|Migration routes and wintering grounds of three northern wheatears breeding in Alaskan (AK) and one in the eastern Canadian Arctic (CN; grey dot, breeding area, blue, autumn migration, orange, spring migration, dashed lines indicate uncertainty in migration routes close to equinoxes). Fifty per cent kernel densities of winter fixes (beginning of December 2009-end of February; purple, bird AK-1; green, bird AK-2; orange, bird AK-3; blue, bird CN-1) are given depending on the sun elevation selected (with 228 for most southern and with 24.58 for most northern densities). Pie charts indicate the proportion of individuals (AK: n 1/4 9, CN: n 1/4 4) originating from one of the three pre-defined wintering regions (red, western; orange, central; yellow, eastern)  based on stable-hydrogen isotope (dD) values in winter grown feathers and the dD values within each wintering region (mean+s.d. shown); Credit: F. Bairlein et al. 'Global migration of wheatears' (doi:10.1098/rsbl.2011.1223) in Biology Letters|
|Female wheatear by Adam Seward. Who couldn't love these birds?!|
|Wheatear pair by Adam Seward. It's cold up north!|
This isn't the longest migration of any bird (Arctic Terns must hold that record), but it probably is for any passerine. And there are a few more interesting details we can fit in too, such as the flight speed. That bird crossing the Atlantic to Britain averaged 850km per day, for four days (perhaps stopping in Greenland), which is pretty remarkable. (Actually, I happen to know that the flight speed of a wheatear is 47kmh, so that's about 18hrs flying time per day, if there's no tail-wind. And I also know they like to make this hop when the wind is in their favour, so if it stopped in Greenland it will have bee a short stop unless there was a strong tailwind). The movement across Asia is also nice to study, showing typical travel times of migrations, being 91 days on the way south (160km / day), and a faster 55 days (250km / day) when the rush is on to get to the breeding grounds in the spring. (Curiously, the Canadian bird was faster in autumn). Knowing that the lean weight of a wheatear is about 20g (not too different from a house sparrow), and what the fuel efficiency rate is, if a wheatear were to do the 14,600km trip in a single flight it would need about 20 times it's own weight in fat. It's unusual to see a wheatear with this much fat(!), with 50-60% of lean body mass the typical departure fuel load for long flights in this species, and much less when there's good feeding to be found en route. In fact, our birds on autumn migration carried loads of fat that would carry them on average about 2000km before needing to stop and refuel, so if we assume this is much more reasonable, 2000km requires about 10g of fat (50% body mass) the total trip might well have been carried out in 7 or 8 stages, interrupted by feeding stops in nice areas. We've found that this race of wheatear can, if the conditions area really good, deposit an average of about 5% of their body mass (1g) of fat per day (exceptionally the Canadian race can get up to 20-25%). There's at least 300hrs of flying time required to do the distance (~13 days). So each stop over to reach 10g and the next flight might be about 10 days, giving about 73 days of travel. Obviously to make the trip in 55 days they've got to be faster, making sure they pick the best places to refuel and average stop overs of only 5 or six days. Pretty had work though, with no time to rest! And imagine repeatedly gaining and losing 50% of your body weight - I think it would kill us!
|How we know the extra stuff! Colour-ringed wheatear weighing itself|
and snacking on mealworks. Big brother is watching... Thanks Adam!
Anyway, what surprises me most about these results is the fact that some wheatears breeding in Alaska seem to have wintered in west Africa (having completed an even loinger migration!) and some Canadian birds here in east Africa. That's particularly interesting to me as it means that some of 'our' colour-ringed birds from Europe and Greenland might have be wintering on the plains near me! I'm going to have keep my eyes open...
PS, thanks to the very nearly Dr Adam for providing the photos of his wheatears! I'm looking forward to cute squirrels next...
Bairlein, F., Norris, D., Nagel, R., Bulte, M., Voigt, C., Fox, J., Hussell, D., & Schmaljohann, H. (2012). Cross-hemisphere migration of a 25 g songbird Biology Letters DOI: 10.1098/rsbl.2011.1223