- Watch these six dots move across the map and be moved yourself: this is a story about coming of age, discovery, hardship, death and survival.
- Each dot is a tag attached to the talon of a Swainson’s Hawk. We follow them on their very first migration, from northern California all the way down to Argentina.
- After one year, only one is still alive.
Discovered: destination Argentina
Young Swainson’s hawks were found to migrate to northern Argentina
The Buteo swainsoni is a slim, graceful hawk that nests from the Great Plains all the way to northern California.
It feeds mainly on insects, but will also prey on rodents, snakes and birds when raising their young. These learn to fly about 45 days after hatching but may remain with their parents until fall migration, building up flying skills and fat reserves.
A common sight in summer over the Prairies and the West, Swainson’s hawks disappear every autumn. While it was assumed they migrated south, it was long unclear precisely where they went.
A group of researchers that has been studying raptors in northern California for over 40 years has now established exactly where young Swainson’s hawks go in winter. The story of their odyssey, summarised in a 30-second clip (scroll down), is both amazing and shocking.
Harnessing the hawks
A Swainson’s hawk, with tracking device.
The team harnessed six Swainson’s hawks in July, as they were six weeks old and just learning to fly. The clip covers 14 months, until next August – so basically, the first year of flight.
Each harness contains a solar-powered tracker and weighs 20 grams, which represents just 3% of the bird’s body weight. To minimise the burden, only females were harnessed: as with most raptors, Swainson’s hawk females generally are bigger than males.
The first shock occurs just one month (or about 2.4 seconds) from the start of the clip: the first dot disappears. The first casualty. A fledgling no more than two months old, who never made it further than 20 miles from its nest.
By that time, the remaining five are well on their way, clustering around the U.S.-Mexico border in Texas. Swainson’s hawks usually travel at around 40 mph (65 km/h) but can almost double that speed when they’re stooping (i.e. dive down, especially when attacking prey).
There’s a strong genetic component to migration. As usual, the Germans have nice single word to summarise this complex concept: Zugunruhe (‘tsook-n-roowa’), literally: ‘migration unrest’ (1). It denotes the seasonal urge of migratory animals – especially birds – to get on their way. Zugunruhe exhibits especially as restless behaviour around nightfall. The number of nights on which it occurs is apparently higher if the distance to be travelled is longer.
The birds may have the urge to go south, but genetics doesn’t tell them the exact route. They have to find that out by trial and error. Hence the circling about by the specimens in this clip: they’re getting a sense of where to find food and which direction to go. Their migratory paths will be refined by experience – if they’re lucky enough to survive that long.
Each bird flies solo: their paths often strongly diverge, and if they seem to meet up occasionally, that’s just an illusion:…
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