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use stored food to survive during the winter
(Pravosudov & Smulders, 2010). The scale
of storing is prodigious. In some titmice an
individual bird may store between 100 000
and 500 000 tiny seeds during a winter,
each in a separate place (Pravosudov, 1985;
Brodin, 1994).
Stored food can be thought of as
analogous to body fat, stored up in times
of plenty and used in times of scarcity
(Hitchcock & Houston, 1994; Pravosudov &
Lucas, 2001). Some species, like the
nutcrackers, use their stores over a whole
season, whilst others, such as some of the
Paridae, use them on a shorter time scale of
days or weeks, perhaps as a strategy for
overnight survival in cold weather.
Ornithologists used to assume that stored
food was communal property that improved
the survival of the group, partly because it
seemed inconceivable that birds could
actually remember the huge number of
places in which they had stored food. After
all, many of us have difficulty remembering
where we have left one bunch of keys!
However, if hoarding has a cost, then free-
loaders that did not pay the cost of hoarding
but reaped the benefits would replace
hoarders in a population (Andersson & Krebs,
1978). Hoarding is advantageous only if the
hoarding individual gains from its hoard
more than do others in the area: one way of
gaining this advantage would be for hoarders
to remember the locations of their stores.
This evolutionary argument has
stimulated many studies of the memory of
scatter hoarding birds (Brodin, 2010) that
have revealed a remarkable story linking
ecology, behaviour and neuroanatomy. In an
ingenious winter field experiment, Anders
Brodin and Jan Ekman (1994) offered
individual willow tits in Sweden 20 sunflower
seeds labelled with a radioactive isotope of
sulfur (
35
S). The birds stored the seeds in their home range. The radioactive sulfur was
incorporated into growing feathers when the individuals retrieved and ate the labelled
seeds, so Brodin and Ekman could work out which birds in the flock recovered the seeds,
and when they did so, by autoradiography of growing feathers (Figure 3.7). The results
Food hoarding for
short-term or
long-term stores
Fig. 3.7
This pair of pictures shows
an autoradiograph (left) and a
photostat (right) of a willow tit tail
feather. The upper edge of the dark
bands on the autoradiographs
indicate that the owner of the feather
ate a radioactive labelled seed on that
day, the sulfur having been
incorporated into a growing feather.
Feathers were induced to grow by
pulling out the original, and a
replacement grows over the next
40 days. The right hand, photostat,
images show faint horizontal lines
that are daily growth bars. (Brodin &
Ekman, 1994). Reprinted with
permission from the Nature
Publishing Group.
