Biology Reference
In-Depth Information
Cartar and Dill (1990) in a study of bumblebee foraging. They augmented or depleted
the energy reserves of the nest, and found that workers switched to risk-prone behaviour
when the reserves were low. In this case the reserves of the colony as a whole were
treated as equivalent to the reserves of an individual.
These experiments suggest that foragers are able to respond to variability in the
amount of reward obtained, and that preference depends on state. However, they do not
investigate the question of whether preference changes with time. Two examples of
how time of day could be important are suggested by Houston and McNamara (1982,
1985). Firstly, if the animal starts off the day risk-prone, but has good luck in its first
few choices, it might be expected to become risk-averse later on. Secondly, as dusk
approaches, for a diurnal forager the long period of enforced overnight fasting might
sometimes favour a switch to risk-prone behaviour to increase the likelihood of
overnight survival (Box 3.3).
Environmental variability, body reserves
and food storing
Small birds in winter often experience large daily fluctuations in body mass: the 20-g
great tit, for example, typically loses 10-15% of its body mass overnight in winter
and regains the mass during the following day (Owen, 1954). The daily gain and
overnight loss is almost entirely made up of fat, which acts as fuel for overnight
survival: thus each day in winter a small bird faces an uphill struggle to build up
sufficient reserves for surviving the next night. Given this observation, should we
expect small birds to carry as much fat as possible at all times, as an insurance
against starvation? Both empirical observation and optimality models suggest that,
in fact, birds usually carry less than the maximum reserves. In winter, birds are
usually heaviest on the coldest/harshest days, suggesting that on other days they are
carrying fewer reserves than the maximum. Furthermore, if the trajectory of weight
gain through the day is examined it is found that birds increase their weight rapidly
in the afternoon (Owen, 1954; Bednekoff & Krebs, 1995), implying that earlier in
the day they do not carry as much fat as they could. Lima (1986) and McNamara
and Houston (1990) explained these observations by hypothesizing that the reserves
carried by a bird reflect an optimal trade-off between costs and benefits. The benefit
of carrying extra reserves is reduced risk of overnight starvation, whilst the cost is
increased danger of death from predation. The danger might arise simply because
heavier birds are less agile at escaping or, more subtly, because birds with more
reserves spend more time foraging rather than hiding from predators. This hypothesis
predicts that the optimal level of reserves will increase (i.e. birds will be heavier)
when the energy cost of overnight survival is higher, or more unpredictable, or when
the danger of predation is lower (Fig. 3.6).
Andy Gosler and colleagues found evidence for these predictions from the long-term
study of great tits in Wytham Woods, Oxford, UK. When sparrowhawks
Accipiter nisus
re-colonized the wood in the 1980s (after a decline due to pesticides), the winter
mass of great tits trapped in the wood declined by about 0.5 g. Mass declines in other
parts of the United Kingdom also coincided with the local re-colonization by hawks
Optimal fat
reserves: trade-off
between
starvation and
predation
