«STEPHEN NOWICKI1,2), WILLIAM A. SEARCY3) and MELISSA HUGHES4,5) (1 Department of Zoology, Duke University, Durham, NC 27708-0325; 3 Department of ...»
THE TERRITORY DEFENSE FUNCTION OF SONG IN SONG SPARROWS:
A TEST WITH THE SPEAKER OCCUPATION DESIGN
STEPHEN NOWICKI1,2), WILLIAM A. SEARCY3) and MELISSA HUGHES4,5)
(1 Department of Zoology, Duke University, Durham, NC 27708-0325;
Department of Biology, University of Miami, Coral Gables, FL33124-0421;
Institut für Verhaltensbiologie, Free University, D-12163 Berlin, Germany) Short title: Territory defense function of bird song
Nowicki et al. – page 2 territory). In all cases in which encroachments or takeovers occurred, these occurred first (or solely) on the control territory of a matched pair, supporting the hypothesis that song functions in territory defense in this species. The incidence of intrusions on control territories was very low, however, posing difficulties for the interpretation of speaker replacement experiments designed to ask more specific questions about how song functions in male-male aggressive competition.
INTRODUCTIONBird song, as with the calls of frogs and songs of acoustic insects, is thought to have two principal functions: attraction and stimulation of females, and territory defense (Catchpole & Slater, 1995; Searcy & Andersson, 1986). Most of the evidence that bird song functions as a “keep out” signal in territory defense is indirect, however, consisting of observations such as the coincidence of seasonal periods of song with periods of territory defense (e.g., Catchpole, 1973), the tendency of territory owners to increase singing rates when faced with an intruder (e.g., Kramer & Lemon, 1983), and the use of song in interactions between neighboring territory owners (e.g., Krebs et al., 1981). All these patterns are suggestive, but none directly demonstrates that song is effective in keeping rival males off a territory.
This lack of direct evidence reflects the difficulty of finding appropriate and tractable methods for experimentally testing the role of song in territory defense. The predominant method for investigating communication between male birds is territorial playback, in which a song is played from a loudspeaker set on a male's territory and the aggressive response of the owner is measured (Falls, 1992). This technique has been highly successful in answering questions about discrimination, for example questions
playback, however, does not directly answer questions about the territory defense function of song. Song is thought to function in territory defense by keeping intruders off the territory, but territorial playback measures the response of owners, rather than intruders, and songs score well if they evoke aggression, rather than avoidance.
Two experimental designs exist that better assess the function of song in territory defense: muting and speaker occupation. In muting experiments, territorial males are rendered unable to sing, either by denervating the syrinx (Peek, 1972) or by puncturing the interclavicular air sac (Smith, 1979). Muting experiments have been performed on three species, red-winged blackbirds (Agelaius phoeniceus) (Peek, 1972; Smith, 1979) seaside sparrows (Ammodramus peninsulae) (McDonald, 1989), and ochre-bellied flycatchers (Mionectes oleagineus) (Westcott 1992); in all three species muted males experience increased rates of intrusion and territory loss relative to unmuted controls, providing direct evidence for the territorial function of song. The utility of this method is limited, however, to testing the general function of song as a “keep out” signal; it does not provide a means for testing more specific functional hypotheses, such as whether large repertoires are more effective than small repertoires in territory defense.
In speaker occupation experiments, owners are removed from their territories and replaced by speakers broadcasting song (Göransson et al., 1974; Krebs, 1977). Such experiments have been performed with four species: thrush nightingales (Luscinia luscinia) (Göransson et al., 1974), great tits (Parus major) (Krebs, 1977; Krebs et al., 1978), red-winged blackbirds (Yasukawa, 1981a, b), and white-throated sparrows (Zonotrichia albicollis) (Falls, 1988). In all cases, territories defended by broadcast song remained unoccupied longer or suffered lower rates of intrusion relative to control territories from which the male is removed but no song is broadcast (although not all these studies used large enough samples to permit statistical analysis of the effect). The important advantage of speaker replacement experiments is that they offer the possibility
because one can compare the relative effectiveness of different kinds or patterns of songs in repelling males from the territory.
Our goal in the present study is to test the territory defense function of song in the song sparrow (Melospiza melodia). Song sparrows are monogamous, territorial passerines, widespread in North America (Nice, 1937). Territorial male song sparrows sing throughout their 3 to 4 month breeding season (Nice, 1943). Each male song sparrow has a repertoire of approximately 5 to 14 discrete song types (Borror, 1965), but also sings an apparently unlimited number of variants of each type (Podos et al., 1992).
Strong evidence already exists for an intersexual function of song in song sparrows, including the observation that singing rates drop dramatically when males attract a female and rebound if that mate is lost (Nice, 1943; Searcy, 1984), and experimental demonstrations that song stimulates females to perform copulation solicitation display (Searcy & Marler, 1981). Support for an intersexual function of song does not, of course, preclude song from also having an intrasexual function in territory defense (Catchpole & Slater, 1995).
