The main conclusion from this chapter concerns the ways in which bird features (such as body weight, wing shape, flight mode and fuel load) interact with one another, and with environmental features (such as wind conditions) to influence bird migratory capability. Such interactions probably account for some of the big differences in migratory performance (speeds, routes, altitudes and distances) that occur between species, and also within species, according to their current fuel loads and ambient conditions. Theory based on aerodynamic principles is increasingly supported by field data. For example, it provides an aerodynamic rationale for the observation that long-range small bird migrants accumulate relatively greater fuel loads for migration than large birds that also travel by flapping flight, and for why many large bird species migrate by soaring-gliding flight, rather than by flapping. Large waterfowl, which travel entirely by flapping flight, are of special interest because of the limited power available to them, and their weight-related restrictions on fuel reserves.
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