Birds

Birds are the most numerous and important vertebrate predators of forest insects. Caterpillars (Lepidoptera) form the basic food supply for many species of forest-dwelling birds (Slide). In addition, some species are adapted to feeding on bark beetles and wood borers (Slide). Insectivorous birds have the following characteristics:

1. They have enormous appetites. An individual bird may kill 20,000 caterpillars in a year. Thus, birds can have a significant impact on the survival of forest defoliators, especially Lepidoptera.
2. They are very mobile. Flocks of birds moving into an area can kill most of the caterpillars in a few hours and thereby eliminate potential outbreak epicenters.
3. They are generalist predators. In other words, although some birds specialize on particular groups of insects (hairy caterpillars, naked caterpillars, wood borers, etc.) they do not specialize on particular insect species. For this reason they tend to switch from one prey species to another as the densities of their prey changes. This, plus their high mobility, leads them to aggregate at places where prey are abundant.
4. They are territorial during the breeding season. Because of this and their general food habits, birds do not tend to have strong reproductive or numerical responses to the density of their prey. In other words, the number of birds in a given area depends more on the availability of territories than on the abundance of food.
5. They have sigmoid functional responses caused by switching and aggregation in response to prey density. Sigmoid functional responses can create -feedback on pest population and, therefore, can stabilize pest populations at very low densities. However, sigmoid functional responses also create escape thresholds so that it is always possible for pest populations to escape from limitation by bird predation. For this reason, populations limited by birds often exhibit eruptive outbreaks.
6. They are most effective on insects exposed to the open environment, and insects which do not have toxic hairs or distasteful chemicals (Note that bright colors are usually intended to warn predators of distasteful or dangerous prey = warning coloration). For these reasons, we should not expect brightly colored and hairy insects to be limited by birds (e.g., the Douglas-fir tussock moth). When insects use camouflage as a defense against visually-searching predators, it indicates that birds can have a severe impact on their survival (e.g., loopers).

EXAMPLE:

Western spruce budworm (Lepidoptera: Tortricidae). There is evidence to suggest that the western spruce budworm is limited, most of the time, at very sparse densities by bird predators. The caterpillars and pupae are relatively naked and exposed on the foliage and are known to be a major food source for many forest birds, such as the dark-eyed junco (Slide). Western budworm outbreaks occur at irregular intervals and usually spread over large forested areas, strong evidence that they are of the eruptive type. This kind of outbreak behavior is expected from bird regulated populations because an escape threshold is often present.

FOR MORE on the importance of birds as destroyers of insect pests tune into the Smithsonian Migratory Bird Center's Homepage and check out Fact Sheet #2 "How birds keep out world safe from plagues of insects".

Mammals

Small mammals such as field mice, voles and shrews, are probably the second most important vertebrate predators of forest insects (Slide). Small mammals seem to be particularly attracted to sawfly (Hymenoptera) cocoons because they are frequently found in the duff and litter layer off the forest floor (Slide). Insectivorous mammals have the following characteristics:

1. They have enormous appetites. An individual shrew may devour up to 10,000 sawfly cocoons in a year.
2. They are generalist predators and tend to switch from one food type to another as densities change. Because they rely on different foods, they do not tend to have strong reproductive or numerical responses to the density of any single food type. In fact, their reproduction may be more strongly related to mast years when trees produce huge amounts of seed, the staple over-wintering diet of many small mammals.
3. They have sigmoid functional responses caused by switching to more abundant prey. As we know, sigmoid functional responses can stabilize pest populations at very low densities but also create escape thresholds. Thus, pest populations can escape from limitation by small mammal predators and undergo eruptive outbreaks.

EXAMPLES:

Sawfly (Hymenoptera) outbreaks are frequently of the eruptive type and their cocoons are known to be a favorite food for small mammals. It seems likely that populations are limited by small mammals between outbreaks.

Gypsy moth (Lepidoptera: Lymantriidae) pupae are also fed upon by small mammals and their populations have many characteristics of eruptive outbreaks. Because small mammals seem to cause much of the mortality to sparse gypsy moth populations (Slide), it is proposed that they have the potential to limit gypsy moth densities in the periods between outbreaks.

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© 1998 Alan A. Berryman