As at breeding and wintering areas, bird densities at stopover sites are often found to correlate with local food supplies, either from place to place or time to time. At some sites, the numbers of migrants vary between migration seasons, or during the course of a single season, in relation to changing food availability. For example, Rufous Hummingbirds Selasphorus rufus, which breed in the northwest of North America, migrate southward in autumn, stopping to feed on nectar from flowers (especially Castilleja linariaefolia) that grow in mountain meadows (Russell et al. 1992). At each stopover, the birds establish individual territories around flower patches. Competition is usually intense, and not all birds manage to acquire territories. Once a bird has a territory, it takes several days to three weeks to reach an appropriate departure weight and move on, leaving its territory vacant for another individual. During seven years of study, the densities of nectar-providing flowers at a study site varied widely due to natural variation in flowering. Territory sizes and population densities of hummingbirds varied accordingly, both between and within years. During dry years of generally poor flowering, the body weights of incoming birds were low, and stopover durations were long. The peak migration date of hummingbirds roughly coincided with the peak date of local flowering, and both events varied by about a month among years, poor nectar supply delaying migration. These findings thus revealed effects of food supplies at stopover sites on the local densities, behaviour, fattening patterns, staging periods and migration dates of hummingbirds. Instances of hummingbirds starving while on migration were said to be 'not uncommon'.
Relationships between food supplies and migrant numbers have been shown in other species, and in widely different situations (e.g. Martin & Karr 1986, Kelly et al. 2002, van Gils et al. 2005). In one study in the Arizona mountains, the relative numbers of insectivorous passage migrants in different habitats varied between seasons, in accordance with seasonal changes in insect densities (Hutto 1985). In this area, 54% of bird species showed shifts in habitat use between seasons which matched the changes in insect availability. At other sites, the numbers of birds have increased or decreased over the years in association with changes in food supplies, as noted for example in various species of cranes and waterfowl (Chapter 21). Such relationships confirm that migrants can respond to food supplies en route, concentrating in greatest densities at times and places where food is plentiful. They also indicate that, at some sites, the numbers present at one time could be limited in relation to food supplies.
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