VIGILANCE AND GROUP SIZE IN OSTRICHES 1
Abstract
Wild ostriches were observed while feeding alone or in groups of up to four birds
and their vigilance (proportion of time with the head up) recorded. Individual vigilance
declined as group size
increased, mainly through a decrease in the frequency with which the head was raised
Males were more vigilant than females, mainly because they kept their heads up for longer
Vigilance was influenced more by the presence than by the vigilance of a companion
When a head would stay down for a long
time was impossible for a predator to predict. It is concluded that grouping
by ostriches when feeding
results in only a slight reduction in the group's vulnerability to successful predator attack,
but in a considerable decrease in individual vulnerability
Ostriches (Struthio camelus) do not bury their heads in the sand. However, they may
bury the head deep in the grass and other low vegetation on which they are feeding.
When they do so the head is invisible to the observer, and correspondingly, the ostrich's
ability to observe anything in
its environment must be exceedingly small. In particular, its ability to detect visually an
approaching
predator must be minute. Adult ostriches are known to be subject occasionally to
predation by lions (Panthera leo) (Schaller 1972).
At intervals during its feeding, the ostrich raises its head, to a height of over 2 m, and
remains still with the neck erect, moving its head at intervals. It is clear that with their very
acute eyesight (personal observation) as well as the high vantage point, an ostrich with
its head up has excellent powers of observation.
After a static period, variable but averaging several seconds in length, the ostrich resumes
feeding. It was clear that an ostrich with its head down did
not detect a distant novel visual stimulus (a hand being waved), whereas a bird with its head
up usually appeared to do so, although it might make no response other than to remain
stationary with its head raised.
Vigilance (defined as standing with the head high) and pecking (with the head near the
ground) are obviously incompatible. Increased vigilance, and therefore presumably a reduced
risk of being preyed upon, involves a reduction in the time spent pecking and therefore a
reduced rate of intake of food. A compromise is needed.
In principle, ostriches may be able to achieve a compromise more beneficial to themselves if
they feed as a group than if they feed alone. Provided that the members of a group can
make use of the vigilance of their companions, a bird *Present address: Curator
of Mammals Zoological
Society of London, Regent's Park, London NW1 4RY.
feeding in a group can benefit in two main ways. It can reduce its own periods of vigilance
and so spend a greater proportion of its time pecking; or it can spend the same amount
of time pecking but gain greater protection against predation. It can also use a compromise
between these two ways. The aims of this paper are to investigate the way in which
ostriches resolve the time allocation
problem when feeding alone or in groups, and to quantify the effects of being in
groups on their vulnerability to predation. There may, of course, be a cost associated
with feeding
in a group, most obviously that of increased competition for food. Such competition
among the two to four widely dispersed members of a feeding group of ostriches
appears to be minimal and is outside the scope of this paper.
Pulliam (1973) constructed a theoretical model in which birds could afford a constant
proportion of their time not feeding but with their heads cocked and alert for approaching
predators. He showed that the group's ability to detect predators would increase rapidly
with group size at first and quickly level off. Several studies have demonstrated
experimentally that larger groups of birds detected predators sooner,
but in these cases the changes in feeding rate were not quantified
(Powell 1974; Siegfried & Underhill 1975; Kenward 1978). Ostriches are particularly
well suited for both feeding rate and vigilance to be measured simultaneously, and thus for
the different ways in which they benefit from grouping to be assessed in
relation to one another.
Methods
Ostriches were observed during their breeding season in the Lake Jipe region of Tsavo West
National Park, Kenya, in July to October 1977 as part of a longer investigation into their
communal nesting system (Bertram 1979). The habitat there consists of open undulating
grasslands with many scattered bushes and a few trees.
Observations were made using binoculars from inside a vehicle at distances
usually of 0.5-1 km (range 100 m-2 km), at which distance the birds did not react
to my presence. Data were recorded
by running commentary on tape and later transcribed.
Ostriches may feed almost anywhere and at any time, and do not have organized feeding
bouts. They were classified as feeding if during the minute before the observation period
began they were largely static and had their heads down and pecking among the
vegetation for more than
50 % of the time. During the observation period I recorded each time each individual
raised or lowered its head. An observation period was temporarily halted (1) if at any
time any of the birds moved out of sight behind a tree, (2) after a bird had kept its head up
for more than 30 s at a time for whatever reason, or (3) if it was clearly disturbed by
some outside influence; the observation
period was continued again when visible feeding was resumed. Nearly half of the observation
periods contained interrruptions for one or other of these reasons. A length of 4 usable rain
was considered the minimum acceptable, and as a result, observation periods averaged 6 usable
rain in length (range 4-8.25 rain).
Observations were made only of adult birds ( > 2 years old) without young. At that period
only a minute proportion of the adult population was accompanied by either chicks or juveniles.
A number of birds could be recognized individually.
Repeat observations of the same individual were rarely made, and whenever they
were it was at a different date, place and group.
I assessed at intervals each bird's distance from its companions, if any. Birds were considered
to be part of a group if they were within 65 m of one another; usually they were 10-40 m
apart, and only once was any other ostrich visible within 300 m. Most ostrich groups are loose
and transient associations, whether or not the birds are feeding, and of variable composition.
Overall during the study, 49 % of first sightings consisted of single birds, 35% of two birds,
and 16% of three or more birds. The distribution of feeding observations corresponds
approximately to these proportions. Birds did not appear to be aggregating mainly in order to
feed, but for other reasons.
No useful attempt could be made at assessing food abundance, partly because of the
problems of seeing what the birds were eating, and partly because ostriches are known to be
both catholic and selective feeders, so it is difficult to categorize any particular piece of plant
material as being food or not food. It appeared that food was not in short supply, since birds
usually took relatively
few paces while feeding (up to a maximum of about 20 per minute), and spent a large
proportion
of the day neither feeding nor apparently looking for food
Feeding conditions for ostriches were presumed not to change markedly during the
period between 2 August and 2 October when the data reported here were collected,
since no trend over time could be detected in the peck rate while the head was down,
in those instances when peck rate could be measured. In addition, no relationship
could be found between the distribution of the observation periods of different groupings
and the date, the time of day or the distance of the observer
Results
Normal 0 false false false EN-US X-NONE AR-SA
Figure 1 shows the percentage of time each bird spent with its head up during the
observation
Fig. 1. The percentage of a feeding observation for which an ostrich's head was raised
when in groups of Different sizes.
period. The birds have been classified by sex, and according to the number of birds in the
group. Because of the small samples, the larger groups of three and four birds have been
lumped together for analysis. All significance values are one-tailed.
The main results are the following: (1) There was an overall decline in the percentage
vigilance as the number of birds in the group increased (Fig. 1). The median percentage
vigilance of single birds was 34.9, that of birds in groups of two was 22.9, and that of birds in
groups of three or four was 14.0. The differences are significant (Mann-Whitney U tests: single