Strategy and constraint in the winter fattening response to temperature in the great tit Parus major

1. For many small birds at temperate latitudes, the low temperatures and long nights of winter are associated with an increased risk of starvation. Birds such as the great tit Parus major L. face this by increasing their fat reserves. Theoretical, correlative and experimental evidence indicates that the amount of fat carried is not maximized for a given set of circumstances, but optimized, at least partly because of a need to trade off starvation risk against predation risk, part of which may be mass-dependent, and therefore increased by carrying increased reserves. Evidence for such a trade-off strategy includes the observed negative correlation between fat reserves and ambient winter temperature operating as a proximate factor. 2. However, the relationship between fat and temperature requires further investigation for two reasons. First, field evidence that temperature acts as a proximate factor is equivocal. Secondly, measures of temperature used (e.g. mean temperature over the previous 24 h) have given little resolution as to the time over which temperature effects operate, and whether low temperatures might constrain the bird's ability to fatten, in addition to signalling the need to fatten further. 3. This paper uses observations of fat from a wild great tit population, studied over 17 years, to address these two areas. First, it demonstrates that temperature acts as a proximate factor influencing fat reserve levels. Secondly, by relating the time of observation more precisely than previously to temperatures prevailing at particular times during the previous 24 h, it is shown that while past temperature (e.g. 5 + h previously) is used to optimize fat reserves (strategic fattening), more immediate temperature may constrain the bird's ability to achieve the optimum. 4. This constraint is demonstrated by a positive correlation between fat reserves and current temperature; probably the first demonstration of such a relationship. However, this constraining effect of temperature has diminished over the last 17 years, presumably because of climate amelioration.