This chapter focuses on the availability of food for birds. Food supply is an important factor that influences the number of birds, but its precise effects are not easy to quantify. One problem is that shortages are not always shown in an obvious way, such as starvation (loss of body condition to the point of death). Territorial and other interactions between individuals can operate to adjust bird densities to local food supplies, causing hungry birds to move elsewhere, where they may survive or die from a variety of causes, not necessarily including starvation. Some birds exposed to food shortage may thus succumb to predation or disease, which conceals the underlying cause of their deaths. Food shortage may also reduce the population size through lowering breeding rates, not necessarily entailing the starvation of full-grown birds. This type of effect may be hard to detect because of the time lag between the food shortage and the resultant decline in the breeding numbers. In some long-lived species, individuals do not normally breed until they are several years old, so it may take several years before the effects of poor breeding are reflected in poor recruitment.

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