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Matt Johnson
Lecture Notes
ORNITHOLOGY
(Humboldt State Univ. WILDLIFE 365)
LECTURE 22 – SOCIAL BEHAVIOR
I.
II.
Intro – birds are both predators and prey. Their needs for food and
for protection thus determine whether they are social or asocial,
cooperative or competitive. This lecture focuses on these patterns,
and we’ll conclude by examining and learning from our “bird bow”
experiment last week.
Territoriality. Territoriality and flocking are both conspicuous
elements of avian life, and are well-studied. Spacing patterns are
indeed better studied in birds than in any other animals.
A. Definition – a territory is a given area that is defended for some
period of time, although that may be for very short period of
time. Use of this area by other birds is discouraged with acts of
display or defense. Thus, primary (often exclusive) use of the
area is limited to the defending individual (and in some cases its
family or cooperative group members).
B. Territoriality is not fixed, it is very flexible in time and space
(example of Great Tits abandoning terr. in cold weather, and
sanderling defending a moving Willet).
C. General patterns
1. Territoriality overall is favored at moderate levels of food
availability (we talked about this when we discussed food
availability)
2. As territory size increases, the costs to defend it rise in
proportion to its perimeter (2r), while the benefits rise in
proportion to its area (r2). [Note, the benefits are in
relation to need…so they do not increase without
limit…they follow a logistic, or sigmoid, type of curve;
thanks for the question Stein]. Thus, defensible territories
are moderate in size.
3. Over the defensible range, territory size is further influenced
by food availability in the environment, the less food
available, the larger the territory.
D. Dominance. Territoriality is one expression of dominance
behavior. Dominant birds are better able to defend territories
than are subordinate birds.
1. Dominance status is directly related to age and body size
(sex only to some extent). Generally, larger birds (males in
songbirds and many waterfowl, females in raptors and
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III.
shorebirds) dominate smaller birds, older birds dominate
younger birds.
2. In polyandrous birds, females dominate. In polygynous
birds, males dominate, in monogamaous…not so clear…but
in some cases males dominate probably because these
“monogamous” systems are more polygynous than we first
thought.
3. Remember, must be careful with term “dominance.” We
mean more likely to “win” behavioral interactions…more
likely to pass on genetic material to future generations. This
does not necessarily mean “better”, for environments are
always changing. Just like ol’ Bobby Dylan sings, “the loser
now will be later to win”
4. Rank has its advantages.
a. Dominant birds defend the best territories, which can
confer survival and/or reproductive advantages.
b. Where territorial behavior is lacking, dominant birds get
first access to preferred resources (food, mates, cover).
c. When in flocks, dominant individuals preferentially
occupy middle positions, which shelters them from
predator attacks…..and that brings us to…
Flocking.
A. Flocking behavior exists as a balance, a trade-off, between costs
and benefits, like many other animal behaviors.
B. The principle cost is increased intra-specific competition.
Flocking is, afterall, the antithesis of territoriality. Although
there are exceptions (we’ll get to them later).
C. The principle benefit is reduced predation risk. This advantage
is realized via several, non-mutually exclusive, routes:
1. The flush of a large flock of birds may confuse an attacker
and reduce its probability of success. Also, a tight flock
may actually pose a threat to a fast-approaching falcon.
2. As long as the predators cannot congregate in proportion to
the flock size (most cannot), then predators may become
satiated and individuals in a flock may thus suffer less
predation risk.
3. As flock size increases, the probability that a single given
forager is killed decreases (as long as attack frequency does
not increase in proportion to flock size).
4. Predator detection improves with increasing flock size.
Many pairs of eyes scanning for a predator are more likely to
spot it early than a single pair. Even without
communication, the flush of the “spotter” will alert other
flock members, but in may cases this group flush response is
enhanced by ritualized alarm calls.
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i.
IV.
alarm calls may appear advantageous to all but the
sucker who gave it, but in fact they are “selfish”
behaviors, for 3 reasons.
ii.
