The behaviour of white-tailed deer on Ossabaw Island, Georgia was examined in three habitats which differed in cover density and forage abundance. Forest had relatively dense cover but low forage availability; wooded pasture had relatively dense cover and abundant forage; open pasture had no woody cover but abundant forage. Deer groups were small in both the forest and the wooded pasture relative to groups in the open pasture. Group size in the open pasture did not vary with seasonal changes in forage supply. Groups were small in open pasture during the autumn rut when food was abundant, but were relatively large at other times of year, even during the winter when forage abundance was comparable to that in the forest. Deer in the open pasture spent more time feeding, less time moving, and less time alert than did deer in the forest. Deer in the wooded pasture were intermediate between deer in the other two habitats for percent time feeding, more similar to deer in the open pasture for percent time moving, and more like deer in the forest for percent time alert. Differences in alert behaviour suggest that deer in dense vegetation (e.g. forest and wooded pasture) were more wary than deer in open areas. In all habitats, deer in larger groups (four or more deer) spent more time feeding and less time alert than deer in smaller groups. More aggressive interactions were seen in the open pasture; more nonaggressive interactions were seen in the forest. The rate of aggressive interaction was not clearly related to group size. Temporal variation in nearest-neighbor distance within groups was greater in the forest than in the open pasture, possibly due to loss of contact between deer under conditions of restricted visibility. Although these data suggest that cover density rather than forage characteristics was related to deer group size, no support was given for the hypothesis that this relationship was a consequence of different anti-predator strategies. Groups may have been smaller in dense vegetation because deer had difficulty maintaining contact with other group members when visibility was restricted.

For many animals, selecting whether to forage during day or night is a critical fitness problem: at night, predation risks are lower but feeding is less efficient. Habitat selection is a closely related problem: the best location for nocturnal foraging could be too risky during daytime, and habitat that is safe and profitable in daytime may be unprofitable at night. We pose a theory that assumes animals select the combination of daytime and night activity (feeding vs. hiding), and habitat, that maximizes expected future fitness. Expected fitness is approximated as the predicted probability of surviving starvation and predation over a future time horizon, multiplied by a function representing the fitness benefits of growth. The theory's usefulness and generality were tested using pattern‐oriented analysis of an individual‐based model (IBM) of stream salmonids and the extensive literature on observed diel behavior patterns of these animals. Simulation experiments showed that the IBM reproduces eight diverse patterns observed in real populations. (1) Diel activity (whether foraging occurs during day and/or night) varies among a population's individuals, and from day to day for each individual. (2) Salmonids feed in shallower and slower water at night. (3) Individuals pack more tightly into the best habitat when feeding at night. (4) Salmonids feed relatively more at night if temperatures (and, therefore, metabolic demands) are low. (5) Daytime feeding is more common for life stages in which potential fitness increases more rapidly with growth. (6) Competition for feeding or hiding sites can shift foraging between day and night. (7) Daytime feeding is more common when food availability is low. (8) Diel activity patterns are affected by the availability of good habitat for feeding or hiding. We can explain many patterns of variation in diel foraging behavior without assuming that populations or individuals vary in how inherently nocturnal or diurnal they are. Instead, these patterns can emerge from the search by individuals for good trade‐offs between growth and survival under different habitat and competitive conditions.

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