Passerine birds, also known as perching birds, that migrate by night tend to fly faster in spring than they do in autumn to reach their destinations. This seasonal difference in flight speed is especially noticeable among birds that only make short migratory flights, says researcher Cecilia Nilsson of Lund University in Sweden, in Springer's journal Behavioral Ecology and Sociobiology.
Nilsson, in a group led by professor Thomas Alerstam, used a tracking radar to measure over three years the speed by which birds flew over Falsterbo Peninsula, a bird migratory hot spot in south-western Sweden. The seasonal differences they found correspond with those recorded for other nocturnal passerine migrants at other sites in southern and northern Sweden.
The seasonal differences in airspeed are more noticeable among short distance migrant birds. Nilsson and colleagues suspect that such birds fly faster in spring because they have a greater urgency to reach their breeding grounds first and to choose the best territories, mates and other resources. While the time savings made in spring might seem miniscule, these remain important because they influence the arrival order of individual birds. In autumn, the birds take things more slowly because they are not as pressured to reach their winter grounds.
Wind is the one weather condition that influences birds' decisions the most about when to take off. In fact, Nilsson and co-authors discovered that passerine birds can actually fine-tune their flights to make full use of winds, making their flying and subsequent migration easier. Short distant migrants have higher ground speeds (speed relative to the ground below) than airspeeds (own speed relative to the air around the bird) in both seasons. Hence, these birds make use of wind assistance. In contrast, long distance migrants often travel with airspeeds exceeding ground speeds, resulting from flying in headwinds, in autumn. These findings corr
|Contact: Laura Zimmermann|