Vocal Response to Predators in Pair-Bonded Carolina Wrens

The Carolina Wren (Thryothorus ludovicicreate a uniform visual background.It was provided anus) has a social system that is much more with a one-way observation window.characteristic of tropical than temperate zone The hawk was trained to fly across the room to an species.Males and females form permanent exposed perch.It flew from a perch concealed from pair bonds (Bent 1948, Thomas 1953, and the wrens directly over the caged wrens to a perch in view of the wrens.The hawk was tethered-to a pers.observ. of a color-banded population monofilament line on a nullv which standardized its over 4 years) and pairs defend a territory flight path.When in flight,-the hawk passed 1.7 m throughout the year.above the cage to a perch 2.5 m from the center of Probably due to this social system, Carolina the wren cage.Wren sounds elicited were tape-recorded via a Wrens use all of their vocal signals throughout microphone in the testing room.Concurrently from the year (Morton, in press).Here we present an adjacent room, we observed wren and hawk be- data derived from field observations and lab-havior and tape-recorded our observations.oratory experiments on one of this wren' s vo-The sounds were analyzed on a Kay Elemetrics cal signals, which we term the chirt.Carolina Sonograph; amplitude and temporal data were analyzed with a B and K graphic level recorder.Wren chirts are structurally variable and are delivered in temporally variable ways.We FIELD STUDY hope to explain here how this call varies and A population varying from 8 to 10 pairs in deciduous how its variation is adaptive within the Caro-forest surrounding one author' s residence was colorlina Wren' s social system.Since chirt calls banded.Nearly daily monitoring of behavioral events depart from the commonly held view that within the population was made by Morton from April 1972 to September 1975.Vocalizations were most bird sounds are rather discrete (Marler tape-recorded together with observations of contextual 1967, Smith 1968, 1969), a discussion of them events (Morton, in press).Naturally occurring chirts may shed new light on sources of selection were included in this study and are discussed here.favoring graded communication systems.Two field experiments were performed using the same laboratory hawk as a stimulus.In one, the hawk was tethered in a wren territory.In the second, the METHODShawk was tethered and the female member of a wild wren pair was placed in the hardware cloth cage 2.5 LABORATORY STUDY m from the hawk while the male was free.We tauerecorded our observations from a blind ca.15-m Eight Carolina Wren pairs were captured from the away.same woodlot near College Park, Maryland, during October, November, and January (non-breeding sea-RESULTS son) and tested in the laboratory for their vocal responses to a live Roadside Hawk (Buteo magni-LABORATORY STUDY rostris ) .To capture mated pairs a hand-raised female wren One or both members of each pair responded was placed in a cage near a tape recorder that played to the hawk presentation with short harsh back male song.The captive female responded to sounds we termed chirts (Figs.l -4).Both the playback with a female "chatter" vocalization. males and females emitted chirts but two ofThe effect was to duplicate an intruding pair of wrens so that both the male and the female wren that we the females also used a second call termed dit wished to capture would fly to the vicinity of a mist interspered with chirts (Fig. 1).There was net where they were captured.Six pairs captured in no consistent sexual difference in chirt structhe morning were used the same afternoon; two pairs ture, but each pair member had a distinctive captured in the afternoon of one day were used the quality to its chirts such that we could difnext afternoon.The pairs were color-banded and released on their territories after use.ferentiate them by ear, but we do not know if Each pair was placed in a .6 x .6 x 1 m hard-wrens recognize individuals on this basis.The ware cloth cage containing two wooden dowel chirt response persisted up to three hours beperches; water and food (mealworms) were available on the cage floor.Each caged pair was isolated fore completely habituating, providing a comin a sound-attenuated room for two to four hours bined total of about 12 h total response from acclimation before exposure to the hawk in that room.the 8 experiments.The room (3.46 x 3.20 x 2.45 m) was completely Two aspects of this vocal response appear lined with white blankets to reduce echoes and to to be unique in avian communication studies: D221