W. Cresswell, DIURNAL AND SEASONAL MASS VARIATION IN BLACKBIRDS TURDUS-MERULA - CONSEQUENCES FOR MASS-DEPENDENT PREDATION RISK, Journal of Animal Ecology, 67(1), 1998, pp. 78-90
1. A bird's mass in winter should reflect the trade-off between the be
nefits of an increased energy store to reduce starvation risk: and its
costs in terms of increasing mass-dependent predation risk. State-dep
endent models of this trade-off predict that as starvation risk increa
ses then energy reserves should (i), increase, and (ii) be acquired ea
rlier in the day. 2. Blackbirds increased their minimum weight by 25 /- 1.5% in midwinter; adults weighed significantly more than juveniles
in midwinter, but weighed less in late winter. Weight for both sexes:
and particularly adults, decreased with increasing daylength, but wei
ght increases with respect to temperature decreases were most pronounc
ed in males. Female blackbirds showed relatively small seasonal weight
changes so that their midwinter mass-dependent predation risk relativ
e to males was independent of diurnal weight changes. 3. Blackbirds lo
st at least 1-9% of their body weight overnight depending on temperatu
re. Over 60% of the weight lost was then regained in the first 3 hours
after dawn, with only 5% being regained after 12.00 h. The pattern of
early morning weight gain was maintained throughout the winter, with
the ratio of weight gain rates in the first 4 h of dawn compared to th
e rest of the day being c. 3:1 in midwinter (when individuals maintain
ed high weights) and in early spring (when individuals were losing wei
ght seasonally). 4. The results agree with the theoretical prediction
that energy reserves should increase as risk of starvation increased (
as measured by seasonal factors such as cold, short midwinter days). T
here was, however, little change in the seasonal pattern of diurnal we
ight gain, with weight always being gained early in the day. In the li
kely absence of foraging constraints within the study system, this res
ult fits the predictions of state-dependent foraging models where anim
als can escape from predation after achieving weight gains, by either
using refuges or adopting low-risk foraging options.