Conclusions On The Role Of Body Size In Bird Migration

It will be evident by now that body size (and the associated features of metabolic rate, power requirement and flight speed) have a major influence on the flight modes and migration capabilities of birds, increasing body size bringing increasing restrictions. The power available for flight declines rapidly with increase in body size, and many large birds may have insufficient muscle power or aerobic capacity to fly at Vmr, so are constrained to fly at some slower speed, such as Vmp. Among birds that migrate by flapping flight, the load-carrying capacity decreases with increasing body mass, including the fuel load (Hedenstrom & Alerstam 1992). This constraint in load-carrying is expected to limit the distance that can be flown non-stop by large flapping birds over terrain in which feeding is not possible, although large birds compensate for this to some extent by relatively long wings and by flying more slowly than maximum range speed

(Rayner 1988). Other large birds, such as albatrosses, vultures and eagles, can travel long distances by gliding, which requires much less fuel, but as explained above, this flight mode in landbirds can restrict their times and routes of travel.

Another major advantage of large body size is that it is in most species associated with greater flight speed. This mainly translates to reduced journey times, but it also enables large birds to migrate in more adverse wind conditions than small ones. Headwinds that would merely slow large birds might be sufficient to stop small ones altogether.

The range a bird can attain on a full load of fuel is of special significance. Large birds are not the best performers in this respect, because they are limited in the fuel they can carry, but nor are small birds, which fly too slowly. Rather some medium-sized birds, such as some shorebirds, can undertake the longest non-stop flights, because they can carry large fuel loads and at the same time fly fast. Both passerines and shorebirds can sustain flapping flight for up to a hundred hours (or more), but because shorebirds fly faster, they can cover greater distances in this time (Chapter 1). These same features may also enable shorebirds to perform better under adverse winds than slower species.

The longest apparent non-stop flights recorded for swans reached around 1700 km, for geese and passerines around 3000 km, and for shorebirds around 4000-7500 km, apart from the 10 400-km flight of Bar-tailed Godwits Limosa lapponica from Alaska to New Zealand (Chapters 1, 6 and 8). The longest overall migrations (including stopovers) for swans reached around 3000 km, for geese around 5000 km, and for passerines and waders 12 000 km or more, the latter covering the distance between the northern parts of the northern continents and the southern parts of the southern ones. Because soaring birds do not need to spend long periods refuelling, they can make longer overall migrations within the same time limits as flapping species of similar body weight. Some Steppe Buzzards Buteo b. vulpinus migrating between Russia and South Africa make journeys of 12 000 km or more, as do Swainson's Hawks Buteo swainsoni travelling between North and South America. Equally remarkable, the Ruby-throated Hummingbird Archilochus colubris weighs about 4.8 g, yet regularly crosses the Gulf of Mexico (1100 km) in a non-stop flight of about 18 hours, requiring an estimated 3.2 million wing-beats (Nachtigall 1993).

The range of body sizes found among flying birds spans four orders of magnitude, from the smallest hummingbirds of about 1.5 g to the largest flying species of around 15 kg, including bustards, pelicans, swans, condors, vultures and albatrosses. Flightless birds can be much heavier, reaching 35 kg in penguins and 150 kg in Ostriches Struthio camelus, while some extinct flightless birds probably reached 400 kg. Large birds become increasingly likely to adopt less strenuous flight modes, such as gliding or formation flying, until at some undefined weight, flight becomes impossible.

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