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The Preening Invitation or Head-Down Display of Parasitic Cowbirds: Ii. Experimental Analysis and Evidence for Behavioural Mimicry

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1) Parasitic cowbirds perform an unusual display in which they rapidly approach other birds and freeze in a bowed, head-down posture. Individuals of other species often respond by preening the cowbird. SELANDER & LA RUE hypothesized that this is an interspecific appeasement display used to reduce the aggressiveness of hosts trying to drive cowbirds from nests. However, intraspecifically directed displays are in fact common. Experiments presented here were designed to elucidate the stimuli eliciting the display and to thereby infer its function. 2) In Experiment 1, cowbirds resident in three cages directed significantly more displays to new male and female cowbirds and female red-winged blackbirds added to their cages than to each other; but introduced female redwings elicited more displays than introduced cowbirds. Experiment 2 indicated that redwings are stronger releasers than cowbirds because they are more submissive and fearful, not because the display per se is preferentially directed to other species; resident cowbirds displayed at equal intensities to mounts of female redwings and female cowbirds. Experiments 1 and 2 showed that the display is aggressive and does not represent appeasement. Introduced cowbirds were nearly always submissive to resident cowbirds yet they rarely displayed to residents as would be the case if the display represents appeasement. Often, residents alternated between attacking and displaying to introduced birds, thus making clear their aggressive intent. 3) Experiment 3 tested directly the hypothesis that the display is aggressive. Cowbirds and redwings from separate cages voluntarily entered each other's cage by flying through a passageway. Cowbirds did no displays while in the redwing cage although they displayed to redwings that "invaded" their cage. But these invading redwings elicited many fewer displays than redwings that were forcefully added to the cowbird cage after being handled and that hence showed higher levels of frightened-submissive behaviour. These results support the hypothesis that the display is motivated largely by aggression. 4) Experiment 4 also demonstrated that the display is aggressive. Resident female cowbirds that displayed intensely when a female redwing was added to their cage were later added to the redwing's cage. Each female cowbird was dominated in the redwing cage and did significantly fewer displays there than in her own cage. 5) Experiments 1-4 showed that cowbirds display preferentially to new birds that exhibit frightened-submissive behaviour. Experiment 5 demonstrated that frightened-submissive behaviour exhibited by a familiar bird increases the individual's releasing power. Cowbirds displayed preferentially to female cage mates that were returned to the cage after having been temporarily removed and handled. The displays ceased when the females no longer showed the stress of having been handled. 6) Experiment 6 tested whether new birds, strangers per se, are releasers or whether only birds that exhibit frightened-submissive behaviour are releasers. Cowbirds from separate cages voluntarily entered each other's cage by flying through a passageway. Because birds were not handled, there were no strong differentials in frightened-submissive behaviour between birds from different cages. Cowbirds from both cages directed nearly all of their displays to birds from the other cage, thus showing that stranger status per se is a releaser for the display. 7) All available evidence indicates that the display is motivated largely by aggression and that it is a threat gesture. The display seems to be a safe way of assessing the aggressiveness of other birds because display recipients never respond with anything more aggressive than a light peck. Cowbirds display most often to birds whose size, aggressiveness and fighting ability closely match their own capabilities. The display is directed to new and familiar birds that exhibit frightened-submissive behaviour because such individuals are at a disadvantage. The display is directed to strangers because selection presumably favors dominating as many other individuals as possible. 8) The preening of cowbirds by other species is probably due to a severe conflict situation. The conflict may be heightened because the display poses a confusing message, combining elements of both threat (rapid approach) and appeasement (bowed head). Overall though, other species often regard the display as representing mostly appeasement and respond by allopreening, a common response to intraspecific appeasement display. Because the display represents threat by the cowbird but other species respond as if it represents appeasement, interspecific displays constiture mimicry because the receiver is deceived. Even though other species respond as if they were being appeased, cowbirds are usually able to derive the benefits of dominance because the display enables them to occupy space previously held only by the other bird. Cowbirds almost never preen one another. As practitioners of the display, they have no difficulty interpreting its true message - threat. 9) The head-down display does not conform well to some of the generalizations used in motivational analyses. It is a threat display yet it does not emphasize the anatomical features used in fighting. The display is also a strong exception to DARWIN'S principle of antithesis. 10) There is no evidence to support SELANDER & LA RUE'S hypothesis that the display is used to reduce the aggressiveness of hosts that attack cowbirds near nests. The species most often displayed to in nature are rarely parasitized. However, the display probably would function just as proposed by SELANDER & LA RUE.

Affiliations: 1: Department of Biological Sciences, University of California, Santa Barbara, U.S.A.


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