In contrast, activity during other seasons of the year indicates a diurnal pattern with distinct crepuscular peaks.

Care of Young

Prior to egg-laying, female ruffed grouse at Cedar Creek Natural History Area exhibited a typical bimodal activity pattern. During incubation hens were on the nest for nearly the entire 24-hour period. Immediately after hatching, the hens altered their activity rhythm, beginning activity as much as several hours later in the morning and ending activity earlier in the evening. Maxson (1977) believed that this delay of onset and early cessation was related to brooding behavior by the hen. Brooding dur­ing the cool morning and evening hours reduced the exposure of chicks, helping them to maintain their body temperature.

In studies of barred owls (Strix varia) at Cedar Creek, Fuller (1979) observed that non-breeding owls exhibited nocturnal activity patterns with peaks of activity occur­ring just after sunset and before sunrise. However, when the female was incubating, the male increased his length of activity time, presumably to obtain food for the female. Fuller (1979) indicated that the marked increase in activ­ity of both the male and female following hatching was the result of increased hunting for food for the growing young.

The demands of caring for young were considered to be responsible for seasonal differences in activity among different sex and age groups of black bears (Ursus americanus) studied in southeastern United States by Garshelis and Pelton (1980). Females with cubs were more active than the other sex and age categories in spring and fall.

Nutritional Status

Activity patterns of goshawks (Accipiter eentilis) studied in Sweden revealed that activity increased as the level of hunger increased. Widen (1982) found that the ratio of activity to rest increased significantly on days following the time of last food intake. Further, the percent of total time active per day increased markedly as hunger level increased.

Food Storage and Fat Reserves

Gray squirrels (Sciurus carolinensis) and muskrats (Ondatra zibethicus) studied at Cedar Creek both showed marked seasonal changes in activity patterns (Tester, 1987). A striking increase in activity of gray squirrels in September and October was related to the collection and storage of food for winter. Similarly, high levels of activity of muskrats in late summer and early fall were believed to be related to the time required to build lodges and store food for winter. After ice formed, preventing further lodge construction and food storage, the minutes of activity per 24 hours were reduced by about half.

ENVIRONMENTAL FACTORS INFLUENCING ACTIVITY RHYTHMS

Weather

The influence of weather on activity rhythms may be manifested in many ways, including energetics of maintaining body temperature, initiation of breeding, availability of food, and change in the environment such as formation of ice on lakes and rivers. Gray squirrels at Cedar Creek showed a marked reduction in activity in winter (Tester, 1987; Figala and Tester, 1986). These squirrels were under a strong influence of cold temperatures in January and February. During these months they spent their resting time in tree cavities or in nests made of dry leaves. Protection provided by these nests was probably necessary to maintain their critical physiological temperature. The squirrels were active only during the time of day when ambient temperatures were at maximum.

Badgers (Meles meles) in Britain also exhibited a shorter alpha in winter (Harris, 1982). However, no explanation for this change in activity rhythm was given.

The seasonal effects of temperature on the activity of black bears in southeastern North America revealed marked seasonal changes. In spring, increases in level of activ-' ity were associated with increases in ambient temperature until the' temperature reached approximately 20°C, where activity appeared to stabilize. In summer, activity also increased as temperature increased, until about 23°C when activity declined. In fall, activity increased during warmer periods and decreased when the temperatures exceeded 20°C (Garshelis and Pelton, 1980).

The effect of freezing of lakes can best be illustrated by data on muskrats at Cedar Creek (Tester, 1987). From July 1969 through February 1970, the number of activity periods per 24 hours varied from two to four. In July and August four periods of activity were about evenly distributed throughout the 24 hours. In subsequent months the number of activity periods was reduced to three and then to two at the time of freeze-up of lakes and streams. The day active periods were eliminated and muskrats became essentially nocturnal with the total amount of activity per 24 hours being reduced to approximately half that exhibited in late summer.