I was sailing across Christchurch Bay off the Dorset coast last Friday and Sunday (2nd & 4th Sept) and observed many small groups of swallows heading NORTH just as one sees in the Spring.
Does anybody know why they would be doing this?
Probably just looking for food. This time of year they seem to feed in large flocks.
I'll put money on the wind that day being from the north. Birds will always fly into the wind in preference, just like a jet has to take off into the wind. They both need the lift that the headwind gives them. This means that when we see birds flying south it just means that the wind is southerly, not that they are necessarily migrating. This makes a nonsense of birders recording "visible migration" - they are probably just seeing feeding movements.
The wind was light and from the west on the 2nd and from the south-west on the 4th. Birds normally take-off into wind for the reason you state but to continue to fly into a headwind will reduce the ground speed by an amount equal to the wind speed and require much more energy to get to their destination.
Sorry Bob, but in my opinon there is very little truth in your comment here.
Many birds will fly into a head wind in preference to flying across the wind if they are heading any distance, but when they are heading to feeding sites (or roosting sites) they will head in the direction they need to regardless of wind direction.
"Visible migration" is something that undoubtably can be recorded, although the observers need to have a reasonable knowledge of the type of species typical seen moving around from their chosen viewpoint, and the numbers concerned. When a typical couple of hours early in the morning will produce sightings of 10-20 Chaffinches flying in random directions, it is reasonable to assume that a count of (for example) 35 moving south-west, with many of them high up, is mainly passage movement - especially if 12 are also recorded moving in other rando directions (this 12, and perhaps a few of the others, are the local birds moving around).
Visible migration of Swallows during September may consist of wave after wave moving through a site during the day, with almost all of them moving in a southerly or south-westerly direction, regardless of the wind direction (though many may head SW, or even W, if this is into the wind). I have observed this on many, many occasions, and see no reason to consider it to be anything other than visible migration (especially as similar movements often continue for several days).
Confused by Christchurch in NZ?
Hope you don't mind facetious suggestion - I couldn't resist it. I hope you get an answer to your question. I was delighted to see swallows still around at the weekend, on Bull Island, in Dublin. I haven't seen any in London, where I live, for a good while.
... London Wetlands Centre at Barns is still reporting swallows, admittedly en route to sunnier climes.
I saw loads of swallows in Mid-Wales (near Lake Clywedog) on Sun 18th Sept, and was puzzled as I thought they should have gone before then. My memories of previous years must be wrong.
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The most likely reason for the observation of Swallows flying north in the original post, is that for whatever reason they had decided to abandon a crossing of the channel and were heading back towards visible land.