General Migration Question (1 Viewer)

Just seen my first Willow Warbler of the year and, as always, marvelled at the possibility that some of these little birds will have found their way from South Africa. However, pondering on the whys and wherefores of migration made me realise that I don't fully understand what is going on; my understanding is that these birds overwinter (our winter!) in warmer climes where life will be easier and food more abundant, hence they choose a South African summer. They return to us as our days lengthen and temperatures rise providing more food and longer to eat it.

So my questions are:

- why don't they breed in South Africa in November/ December?

- Why do all British migrant birds move North to breed in spring /summer, (whether they go from Britain to the Arctic or from Africa to Britain?)

- Is the assumption in the last question correct: are there any birds which visit Britain in our summer and fly South to breed in a Southern Hemisphere summer?

I'm continually amazed at how much I don't know or understand!

Roger

I've always understood that it was longer daylight hours and less competition for food and nesting places that was the benefit of migration. So staying in Africa... actually west and North west Africa not South Africa.. doesn't offer the same opportunities as flying north.
Migration is thought to have started (or expanded) at the end of the last ice age with the retreat of the ice sheet allowing birds to fly progressively further north to take advantage of the openings being uncovered...literally.
Good book on migration is 'Time to Fly' by Jim Flegg isbn 1-904870-08-2

You are kind of right, and if you grab a copy of the last BirdWatch magazine there's a bit in their I wrote on why birds migrate! Basically migrants are really forced out of the country by the lack of winter resources, as they can't compete with our residents. Similarly, there are more resources here in the summer than in Africa, so it is worth the move north.

In answer to your second question, if you look at an Atlas, you'll see why things come north and don't go south. At the same latitude south as we are north, there really isn't much, so its partly a case of having room to breed. Add to that our Gulf Stream warmed climate and we are rather lucky here. In the Americas where the southern habitats are much lusher over a large area (compare the lush Amazon with the rather barren Sahara at a similar latitude) then southward (austral) migration is commonplace.

I know this is all very brief, but hopefully it will answer the basics of your question.

Mark Grantham
BirdTrack Organiser
www.birdtrack.net

Thanks both of you, I realise now that looking at a globe or atlas would have been useful, never really thought that there was nowhere to go in the south eastern(ish) hemisphere. The fact that birds in the American continent do go south answers my question.
Are there any birds that breed twice a year, once in each hemisphere? Do we assume that that would simply be too exhausting to be worth the extra benefit of twice the number of offspring?

Mark Grantham said:

In answer to your second question, if you look at an Atlas, you'll see why things come north and don't go south. At the same latitude south as we are north, there really isn't much, so its partly a case of having room to breed.

Click to expand...

Actually, the same principle seems to operate in the western hemisphere as well, at least where austral migrants are concerned. Many birds that breed in the southern half of South America are partial migrants, in that some members of the population remain in the tropics year-round, while others migrate south into Argentina or Chile to breed. One reason that has been put forth is that because there is relatively little land mass in the southern half of the continent compared to the wide northern half, there isn't enough room or resources for entire populations to migrate and breed in this area.