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Stupid question time: why do birds migrate to breed?

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Old Friday 24th October 2014, 10:16   #1
lazza
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Stupid question time: why do birds migrate to breed?

OK, as I've become more interested in birding, a few questions about bird migration have started to puzzle me.

So, I understand that some bird species migrate away from the winter, and that this is a case of survival due to potentially fatal weather conditions and lack of food sources.

But why do the species that migrate north to breed do so in the first place? Are the places that they migrate from not suitable for breeding? Surely many areas maintain an abundant source of food year-round, so why do so many species go through this?!

And I realise, as a UK birder, I've been programmed to think of this cycle whereby many species depart in our Autumn and then return to breed in our Spring. But if the birds migrate south, are they not migrating to a second summer (during our winter) and could therefore breed again? Are there species that do this (migrate to the northern hemisphere, breed, migrate to the southern hemisphere, breed, etc.)? Or even for those birds that do not cross the equator, conditions in Ghana, for example, during the European winter, must be quite conducive to breeding...?

And what makes some migrants stop in (say) Spain, whereas other members of the same species might travel to Finland? Or on the other hand, what makes other species not stop in Spain, so that they all end up in northern Europe to breed?

I'm sure these are pretty stupid questions, the answers to which are taught in Year 2 at birding school, but I skipped a few years, so I'm none-the-wiser!!
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Old Friday 24th October 2014, 10:43   #2
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I can't answer most of your questions Lazza, but one thing comes to mind and that's daylight length. In the north there's hardly any hours of darkness through the summer months, so the likes of Ospreys can continue hunting for fish to feed their growing brood for most of the day. If they stayed near the equator, they'd have just around 12 hours of light each day.
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Old Friday 24th October 2014, 11:38   #3
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It's worth looking at the counterfactual case - what if there was no migration? Resident bird populations in e.g. the UK would be much smaller due to the shortage of food in the winter months (plus colder temperature and shorter daylength). But in the summer months there would be a large unexploited surplus of food, especially insects. By migrating, birds can take advantage of the seasonal abundance to breed, while back in e.g. Africa there is no such surplus, because (1) the seasonality is different, and (2) the resident bird populations are much higher so there's much greater competition for food. There's enough food in Africa for non-breeding birds to survive the northern winter, but not to meet the much greater food requirements of breeding. Essentially, birds can have greater breeding success by migrating to breed rather than staying all year in their winter range.

Apart from food, another factor might be that predation is lower in the north, due to predator numbers (both birds and mammals) being limited by lower prey availability in winter.
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Old Friday 24th October 2014, 13:49   #4
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Originally Posted by lazza View Post
But why do the species that migrate north to breed do so in the first place? Are the places that they migrate from not suitable for breeding?
Hi, yes.

Tropical regions are full of local birds adapted to local conditions which would out-compete them. Most northern birds on their wintering grounds feed on temporary, local or specialized food supplies, which would not support breeding birds.

Imagine what would happen if wintering birds in Britain would try to breed. Would flocks of Pink-footed Geese manage to raise goslings in English countryside? Would Sandpipers raise chicks on periodically flooded tidal flats? Or Snow Buntings breed in summer fields?

There should be more scientific or semi-popular material on the internet, too.
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Old Friday 24th October 2014, 20:38   #5
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Originally Posted by delia todd View Post
I can't answer most of your questions Lazza, but one thing comes to mind and that's daylight length. In the north there's hardly any hours of darkness through the summer months, so the likes of Ospreys can continue hunting for fish to feed their growing brood for most of the day. If they stayed near the equator, they'd have just around 12 hours of light each day.
I read somewhere that certain species need the extended day of an Arctic summer to get enough food to feed their young. A 12 hour day would not give them enough food to grow. But logically, they have to grow very quickly because they have to be on the wing before the season turns, so it is probably that evolution has selected the birds that can use a 20-24 hour day to feed up their young quickly and any that took longer died out.
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Old Friday 24th October 2014, 20:58   #6
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Imagine what would happen if wintering birds in Britain would try to breed. Would flocks of Pink-footed Geese manage to raise goslings in English countryside? ....
In this instance, yes, feral Pinkfeet can, and do. But the other examples you gave don't work
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Old Tuesday 28th October 2014, 09:14   #7
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Well, I guess that's all fairly logical, although I hadn't thought about day length being a factor.

Do any species breed twice, once in the northern summer and again in the southern summer, or is this just too draining?
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Old Wednesday 29th October 2014, 09:39   #8
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Do any species breed twice, once in the northern summer and again in the southern summer, or is this just too draining?
Never heard of this happening, but I don't see why it couldn't in principle at least with species with a fast brood turnover. There's claims that Quail can raise a first brood in the Mediterranean and a second in northern Europe, which is as close as I've heard of to separate-location breeding like this. With larger species it couldn't happen, as raising a brood and completing a full moult takes more than six months. Also some species (e.g. Barn Swallow) moult in the wintering area, which would make breeding there unworkable.
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Old Wednesday 29th October 2014, 20:23   #9
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Well, I guess that's all fairly logical, although I hadn't thought about day length being a factor.

Do any species breed twice, once in the northern summer and again in the southern summer, or is this just too draining?
As a general rule no, and I don't know of any exceptions. While resident tropical birds may have extended breeding seasons, and some species in the wet Neotropics have the capacity to breed at any time of the year, migrant birds have to spend a great amount of energy during both spring and fall migration and thus restrict their breeding activities to the summer period. In fact, to reduce weight and thus energy expenditure during migration, the reproductive organs in migrants shrink to a fraction of their breeding-season size.

It's generally accepted that migration in most species evolved as tropical ancestors moved toward temperate regions to take advantage of the brief superabundance of food (and extra daylight hours) and reduced competition during the summer, but there aren't enough resources to support all those species during the winter so most of them return to the tropics for overwintering.
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