ECOL 487L, Animal Behavior Lab
Lab#7: Vigilance- foraging tradeoff
During the JWatcher lab two weeks ago, we watched several videos of foraging animals
to compare the time they allocate to foraging and vigilance. By watching house finches,
we ask a similar question this week. We will be observing how risk of predation affects
foraging behavior, and whether different group sizes influence how much time animals
allocate to foraging versus vigilance.
Group living, vigilance and foraging
The behaviors that maximize animals energy gain and fitness, such as foraging and
mating often make them vulnerable to predators. In order to avoid being attacked,
animals frequently raise their heads to scan for predators. However, this reduces the
amount of time they can spend on foraging. A strategy that would increase foraging time
while reducing scanning time without increasing the risk of predation would be
beneficial.
One of the advantages of living in groups is the increased safety from predation. This
advantage is so important to the animals that it has even been recognized as a possible
reason for social evolution (Wilson 1975). Increased safety can either be due to the
dilusion effect, which predicts that any individual in a large group has a lower chance of
being preyed on (summarized in Roberts 1996), or due to an increased chance of predator
detection when more animals are scanning for predators.
A large number of studies have documented that in group living animals, individuals
foraging in groups will forage for longer periods while scanning for predators less
frequently than individuals that forage alone. The observed decrease in vigilant behavior
is even more pronounced when individuals forage in larger groups (Roberts 1996).
Several theoratical models and some empirical data has also shown that the physical
location of individuals within the group also influences whether they are vigilant or not
(Fernandez-Juricic et al. 2004)
Questions
We will be watching house finches at feeders to address the following questions:
1) Does the time allocated to foraging and vigilance change when individuals are in
a group?
2) What influence does the group size have on foraging and vigilance?
3) Does location of individuals in the group affect their foraging and vigilance
behavior?
Based on the information above, what would your predictions for each of these questions
be?
For more information on the vigilance-foraging tradeoff, you can read two papers that are
among the most influential papers on the topic: Bednekoff and Lima (1998) and Gilberts
(1996), see references below.
Instructions
We will divide into four groups and visit bird feeders on campus. Hawks are known to
prey on birds at feeders, and two of the feeders we will visit are at high predation areas.
For the first few minutes at a feeder, watch the birds without recording any data to
familiarize yourself with the amount of turnover that happens in the feeders, and to give
the birds some time to adjust to your presence. You can watch the birds that are at the
feeders as well as the birds that are on the ground around the feeders. Then, pick one
individual, start your stopwatch, and record data on:
1) Time of observation
2) Whether it is feeding or vigilant. Define vigilant behavior as not picking seeds
from the feeder and looking away from the feeder (may be looking towards you).
3) How many birds are at the feeder (and/or on the ground)
4) The location of the bird. If it’s on the feeder, are any other birds facing the same
way as it is? If it’s on the ground, is it close to the feeder, or away from the
feeder? If there are several birds on the ground, where is the one you’re watching
located in comparison to others? Are the birds feeding and eating at the same
time – how mutually exclusive are these behaviors?
After about 15 minutes of taking detailed observations on individual birds, focus your
attention on taking data for the analysis. Work together as a group to do this. As a
group, decide if you wish to focus on one species or several, and birds on the ground or
feeder. This may vary with the activity level of your feeder. Also determine a definition
of “flock:” for instance, all birds within a 2-foot radius, or all birds directly adjacent to
one another. Record these group-specific data on the top of your data sheet and please
stick to this information for the rest of the lab. Next, try each of two data collection
methods to record flock size and vigilance.
First, use focal individual sampling. Choose a focal bird and record data on this bird for
2 minutes – record the total number of separate times the birds look up and scan for
predators (birds on the ground may tilt their head). At the start and ending of the two
minute period, count the number of birds in the flock. Aim to get focal sampling data on
at least two birds (per person): following scan sampling, compile the data within your
group.
Second, shift your attention to the overall dynamics of the group and try scan sampling.
Every two minutes, count the number of birds present, and record the number of birds
engaged in each behavior (vigilant/foraging). You may wish to, as a group, divide the
space into focal areas (e.g., one person watches the area to the left, etc.).
After observations, compile the data for your group, and turn into the TA.
Statistics:
In order to address the question of how group size influences the time allocated to
foraging and vigilance separately, we will use linear regression. For examples of graphs
of data analyzed by these tests, refer to the statistics handout from the Desert Museum
lab. Stay tuned for another handout (available on Thursday) with more details on how to
analyze your data for your lab report.
Note:
While you are watching the birds, keep in mind another aspect of vigilant behavior. Some
animals have long bouts of vigilant behavior, where they cease foraging for a long time to
watch for predators. This behavior is called sentinel behavior, and has been the subject of
much research in birds and mammals. Results of some studies showed that animals
became more sentinel when they were well-fed (Bednekoff and Woolfenden 2003). Do
you think that watching birds forage on feeders where they have constant access to food
could have influenced the results of our study? If we were to repeat the study in an
environment with less resources, how different would our results be?
For the lab report: In addition to your graphs and statistical results, include (in
introduction) your predictions for the three questions we addressed today. Question #2 is
the one we hope to address with the analysis detailed above, but your preliminary
observations should be applicable to questions 1 and 3.
In discussion, you might include your ideas on the sentinel behavior issue and whether
watching animals at feeders may have biased our results. You may also address possible
biases from our experimental design – for instance, we were interested in exposing you to
multiple methods, but in doing do, how might this have affects the comparisons over
time?
We addressed one of the benefits of group living today. What are some of the costs of
group living? Do you think that some of those costs could be the reason why some
animals do not live in groups? Why or why not?
References:
Bednekoff, P.A.& Woolfenden, G.E. 2003. Florida scrub-jays (Aphelocoma
coerulescens) are sentinels more when well-fed (even with no kin nearby). Ethology, 109,
895-903
Bednekoff, P.A.& Lima, S.L. 1998. Randomness, chaos and confusion in the study of
antipredator vigilance. Trends in Ecology & Evolution, 13, 284-287
Fernandez- Juricic, E., Siller, S. & Kacelnik, A. 2004. Flock density, social foraging, and
scanning: an experiment with startlings. Behavioral Ecology 15, 371-379
Gilberts, G. 1996. Why individual vigilance declines as group size increases. Animal
Behavior, 51, 1077-1086
Wilson, E.O. 1975. Sociobiology: The new synthesis, Harvard University Press.