Some patterns of singing behavior shown by male song sparrows have been interpreted as aiding in defense of territory. Kramer and Lemon (1983; Kramer et al.,
1985) showed that male song sparrows increase the frequency with which they switch between song types as the context of singing becomes more aggressive, and suggested that switching rate is used as a graded aggressive signal aimed at other males. Beecher et al. (1996) found that male song sparrows in a Washington population interact with neighbors by "repertoire matching," replying to a neighbor with a song in that neighbor's repertoire. Matching is thought to be a tactic that allows an aggressive message to be aimed at a specific individual (Brémond, 1968), and thus is also interpreted as functioning in male-male aggressive competition for territories. These interpretations of song switching and song matching presuppose that song functions in territory defense, an
We chose speaker occupation to test the territory defense function of song because this design can be elaborated to test the relative effectiveness of different types of song or singing behaviors. If initial experiments demonstrated that presence of song is more effective in limiting intrusion onto territories than is absence of song, we planned to move on to look at more complex questions, such as whether variable song is more effective in territory defense than is invariant song, whether faster song rates are more effective than slower rates, and so forth.
METHODS Experiments were performed during June, 1995, May and June, 1996, and May, 1997 on Pennsylvania State Gamelands No. 285 in Crawford County, Pennsylvania. The song sparrows used in the experiments held territories on the margins of old fields, in some cases where the old fields bordered second growth deciduous forest, and in other cases where the old fields bordered a lake or marsh.
Our basic procedure was as follows: 1) We chose a pair of nearby territories, and randomly designated one as a control and one as an experimental territory. 2) We removed the owners from both territories as simultaneously as possible. 3) We set out two speakers on the experimental territory, and played recorded songs of the original owner alternately from one speaker and then the other throughout the remainder of the trial, while leaving the second territory as a silent control, unoccupied by speakers. 4) We observed which of the two territories (if either) was first to be wholly or partially taken over by another male song sparrow. We expand on each of these points below.
1) Each pair of experimental and control territories was located in the same old field, and was chosen so that the two territories would be matched as much as possible for size, habitat quality and for the density of local territory owners and floaters. In no
any boundary. In all cases we had previously color banded the owners of both territories in a pair, so that we could map territory boundaries prior to the trial. Finally, in all cases we had previously recorded songs from the owners of both territories. We recorded owners of control as well as experimental territories so that there could be no bias towards using better singers as experimental rather than control males. Only after recordings of both birds were complete did we choose one of the pair as the experimental territory using a coin flip.
2) We removed owners of experimental and control territories by attracting them to mist nets using playback of song sparrow song. For this purpose, we constructed a playback tape for each experimental and control territory using one song of the owner of that territory; by playing only the owner’s song during capture we sought to minimize the chance that playback would seem to neighboring males to represent an intrusion onto the territory. We attempted to capture both the experimental and control males as simultaneously as possible early in the morning on the first day of the trial, using as few playback songs as possible. On two occasions we succeeded in capturing an owner using only three playback songs, whereas at the other extreme we once continued playback intermittently for 70 minutes before succeeding. More typically males were captured with 2-10 minutes of song. During two trials we used playback of swamp sparrow (Melospiza georgiana) distress screams as well as song sparrow song in capturing males.
During 3 of 11 pairs of trials we failed to capture one of the two target owners on the initial day, and therefore performed the second removal one or two days later (see Table 1). Experimental males were removed on average at 06:12 (EDT) and control males on average at 06:21. Removed males were held in cages and provided with mealworms, grain, and water, and all were released in good condition at the end of the trial. Most immediately reoccupied their territories.
3) As soon as an experimental male was removed, we set up two speakers 15 m
Sony TC-D5M stereo cassette recorder and two matched SME speakers (Saul Mineroff Electronics, Elmont, NY) were used for playback. Playback tapes were constructed so that 3 minutes of song would be played from one speaker, followed by 1 minute of silence, 3 minutes of song from the second speaker, 1 minute of silence, etc. During the 3 minutes of song, songs were presented at the rate of 1 song per 10 sec. Songs consisted of six variants (Podos et al., 1992) of one song type recorded from the owner of the territory; thus a different playback tape was used in each of the 11 experimental trials.
Playback tapes were made using digitized songs recorded previously from the experimental male (25kpts/s, SIGNAL sound analysis software; Beeman, 1996).
Amplitude was set at a level that seemed by ear to match that of singing males in the field, and was measured as 89-92 dB (depending on the song) at 1 m. We started playback on the experimental territory on average about 20 minutes after the male was removed.
4) During the eight pairs of trials for which we removed experimental and control males on the same day, one observer watched both territories alternately, spending an equal amount of time watching each during successive 30 min periods. During the three pairs of trials for which the experimental and control removals occurred on separate days, an observer watched the relevant territory continually. During 1995, we continued playback and observations until 20:00 on the day of the removal, and then discontinued the trial whether or not any takeover had occurred on either territory. During 1996 and 1997, we continued trials into a second day if no takeover occurred during the first day.
In these cases, we discontinued playback at dusk (ca. 20:30) on the first day, resumed playback at dawn (ca. 05:30) on the second day, and continued the trial to 10:00 on the second day.
We used the presence of a singing male on a removal territory as our criterion for defining a takeover or encroachment. If the intruding male sang throughout the territory,