Biology Reference
In-Depth Information
(a)
(b)
20.0
2
High variance
Low variance
High variance
Low variance
1
19.0
10
0
18.0
-1
Variable environment
Constant environment
17.0
0
-2
0
6
12
18
24
8
10
12
14
16
Days
Time of day
Fig. 3.6
(a) Body reserves and environmental variability. The graph shows the body mass of a captive great tit
(one of eight in the experiment) which was transferred from a constant to a variable environment for 12 days
before returning to the constant environment. Variability in this experiment was produced by randomly altering
the length of the night-time period of no foraging. From Bednekoff & Krebs (1995). (b) Food storing and
variability. In this experiment, captive marsh tits (one example is shown) stored more food (left), but did not put
on more body reserves (right), in a more variable environment. These results suggest that food storing, like fat
storage, is a method of coping with environmental variability: whilst great tits, which do not store food, cope
with environmental variability by putting on extra fat reserves, marsh tits store extra food in the environment.
The right-hand graph also shows the daily weight trajectory of a marsh tit. In the afternoon, the bird transfers
food from its hoards to its body, so reserves rise steeply towards the end of the day. From Hurly (1992).
(Gosler
et al
., 1995). Experimental presentation of model hawks at feeders led to a
similar mass decline in individual birds, so it is likely that the mass change reflects a
strategic choice by individuals to carry less fat reserves in the face of increased
predation risk. Furthermore, dominant birds (which had priority of access to food)
tended to be lighter than subordinates. These results suggest that fat reserves are an
insurance and the amount carried is tempered by both starvation risk and predation
costs (Gentle & Gosler, 2001).
Food storing birds: from behavioural
ecology to neuroscience
The Clark's nutcracker (
Nucifraga columbiana
) of Western North America collects seeds
from a variety of pine (
Pinus
) tree species in the autumn, carrying them in a special
throat pouch, and stores them in scattered places on a steep hill side, hidden a few
centimetres below the soil surface. The seeds are then retrieved as a source of food during
the winter and spring, and even in the following breeding season for feeding young. Each
nutcracker, it is estimated, stores around 30 000 seeds in 2500-4000 separate places
(VanderWall, 1990). Many members of the chickadee and titmice family (Paridae) also
