A question for those in the know?

[astarsdad]

Why don't our summer visitors (swifts, swallows, warblers) from warmer climates breed when they migrate back to those warmer climates?
Also, those warmer climes have non-breeding visitors in their summer, such as the forementioned, so why do we not have non-breeding summer visitors?

Terry

[Adam Cheeseman]

Originally Posted by astarsdad

Why don't our summer visitors (swifts, swallows, warblers) from warmer climates breed when they migrate back to those warmer climates?
Also, those warmer climes have non-breeding visitors in their summer, such as the forementioned, so why do we not have non-breeding summer visitors?

Terry

Purely guesswork but the first one I would say after migrating from and to their wintering grounds and raising one or more broods in between I would say they would welcome a rest and be able to recouperate and moult.

Second the non-breeders in warmer climes are most likely wintering birds that have come from their summer breeding grounds e.g. Swallows.
Not summering birds, but birds that come here in winter from the North are effectively doing the same as our summer migratory birds by moving south to warmer climes for the non-breeding season.


Cheers,

Adam

[astarsdad]

Thanks for the reply Adam.

I did think of the opportunity to recuperate as a reason, but what about the species that have more than one brood here in the northern tropics? Having more than one brood would suggest that that is not the answer.

The second part of my question is...why is it not visa versa that we have non breeding birds in our summer as Africa has non breeding birds in their summer (i.e. our summer breeding birds)

Terry

[david156]

I'm sure there is one summer visitor that doesn't breed here, but breeds in its wintering grounds but can't think what it is. Will do some research

[Adam Cheeseman]

Originally Posted by astarsdad

Thanks for the reply Adam.

I did think of the opportunity to recuperate as a reason, but what about the species that have more than one brood here in the northern tropics? Having more than one brood would suggest that that is not the answer.

The second part of my question is...why is it not visa versa that we have non breeding birds in our summer as Africa has non breeding birds in their summer (i.e. our summer breeding birds)

Terry

Some birds will only have one brood regardless, whilst others will take advantage of the conditions in their summering grounds if they are favourable to have more broods.

Non-breeding birds in Africa in their summer is our winter. I expect the same applies to birds that breed further south in the southern hemisphere and migrate north to their wintering grounds.

Cheers,

Adam

[poschiavanus]

The basic reason is competition. There are resources available in the Northern temperate zone in summer which can be exploited by those species able to do so. Migrants are therefore able to exploit a resource for breeding which non-migrants can't.

The advantage of moving into an area with lots of residents and competing for the same limited food source during the breeding season will be far less.

Note that insect feeders will have to move far enough south for insects to be on the wing throughout the winter anyway. Some hirundines (thinking of the non-British Crag Martin) only move to the Med or N. Africa.

There are probably more complex aspects of this: reflecting paleo-geographies, and where given groups of birds evolved.

[david156]

Originally Posted by david156

I'm sure there is one summer visitor that doesn't breed here, but breeds in its wintering grounds but can't think what it is. Will do some research

I remembered!

Sooty Shearwater is a summer visitor to our seas but breeds in the southern hemisphere during our winter (there summer). Another is the Great Shearwater which winters during our summer of the coast of NE america but breeds on islands in the south atlantic during our winter.

Cheers David

[Sledge]

When the young are here, after hatching and fledging, the parents look after them and make sure they survive. Perhaps then moving to a warmer clime gives the now-solitary young a comfortable period to feed and strengthen while they grow to adulthood and become even more able to cope with life.

[astarsdad]

Thank you for all the replies.

Ialso posted this question on the BBC Springwatch forum. I have come to the conclusion that because there being a much greater proportion of land in the northern hemisphere than in the southern hemisphere, there is a greater availability of food for their broods and less competition.

Terry