Effects of food supplementation, song playback, and temperature on vocal territorial behavior of Carolina Wrens

--We found both the song rate and the rate of song-type change of male Carolina Wrens (Thryothorus ludovicianus) in winter were positively correlated with ambient temperature. When the effect of temperature was controlled statistically, food supplementation significantly increased both the song rate and the rate of song-type change. Song playback did not significantly increase either song rate or rate of song-type change, however. Because foraging and singing are mutually exclusive behaviors in Carolina Wrens, the increase in vocal territorial behavior associated with warmer temperatures and food supplementation may reflect a decrease in the time required for foraging. The rate of vocal territorial behavior in winter may be more dependent on the amount of food available to wrens than on the presence of intruders. Received 23 December 1986, accepted 14 July 1987. THE establishment and maintenance of territories requires significant investments of time and energy (Wittenberger 1981, Davies and Houston 1984). Because this cost of territoriality may reduce the amount of time individuals have to devote to foraging and breeding, a variety of ecological factors may influence the amount of time individuals devote to territory maintenance (e.g. Ydenberg 1984, Tatum 1985, Ydenberg and Houston 1986). We attempted to evaluate the effects of three ecological factors, food availability, intruder pressure, and temperature, on the amount of time male Carolina Wrens (Thryothorus ludovicianus) devote to vocal advertisement and defense of their territories during the winter. Increased food availability has been shown to enhance territorial behavior in many birds, including the Great Tit (Parus major; Ydenberg 1984), the Red-winged Blackbird (Agelaius phoeniceus; Searcy 1979), and the Calliope Hummingbird (Stellula calliope; Tatum 1985). Searcy (1979) proposed that the increase in display rates observed in food-supplemented Red-winged Blackbirds was due to food supplementation and not to intruder pressure, because he measured no increase in intrusion rate. In wintering Sanderlings (Calidris alba) territory size was in• Present address: 994 Sycamore NW, Corvallis, Oregon 97330 USA. 2 Present address: Section of Neurobiology and Behavior, Cornell University, Ithaca, New York 14853 USA. 11 versely related to intruder pressure, which was in turn positively correlated with food abundance (Myers et al. 1979). When intruder pressure declined and food abundance remained the same, territory size increased, indicating that intruder pressure and not food abundance directly influenced territory size. Norton et al. (1982) manipulated intruder pressure on the territories of Black-chinned Hummingbirds (Archilochus alexandri) while food availability was held constant and found that territory size decreased as intruder pressure increased. Few studies are reported in which food availability and intruder pressure were manipulated simultaneously (for an exception see Tatum 1985). Because Carolina Wrens in North America are permanently monogamous and permanently territorial (Morton 1982, Simpson 1985), they are ideal subjects for a study of the effects of food availability and intruder pressure on territorial behavior. During winters when preferred foraging microhabitats are covered by deep snow, winter mortality can exceed 90% (Morton 1982). Therefore, the ability of the male to establish and maintain a territory on which sufficient food is available during the winter often determines which pairs survive. Male Carolina Wrens defend territories primarily by singing, and they sing vigorously throughout the year. Because males form pair bonds when they are too young to sing a fully developed song, song seems to be more important in territory maintenance and defense than in mate attraction (Morton 1982). By conducting the The Auk 105: 11-16. January 1988 12 STRAIN AND MUMME [Auk, Vol. 105 study in winter, we avoided the possibility of any confounding effects of mate attraction or other breeding behavior. MATERIALS AND METHODS The study was conducted at Memphis State University's Meeman Biological Field Station, 20 km north of Memphis in Shelby Co., Tennessee. In late October and early November 1985, playbacks of male and female songs and calls were used to attract Carolina Wrens into mist nets. Eighteen birds were banded with unique color combinations and released. From this population of color-banded wrens we chose 8 pairs for intensive study. The male and female were banded in 5 of the pairs, and only the male was banded in the other 3 pairs. The 8 study pairs were observed extensively in late November and early December 1985. During these observations sex was determined on the basis of vocal behavior (Morton 1982), and the territories were mapped. Territory size ranged from 2.1 to 8.1 