SEB Bulletin July 2006 - Getting in Shape for the Winter Break
Migrating birds have received bad press recently because of their potential to spread bird-flu, but actually these animals should be respected for the amazing flying and physiological feats they perform during their long biannual journeys. Research presented at the 2006 SEB Annual Main Meeting in Canterbury highlights the trade off birds face between having enough fat reserves to fuel the long flight, but not becoming so heavy that a lot of energy is required for movement.
Fueling up and slimming down: streamlining for success
If you were going on a long journey would you drive as fast as you could, vary your speed, or drive slowly? For some migrating birds the decision could determine whether they have enough energy to complete their flight-passage, however, for other species speed is less of an energetic cost than body mass. Sophia Engel at the Max Planck Institute for Ornithology3 has found that for Rose Coloured Starlings (Sternus roseus) the energetic costs of long-distance flight are independent of how fast the birds fly during simulated migrations in windtunnels. “In my study the effect of body mass was much more pronounced than that of flight speed”, says Sophia.
The metabolic costs of flying in the wind-tunnel are measured by using doubly labelled water. “The isotopes of oxygen and hydrogen present in the water molecule can be tracked in an organism’s body”, explains Sophia. “From the relative turnover rates of both isotopes we can calculate CO2 production.” In the case of these starlings, flight-speed does not seem to change CO2 production, but studies on other species of bird show that avian flight costs do depend on how fast the birds fly. “I think that medium-sized species with average-shaped wings generally have constant metabolic flight costs, whereas birds with very pointed wings or special tails are probably more sensitive to flight speed”, predicts Sophia.
Northern birds are fatter!
“The further birds migrate north for the summer, the faster they put on weight”, says Dr Tony Williams (Simon Fraser University, Canada)1 who has been tracking migrating birds for several years. “This research may have implications for the designation of protected areas which will ensure birds can complete their spring and autumn migrations.”
“Our data can be used to assess habitat quality and the importance of specific sites for migratory birds, and this can contribute to decisions about whether migratory sites are protected and which sites are prioritised for protection”, explains Williams.
Two techniques were used to study Western Sandpipers on their spring journey along the Pacific ‘Flyway’ from Mexico to Alaska: 80 birds were fitted with radio-telemetry tracking devices and a further 400 had blood samples taken to give measurements of fattening rate. Williams found that birds fatten more rapidly as they move further north – as they get closer to the breeding grounds - and that the longer the birds spend hanging around at San Francisco Bay (one of the more southerly refuelling sites), the lower their fattening rates.
Such differences in fattening rates cannot simply be explained by differences in the availability of food. “Our current thinking is that the difference in fattening could be caused by differences in behaviour (birds simply feed more intensively in the north) or changes in physiology that the birds experience as they move further north”, says Williams. “We know there are major differences in gut structure and digestive enzymes between non-migrating and migrating birds, so there might also be similar alterations in migrating bird’s physiology further north, which allow more efficient digestion.”
Birds of a feather lose weight together
Arctic Geese lose a massive 25% of their body mass in late summer, approximately 6 weeks before embarking on their 3,000 km journey from the Arctic to Scotland. It is a race against time to replace that lost body mass, mainly as fat, before the migration. Even though they will be staying put, the body mass of their captive counterparts does the same thing, in preparation for a migration the birds will never make! “You can take the bird out of the Arctic, but you can’t take the Arctic out of the bird”, says Steven Portugal2 who works with Prof. Pat Butler at the University of Birmingham.
When the geese have completed their mammoth journey to Caerlaverock, Scotland, they change from having a constant heart rate and body temperature in the continual light of the Arctic, to a circadian rhythm entrained to the daynight cycle of Scotland. Amazingly, the captive-geese also show similar changes, even though they remain in the UK all year round. “Geese that have been in captivity for more than five generations and experience very different environmental cues compared to their wild counterparts still appear to carry on as if they were summering in the Arctic and performing the migrations like the wild-geese”.
They propose to test a number of hypotheses as to why the geese exper-ience such massive weight loss during wing moult, prior to their migration in the Autumn: “The weight loss may be to allow the geese to regain flight quicker on partially developed flight feathers, which is useful for finding food and laying down fat in preparation for the long flight ahead”. During the moult the oxygen consumption of the geese increases by 50%, and an increase in heart rate is also seen. “It is not clear if this increase in oxygen consumption is needed for the replacement of feathers, to keep the barewinged goose warm, or to help burn fat and thus lose mass, in order to regain flight more quickly”, they say. Portugal and Butler’s results will improve the understanding of the relationship between resting oxygen uptake, body temperature and the laying down of fat, something of fundamental importance in animal physiology.
Lucy Moore
SEB press officer
References:
1. http://www.biosciences.bham.ac.uk/labs/butler/ Steve1.htm
2. http://www.sfu.ca/biology/faculty/williams/)
3. http://www.orn.mpg.de/
Notes:
*Radio telemetry: animals carry tiny harmless transmitters, which signal to receivers that transmit data to the laboratory about the position of that animal.
* Dr Williams’ research was conducted with colleagues from the USGS, US Forest Service, and PRBO Conservation Science.
