4.2 | The effect of abient factors on fat score change
in winter
For individual birds, adaptation and flexibility were required to face a
volatile environment that includes changes in weather and availability
of food resources (Cavieres and Sabat, 2008; Laplante et al., 2019). As
one of the most well-discussed factors, the influence of temperature on
the fat reserve of robins in the short- and long-term showed a
paradoxical pattern. On the capture day, Tmax had a significant positive
relationship with the fat score residual, indicating that birds would
get fatter during warmer days. This may be due to the mildness and
stability of the weather making foraging and other activities easier for
birds as insects also become more active. While on the model of 7 days
preceding capture day, the effect of MTmax reversed, birds tended to
have less fat reserve during the warmer period, this may be as a result
of the reduced flight burden for birds in a stable environment. The
index representing the volatility of temperature in a time period,
MTran, had a positive effect on the birds’ fat score, suggesting that
birds tended to store more fat when the weather was more unstable. In
the long-term, birds may adjust the fattening process in winter to reach
a higher fitness level. When the temperature turns colder and unstable,
the risk of starvation and hypothermia increases, and therefore birds
store more fat to increase their fitness.
Compared with temperature, all humidity related factors were found to
have a constant negative effect on the fat score residual in all three
models. This result was consistent with previous studies on snow bunting
(Laplante et al., 2019) and black-capped chickadee (Petit et al., 2013).
Humid air may increase heat loss through the evaporative cooling of
water condensing on the body, and thus periods of higher humidity could
lead to higher daily energy expenditure and a lower fat reserve (Petit
et al., 2013).
The influence of snowfall events was not as strong as expected,
exhibiting a negative effect on the fat score residual only on capture
day. According to studies in North America, snowfall can be a very
important predictor of a bird’s fat reserve, as its occurrence is
usually accompanied by extreme cold weather and limited food resources.
Small birds in winter tended to store more fat during periods of
long-term snowfall (Rogers and Heath-Coss, 2003; Laplante et al., 2019).
Our differing result might be explained by the much milder winter
climate in the subtropical forest where the study was conducted. In
Dujiangyan, snowfall usually does not last for more than one week, and
the mean daily temperature seldomly drops below zero. Hence, the cold
stress triggered by snowfall events in Dujiangyan can only affect
short-term or same day fat storage.