Swallow populations fluctuate and are affected greatly by weather.
Why swallow populations fluctuate
Swallows require rain for wet mud for nest building and for encouraging the abundance of insect prey. Cold periods and prolonged rain during the breeding season can however, reduce the numbers of flying insects and lead to high levels of chick starvation.
Independent of weather-related fluctuations, there have been widespread declines in swallow numbers across Europe since 1970. The cause(s) of these declines is unknown but potential factors are discussed below.
- Climatic changes in the swallows' African winter quarters and on migration routes may be having a serious impact. Research has shown that swallows are returning to their breeding areas in poor condition and are laying fewer eggs than previously.
- Adverse climatic conditions in Europe may also be having a detrimental effect. Cold springs with late frosts can cause problems for swallows, as do exceptionally hot and dry summers. In the latter case, pools dry out, reducing the numbers of emerging insects, and nestlings die from heat exhaustion and dehydration. The expansion of the Sahara desert may be making this formidable barrier increasingly difficult for swallows to cross.
- Changes in farming practices throughout Europe may be reducing the numbers of nest sites and the quantity of flying insects. Swallows like to forage over grazed pastures, and the loss of cattle grazing has negatively affected swallows in some regions of Europe.