ha (2 = 4.1 ha). To avoid habituation, the study pairs were not subjected to playbacks during territory mapping. Food availability and intruder pressure were manipulated to determine if either variable had a significant positive effect on vocal behavior. Food availability was manipulated by food supplementation, and increased intruder pressure was simulated by the use of playbacks of recorded Carolina Wren vocalizations. Each of the 8 study pairs was subjected to each of four possible treatments: (1) food supplementation with playbacks, (2) food supplementation without playbacks, (3) no food supplementation with playbacks, and (4) no food supplementation without playbacks (Table 1). This produced 32 separate observation periods. The study pairs were divided randomly into two groups: those that received food supplementation during the first block of observations (10 December 1985 to 6 January 1986) and those that received food during the second block of observations (12-23 January 1986). Four days before the beginning of each block of observations, two feeding stations were established on each appropriate territory. Feeding stations consisted of cans measuring 17 x 15 cm and open on one end. The cans were placed on their sides and partially filled with dirt to hold them in place and provide a natural surface for foraging birds. A 14 x 18 x 2.5 cm board was placed in front of the opening to the feeding station and partially covered with leaf litter. Feeding stations were placed in vine tangles and tree falls to take advantage of the Carolina Wren's preference for feeding in concealed areas. Each day throughout the observation period 100 mealworms (larvae of Tenebrio sp.), approximately 11 g total, were added to each feeding station. The mealworms were placed primarily inside the cans, with a few scattered on the board to attract birds to the feeding stations. We observed Carolina Wrens near the feeding stations on 7 of the 8 experimental territories and saw birds enter the cans on 4 territories. During the observation periods the first playback began 30 min after observations began, with additional playbacks hourly for a total of four playbacks. For each playback the tape player was placed on the ground near the center of the territory. Each playback lasted 2 min and began with 30 s of silence, during which the observer retreated approximately 5 m and made himself as inconspicuous as possible. The remainder of the playback consisted of 30 s containing 8 songs by a male from the local population followed by 1 min of 15 male songs and 6 female chatters (see Morton 1982 for a description of chatters) from a population of Carolina Wrens in North Carolina (see Simpson 1985 for North Carolina localities). Vocalizations from local and distant populations were used to avoid any possible difference in singing response to familiar and unfamiliar songs (Shy and Morton 1986). The same tape was played at the same volume for all playbacks. Although all male and female vocalizations were noted, male vocalizations used in long-distance territory maintenance and defense (song, song-type change, and "cheer" calls) were of particular interest. Individual male Carolina Wrens have song repertoires of 27-41 different song types (Morton 1982), each consisting of a single component repeated several times (Kroodsma 1977). Switching song types is often a response to intruders on or near the territory (Simpson 1985). Because song-type change was noted only when detected aurally, our observed rate of songtype change probably underestimates the true rate. It is, however, suitable for the comparative purposes of this study, In addition to song and song-type change, we noted the number of "cheer" calls. "Cheer" is the onomatopoeic name for a middleto long-range male vocalization used in territory defense and maintenance (Morton 1982). Daily observations began approximately 30 min before sunrise and continued for 4 h. Observations were made on 2 territories simultaneously, with one observer on each territory. Temperature was recorded at the beginning of and hourly throughout the observation period. Efforts were made to keep the resident Carolina Wrens in sight throughout the observation period, although this was not always possible. Behavior, type and number of vocalizations produced, microhabitat, height, and location within the territory were recorded for each wren at 2-min intervals. All songs and other vocalizations produced in response to the playbacks were noted. In addition, response to playbacks was measured as time to the first song in response to playbacks and as the resident male's closest approach to the source of the playback. We used matched-pairs analysis to analyze the effects of food supplementation and playback. A January 1988] Vocal Territorial Behavior of Wrens 13 TABLE 1. Vocal behavior of male Carolina Wrens exposed to the four experimental treatments. Values are œ+ SE.