some of the warblers etc, but not far short. Also, bear in mind that many will be ship-assisted.

There have also been things like American Robins that have wintered in the UK (didn't that Grimsby bird last until spring before getting eaten?). There have also been several wintering Yellow-broweds that have lasted until spring and then presumably deprted too.

The thing about Spring migration is that most birds make it in a very few giant hops, as they're in a rush to get back to a breeding site and claim a territory. So, if something leaves Western Europe/NW Africa then it can be in Northern Europe, where it probably wont be found, before it might land again.