The North American deserts

The western North American deserts are easier to cross than the Eurasian-African ones, because they are generally smaller, better vegetated, and better endowed with oases and riparian vegetation where birds can feed. Some of the desert towns, with their well-watered gardens, also offer good feeding for some kinds of birds, but many former rivers are now dry for most of the year, through excess water abstraction. Nevertheless, some oases attract many passing migrants, and hotspots for bird-watching can be found scattered through all the main desert areas. In southern California, such hotspots include the Forty-nine Springs Oasis in Joshua National Park and the Big Morongo Canyon Preserve (which includes many hectares of riparian cottonwoods and marsh) in Morongo Valley, each attracting more than 100 regular passage species at migration times. Again, however, it is likely that most migrating birds fly over without stopping.

One of the most interesting birds to cross the western American deserts each year is the Eared Grebe Podiceps nigricollis, as it travels from the Great Salt Lake in Utah to the Gulf of California (Jehl et al. 2003). The birds develop flight seasonally, especially for migration, losing body weight but building pectoral muscles in order to do so (Chapter 5). The body shape of these birds, which is adapted to foot-propelled diving, is not ideal for flight, and the energetic costs are the highest recorded for any bird species, at around 25 X BMR (basal metabolic rate). The birds take off from water, after a long, foot-pattering taxi. They fly only at night (perhaps chiefly to avoid predation), and have to cross more than 1000 km of mainly desert in nonstop flight, potential stopping places being almost non-existent. To judge from radar-based observations, the flight is direct and fast, taking about 17 hours, and ideally completed within a single night. By storing massive body reserves on the Great Salt Lake in autumn, grebes can postpone their migration beyond the date in autumn when their local food supplies collapse, waiting until nights are longer and thereby benefiting from safer flying time. The grebes thus migrate to their wintering areas later than any other North American bird, and on body reserves accumulated weeks previously (Chapter 5).

The grebes depart in periods of stable air or light tailwinds, avoiding the prevailing northwesterlies. Not every year do conditions remain favourable throughout the flight, however, and in some years thousands of birds are drifted off course or crash-land in desert snow storms (Chapter 28). If the birds encounter rain or headwinds within an hour or so after take-off, they can return to the staging lakes, as confirmed by radar observations, but they then suffer further depletion of body reserves until conditions again become suitable, for at that time of year spent reserves cannot be replenished locally. Behaviour during departure also falls short of ideal. The birds assemble in large dense flocks, and on appropriate nights, tens of thousands or hundreds of thousands take off en masse, shortly after sunset. This early departure makes best use of the available dark period, but the numbers involved produce mid-air collisions. On some nights hundreds crash into one another, come skittling down and die within moments of taking off (Jehl et al. 2003). The return journey is somewhat shorter, as birds can stop and feed en route at the Salton Sea, which in spring offers abundant food.

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