Kin Selection. First, alarm calls are often given in
flocks of related birds. By helping its relatives
survive, an individual actually helps pass some its
own genetic material onto the next generation, since
it shares genes with its relatives. Thus, natural
selection favors helping ones kin – that is called kin
selection.
iii.
But even in flocks of unrelated individuals, by
getting the whole flock to flush simultaneously, the
caller reduces its own probability of being nabbed
for the reason above.
iv.
Third, alarm calling may actually attract other flock
members. This attraction would be particularly
advantageous for a dominant birds, which would
then occupy the safe center of the flock.
Test of the trade-off with our data.
A. First, lets think about the trade-off.
1. The cost of flocking is higher intra-specific competition for
food, and the benefit is reduced predation risk.
2. The effects of competition for food are strongest when food
is scarce (costs of flocking go up with decreasing food).
3. Thus, flocking should be favored (benefits>costs) when
food availability is high, but discouraged when food is
scarce.
B. Before we get into testing that, let’s look at our overall patterns.
C. Foraging Efficiency.
1. First, foraging efficiency (measured as the time to find 5
macaroni) increased as food availability declined.
2. Also notice that, in general, the flocking experiments had
lower foraging efficiencies than the territorial experiments
 this is because instead of searching for macaroni,
foragers were busy chasing each other around and pulling
flags. This finding illustrates the cost of flocking -enhanced competition means lower foraging efficiency.
D. Starvation.
1. Second, notice that starvation increased in later days when food was less
available. Fewer macaroni means more starvation (good thing we had the
M&Ms).
2. More interestingly, note that starvation was higher in the experiments lacking
predation than those with predation, probably because predation is a "killing
agent" that is "compensatory," which simply means that the falcons killed some
foragers that probably would have died anyway due to starvation. [This simple
fact, that predation is compensatory, allows the hunting (a form of predation) of
some species to continue without detrimental effects to their populations.]
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V.
VI.
E. Predation.
1. Third, notice that predation is (overall) higher for territorial foragers than for
foragers in a flock.
2. This is because a flock can more easily detect an approaching predator due to
cooperative vigilance -- in our model this was simulated by a single member of
the flock yelling a falcon's name to protect the entire flock (a.k.a., the "Jose Mata
Effect", man that guy was vigilant!!!). This finding illustrates the benefit of
flocking -- reduced predation risk.
F. Survival.
1. Overall.
a. Fourth, examine the probabilities of survival for the four experiments. Notice
that in the absence of predation (green and yellow lines), flocking individuals
fared worse (lower survival) than did territorial foragers. This makes sense,
these two experiments modeled the costs (competition) but not the benefits
(reduced predation) of flocks, so of course it was better not to flock.
b. Notice also that survival rates in the non-predation experiments (green and
yellow) were high at first, then plummeted well below the survival rates of the
experiments with predation (blue and red lines). This is because predation, by
removing foragers from the population early in the study, effectively kept food
from becoming overly depleted late in the study. In fact, in the presence of
predation, some foragers managed to still find 5 macaroni on late days because
some other hapless foragers (read: Matt) kept getting picked off by the falcons
(a.k.a. the "Eva Spens Effect"!!), and food remained relatively abundant.
c. This finding illustrates how predation can help stabilize populations which
may otherwise crash as a result of exhausted local food supplies.
2. To flock or not to flock??
a. Lastly, in the presence of predation, we ask the big question: To flock or not to
flock? From our data, we can see that the flocking foragers had higher
survival than territorial foragers up to about day 13 or 14, after which
territorial birds fare better...the lines cross!!
b. This finding fits our prediction: on day 13 or 14, macaroni became so scarce
that the cost of high competition in a flock outweighed the benefit of reduced
predation risk. Take home message: flock until food becomes scarce, then
defend a territory!! Alternate take home message: If you find yourself, for
whatever reason, searching the grass for pasta and they're lots around, find a
friend so you don't get killed by falcons.
Flock size (see figure 14-8 in text)
Mixed species flocks...I just have hand written notes on this….it’s fairly
straightforward…see text pp 343-345)