Saturday, October 29, 2011

It's all in the fat

Much about bird migration remains a riddle, and the puzzle persists when, for example, landbirds undertake open-water crossings that require hour upon hour of sustained flight despite the availability of seemingly safer terrestrial routes or man-made structures on which to land. In the Gulf of Mexico, there are hundreds of active oil platforms and peregrines are known to use them, leap-frogging from one to next using them to rest, cool off or as place from which to capture prey.

Swordpress map and YouTube video showing rigs 1942-2005

Despite the presence and safety of the platforms, some birds crossing the Gulf of Mexico apparently ignore them. For example, after spending 3 days on the Louisiana Gulf coast, one of our birds (28 E) crossed the Gulf flying 540 miles in 11 hours. She spent that night on the Yucatan Peninsular, then once again made a non-stop flight of 430 miles in 12 hours crossing the Caribbean sea to Honduras where she remained relatively sedentary for the next 36 hours. Then ignoring Nicaragua, Costa Rica and Panama, she again traversed the Caribbean Sea making landfall in Colombia 650 miles and 28 non-stop-hours later.

Between October 19 and 24, she flew more than 1850 miles and crossed 1620 miles of open ocean in 51 hours of sustained flight.

1850 miles and three open water crossings (totaling 1620 miles) in 5 days

Fat reserves are critical to lowering the risks of migration in general and managing the risks of strategies like open water crossings in particular. Fat carries twice the energy compared to the same amount of protein or carbohydrate. So storing fuel as fat is smart - birds can reduce the weight they have to carry despite loading up on their on-board fuel. Smart too, because burning fat produces water - not enough to prevent dehydration, but enough to slow it down. For Arctic-nesting peregrines that lose body mass whilst raising broods of young, getting fat fast could make open-water crossing safer than it appears.

28 E feeding her 3 nestlings