Thursday, 24 May 2012

The hazards of migration

The hazards of migration were brought home to me the other day by a photograph that had been sent in to our offices. The photograph showed a large number of swallows and house martins, all huddled on the narrow ledges of a building. These birds, newly arrived in the west country, had flown into bad weather and so had sought what little shelter they could find in order to sit out the heavy rain that was falling.

More widely this spring, we have seen birds held up on migration by poor weather in the Mediterranean. A blocking weather system had halted the migratory journey and forced the birds to sit out the bad weather, while using up vital reserves. At least one of the BTO’s satellite-tagged cuckoos seems likely to have succumbed to the weather. It was charted heading North on its spring migration but ran into bad weather in southern Spain; there it stayed, the signal from the tag suggesting that the bird had died.

You might think that migration was a fairly routine affair for birds, like swallows and martins, able to feed as they went along. Research suggests that even these species have to fatten up ahead of their migration, taking on reserves to help get them across barriers like the English Channel, the Mediterranean and the Sahara Desert, where feeding opportunities are presumably scarce. It is harder still for those species that do not feed on the wing and which have to stop-over to take on more ‘fuel’ at various points along their route. These ‘stop-over’ sites are important and many will be used on a traditional basis. For example, two of the BTO’s cuckoos, taking an easterly route, stopped at the Po Delta in Northern Italy on their way south. This suggests that we should look in more detail at the region to establish its role as a stop-over site for cuckoos and other migrants.

Even when the birds reach their destination they may face problems. There is a pressure to arrive early on the breeding grounds in order to get the best territories, but if you arrive too early then the weather may prove unsuitable and you may die. This year, the cold and wet spring has caught many birds out and even resident species, like tits, seem to be struggling. This time last year, we had large numbers of reed warblers and whitethroats on well-advanced nesting attempts; this year they have only just started to get going. Of course, it is only at the end of the year that we will really know just how poor the season has been. Until then, it is fingers’ crossed for some better weather.  

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