Migration can be quite a challenge for birds traversing large oceans or vast deserts. Many migrants fatten up before they start their journey. Still,especially small birds also need to refuel once they are underway. If you can deposit energy quickly during stopovers, you minimize travel time and benefit from arriving early to pick the best territories at your destination. Several Sylvia warblers, such as blackcap and lesser whitethroat, breed in Europe and western Asia and winter in Africa, but they must cross the inhospitable Sahara Desert when travelling between breeding and wintering grounds. With poor prospects of finding food in this dry land, the migrants prepare for their journey at the edge of the desert. While our knowledge of energy requirements during this time is substantial, the importance of water during their preparation or flight is not yet clear. Some studies have suggested that maintaining water balance poses serious constraints on migratory birds, while others discount the role of water. Puzzled by this question, Nir Sapir and colleagues from the Ben-Gurion University in Israel began exploring the importance of water for the fuel deposition rates of blackcaps and lesser whitethroats during their stopover in the Negev Desert.
The team provided troughs of water for the birds just before they crossed the desert en route to their wintering grounds. In their first experiment, the team examined the amounts of fuel that the birds carried on days with and without water provisioning. Fuel loads were measured as the amount of fat relative to lean body mass. In addition, fat reserves were scored visually into categories. A second experiment measured how quickly individual birds could fatten up in the absence or presence of drinking water. This experiment relied on capturing and recapturing individual birds during their staging period.
The results showed that staging blackcaps and lesser whitethroats respond differently to water provisioning. In the first experiment, blackcaps carried more fat on days with water than on days without, while lesser whitethroats had similar fuel loads under both circumstances. Also, water attracted blackcaps, increasing their numbers in the area, but it did not affect the densities of lesser whitethroats. In the second experiment, the blackcaps'fuel deposition rates were higher when water was supplemented. In fact,without water, blackcaps did not deposit any new fuel. However, the fattening rates of lesser whitethroats did not change with the presence of water.
The increased fuel deposition rates of watered blackcaps may be the result of several factors. Possibly, drinking water buys time: it allows foraging activity during the period of midday heat, thereby increasing food intake. Besides this behavioural effect, drinking water may improve digestion and food utilization, as found in some non-migratory birds. The fact that lesser whitethroats are less dependent on water for refuelling may be understood by their general adaptation to drier food-poor habitats.
Does this mean that water constrains migratory patterns in some species and not in others? We may well find out, but only if we keep travelling with the birds.
