period, colonially breeding swallows—which are more vulnerable to being
cuckolded—produced high rates of alarm calls almost exclusively during
the egg-laying period, when females are most likely to engage in extra-pair
matings.
To determine whether the difference between solitarily and colonially
breeding swallows reflects a behavioral polymorphism, Møller conducted
a second experiment involving the presentation of a model male swallow.
Males were more likely to produce false alarm calls to a model male
swallow during the nest-building and egg-laying periods than in the incubation period and were more likely to produce false alarm calls to the
model swallow than to the control, a model willow warbler. This shows that
solitarily breeding male swallows are sensitive to the risks of extra-pair
copulations and are most responsive to this risk when their mates are fertile.
Once again, Møller’s observations and experiments on barn swallows fit
our three definitional conditions but appear inconsistent with the prediction of rarity. The barn swallows’ alarm call is generally given in the context
of predator detection and elicits a flight response. Males have evolved the
capacity to use this signal to manipulate the responses of their mates,
thereby fending off the threat of extra-pair copulations. False alarm calls
therefore provide barn swallows with a mechanism to decrease paternity
uncertainty. Like Munn’s antshrikes and tanagers, it appears that the cost
of ignoring the alarm call is high relative to the benefit of an extra-pair
mating. Even if the male has made an error, falsely signaling the presence
of a predator, it is to the female’s advantage to flee and then return at a
later time to mate. This economic imbalance may enable males to produce
false alarm calls at high rates.
Møller’s experiments reveal that barn swallows are not acting reflexively.
The use of false alarm calls appears to be under facultative control, sensitive to the risks of extra-pair copulations and the female’s reproductive
cycle. Several questions remain, however. For example, although the male’s
false alarm call temporarily breaks up a covert mating, does the female in
fact obtain fewer extra-pair matings? If a male produces a false alarm call
and the female fails to return to the nest, does he try again, perhaps even
more frenetically? Ristau (1991), in her work on the broken-wing distraction display in plovers, has noted that when a predator ignores the plover’s
first try with an injury-feigning display, the plover tries again, and does so
more dramatically, swooping at the predator in order to grab its attention.
Although male barn swallows distinguish between a model swallow and a
model warbler, do they distinguish between a model male swallow who is
in the company of a model female swallow? Do they perceive a potentially
mated pair as a lower risk? What about an anesthetized swallow who looks
dead? There is clearly no risk, but only if swallows make a clean distinction
between living and dead. If we artificially escalate the rate of false alarm
calls, and do so in a situation where the female can see her mate, will she
abandon him in search of a more honest mate? What are the acoustic cues
112
W.T. Fitch and M.D. Hauser
cuckolded—produced high rates of alarm calls almost exclusively during
the egg-laying period, when females are most likely to engage in extra-pair
matings.
To determine whether the difference between solitarily and colonially
breeding swallows reflects a behavioral polymorphism, Møller conducted
a second experiment involving the presentation of a model male swallow.
Males were more likely to produce false alarm calls to a model male
swallow during the nest-building and egg-laying periods than in the incubation period and were more likely to produce false alarm calls to the
model swallow than to the control, a model willow warbler. This shows that
solitarily breeding male swallows are sensitive to the risks of extra-pair
copulations and are most responsive to this risk when their mates are fertile.
Once again, Møller’s observations and experiments on barn swallows fit
our three definitional conditions but appear inconsistent with the prediction of rarity. The barn swallows’ alarm call is generally given in the context
of predator detection and elicits a flight response. Males have evolved the
capacity to use this signal to manipulate the responses of their mates,
thereby fending off the threat of extra-pair copulations. False alarm calls
therefore provide barn swallows with a mechanism to decrease paternity
uncertainty. Like Munn’s antshrikes and tanagers, it appears that the cost
of ignoring the alarm call is high relative to the benefit of an extra-pair
mating. Even if the male has made an error, falsely signaling the presence
of a predator, it is to the female’s advantage to flee and then return at a
later time to mate. This economic imbalance may enable males to produce
false alarm calls at high rates.
Møller’s experiments reveal that barn swallows are not acting reflexively.
The use of false alarm calls appears to be under facultative control, sensitive to the risks of extra-pair copulations and the female’s reproductive
cycle. Several questions remain, however. For example, although the male’s
false alarm call temporarily breaks up a covert mating, does the female in
fact obtain fewer extra-pair matings? If a male produces a false alarm call
and the female fails to return to the nest, does he try again, perhaps even
more frenetically? Ristau (1991), in her work on the broken-wing distraction display in plovers, has noted that when a predator ignores the plover’s
first try with an injury-feigning display, the plover tries again, and does so
more dramatically, swooping at the predator in order to grab its attention.
Although male barn swallows distinguish between a model swallow and a
model warbler, do they distinguish between a model male swallow who is
in the company of a model female swallow? Do they perceive a potentially
mated pair as a lower risk? What about an anesthetized swallow who looks
dead? There is clearly no risk, but only if swallows make a clean distinction
between living and dead. If we artificially escalate the rate of false alarm
calls, and do so in a situation where the female can see her mate, will she
abandon him in search of a more honest mate? What are the acoustic cues
112
W.T. Fitch and M.D. Hauser