Effects of season and photoperiod on food storing by black-capped chickadees, Parus atricapillus

The effects of season and photoperiod on food storing by black-capped chickadees were examined in two experiments. Chickadees were captured in March (experiment 1) or November (experiment 2), and food storing was tested regularly in the birds’ home cages. In both experiments, two groups were matched for initial level of food storing. The long-day group was then exposed to a summer photoperiod (16:8 h light:dark cycle) and the short-day group to a winter photoperiod (LD 8:16 h) for 2–3 months, followed by 2–3 months in identical conditions for both groups. Birds in experiment 1 stored very little at first, but those maintained on long days moulted and began to store after 6–8 weeks; the group maintained on short days eventually increased storing as well. Birds in experiment 2 began by storing about three times as much food as those in experiment 1; storing declined over time in both groups and was not affected by photoperiod. The results suggest that food storing in parids is controlled by photoperiod in a similar way to other annually varying behaviour patterns such as breeding and migration in birds. The proximate causation of food hoarding in birds is not well understood (Vander Wall 1990). Observations of food-storing chickadees and titmice (Paridae) under natural conditions suggest that they store most food in the autumn or early winter (Odum 1942; Haftorn 1956). However, these observations do not permit strong conclusions about the proximate causes of food storing because many factors that might influence it vary seasonally in the wild. For example, storable foods may be most abundant in the autumn (cf. Haftorn 1956). Birds not occupied with defending a territory or feeding young, as in autumn and winter, may devote more time to storing food. Ludescher (1980) documented annual cycles of food storing by testing captive willow tits, Parus montanus, in a standard way throughout the year, but his birds lived in outdoor aviaries, so they experienced natural changes in temperature and daylength which could have directly influenced hoarding intensity. Short days and cool temperatures induce hoarding almost immediately in several species of small mammals (e.g. rats, Fantino & Cabanac 1984; Djungarian hamsters, Phodopus sungorus, Masuda & Oishi 1988; reviewed in Vander Wall 1990). Animals are more energetically stressed in short days and low temperatures, and greater energetic demands increase hoarding in marsh tits, P. palustris (Hurly 1992), Carolina chickadees, P. carolinensis (Lucas & Walter 1991) and tufted titmice, P. bicolor (Lucas et al. 1993). For example, captive marsh tits store more when they have unpredictable access to food once an hour than when food is available once a minute (Hurly 1992). Like laboratory rats (Cabanac & Swiergiel 1989), Carolina chickadees and tufted titmice store more as their body weights fall (Lucas & Walter 1991; Lucas et al. 1993; Lucas 1994). As well as being influenced by energetic factors and patterns of food availability, food storing may be linked to an annual rhythm. Annual changes in reproductive hormones, breeding behaviour and migration in birds are influenced primarily by photoperiod (Gwinner 1975, 1989). In many species that breed in temperate areas, increasing daylength in late winter stimulates the hormonal and behavioural changes associated with breeding. At the end of the breeding season, in July or August, birds become refractory to the effects of long days, the gonads regress, and moulting and migration may occur. Exposure to short photoperiods is then necessary to restore sensitivity to long days (Nicholls et al. 1988). Thus, autumn and early winter, the times when most food storing is observed in parids in the wild, are characterized by 0003–3472/95/040989+10 $08.00/0 ? 1995 The Association for the Study of Animal Behaviour