Wednesday, 28 April 2010

Eight things I didn't know about swifts

The first Swifts are back! These extremely aerial birds spend just three months of each year with us, from early May to early August - their breeding season - but their continuous flight and noisy chases makes them obvious and fascinating birds to watch. They eat, sleep and mate and collect their nest materials on the wing. I recently read "Swifts on a Tower" by David Lack. A very informative and entertaining book, it is based on the observations of David and Elizabeth Lack and their students. They observed nesting swifts on modified nesting holes in the tower of the Oxford University Museum of Natural History, a building of fame due to being the place where the famous debate on evolution took place between Samuel Wilferforce and Thomas Henry Huxley seven months after the publication of the Origin of Species.
Museum of Natural History (Oxford) the central tower ventilation shafts (10 on each side) are the entrances to the Swift nests. Photo by Michael Reeve, licensed under Creative Commons, from Wikipedia.

Based on this book and more recent research I have compiled this list of facts that I didn't know about swifts.
  1. Their young can survive up to 21 days (!!!) with no food. Their growth patterns match the weather so that they stop growing during cold, wet or windy weather. The length of time they take to leave their nests depends as well on the weather, the earliest leave at 5 weeks, the latest at 8 weeks.
  2. Eggs can get cold (i.e. left by the adults without incubation) for at least two days and survive.
  3. Swifts - and possibly other bee-eating insects - can distinguish drones from bees and drones are the ones they eat, avoiding getting stung by workers.
  4. Swifts fight for their nests cavities by grasping each other's claws. The winner is the individual that can get on his back, so that it can push the other out of the nest.
  5. Adult healthy swifts can indeed take off from the ground. It is the recently fledged individuals in not so good condition that can't (they leave the nest early and their wings are not fully grown.
  6. Once fledged, young swifts are completely independent and receive no further parental care. They return to their summer grounds to breed when they are 3-4 years of age.
  7. The young join non-reproducing birds to sleep on the wing high up on the sky.
  8. Swifts are the fastest birds during self-powered flight (the peregrine "free-falls" using gravity to accelerate) reaching over 110 km/h in their 'screaming parties'. More info here.
More information:
For an informative article by the 'guardians of the tower' see here.

Henningsson, Per, Johansson, L. Christoffer, Hedenström, Anders (2010) How swift are swifts Apus apus? Journal of Avian Biology, 41: 94-98. Here.
Check the fantastic photos of swifts on flight by Jean-Francois Cornuet here.