Warmer spring temperatures and a longer growing season are proving to be good news for barn swallows. Over the past 35 years these birds have responded to global warming by taking more time over rearing their chicks.
Around two-thirds of swallows produce two broods of chicks per year, one in April and another in July or August. Anders M酶ller from the Pierre and Marie Curie University in Paris has monitored barn swallows in the Kraghede region of northern Denmark since 1971, noting the date eggs were laid, and clutch and brood size.
Based on 2705 pairs of birds, M酶ller found that the time between clutches increased between 1971 and 2005. The birds started to breed earlier and gave themselves more time before the second brood. Timing between clutches increased by an average of 8 days (19 per cent) and swallows tended to produce more fledglings (Behavioral Ecology, DOI: 10.1093/beheco/arl051).
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M酶ller believes that this relaxed breeding behaviour is down to climate change. Between 1971 and 2005 the mean April temperature in Denmark rose by 2.2 掳C. The growing season, too, is longer.
鈥淭he [temperature] changes we are seeing today are much more rapid than in the past and the question is whether these birds will be able to keep up,鈥� says Stuart Bearhop, an ornithologist at the University of Exeter, UK.
