Birds migrating long distances over seas or deserts cannot drink on their journeys. This could give rise to dehydration, especially in hot conditions, where the birds must pant in order to remain cool. By panting, birds lose heat through the evaporation of water from the damp inside surfaces of the mouth and nostrils. The potential problem of dehydration has raised interest in the water content of migratory birds. Because fat is stored anhydrously, body water as a fraction of body weight declines as fat is deposited. For this reason, water content is best expressed in relation to lean dry mass. The ratio between the two is highly variable, but in healthy passerines is usually in the range 2:1-2.4:1 (that is, on a percentage scale water forms 200-240% of lean dry mass). In a study of Eurasian Reed Warblers Acrocephalus scirpaceus, Fogden (1972a) took water levels lower than 2:1 as indicative of dehydration, and found that 11 out of 80 individuals caught at spring migration time in Uganda had water contents below this level. He also calculated that, of 409 birds of 11 species obtained on the Egyptian coast after crossing the Mediterranean Sea in autumn (and analysed by Moreau & Dolp 1970), 78% had water indices lower than 2:1, while as many as 12% had ratios lower than 1.4:1. On the same basis, reduced water levels were also apparent among migrants that had crossed the Gulf of Mexico in spring (Odum 1960). While these findings would seem to indicate severe dehydration after long flights, Fogden (1972a) suggested an alternative explanation, namely that in some conditions migrants lower their water levels before departure in order to reduce total body weight, enabling them to cover greater distance on their body fat. This view was later reiterated by Johnson et al. (1989), on the basis of the pre-migratory changes in body composition found among Lesser Golden Plovers Pluvialis fulva on the Hawaiian Islands, preparing for their flight to Alaska. The question whether low water levels reflect dehydration or adaptation to long flights remains open, but birds with such low water levels would surely have little leeway to counter any overheating by panting (Chapter 6).
The body temperatures of birds measured during active flight are generally greater than 41°C, up to 4°C higher than normal (Gessaman 1990). This may improve muscle efficiency and increase maximum power output (Butler & Woakes 1990). But during normal sustained flight, birds must dissipate more than eight times as much heat as during rest in order not to become overheated. Overheating is a potential problem for migrants during continuous flights at high ambient temperatures, as occur by day in deserts (Chapter 6). Flying birds lose some heat through convection and radiation, especially through their underwings and unfeathered legs and feet, but the amount of heat that can be lost in this way is limited by simple physical processes. So at higher ambient temperatures, evaporative mechanisms (mainly through panting) play an increasing role. The resulting risk of dehydration is offset either by metabolic water production (from fat and protein catabolism), by ascent to altitudes where the air is cool enough to keep evaporative heat loss at the required level, or by flight at night when temperatures are lower than during the day (up to about 10°C). The bird might also switch to other energy sources: per unit of energy released, protein yields about five times as much water as fat, and glycogen at least seven times as much (Table 5.1).
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