The Condor93x666-675 0 TheCooper Ornithological Society 199 1 COLONIALITY AND BREEDING BIOLOGY OF PURPLE MARTINS (HWGNE SUBIS HESPERIA) IN SAGUARO CACTI’ BRIDGET J. STUTCHBURY~ Department of Biology, Yale University,New Haven, CT 06511 Abstract. Purple Martins (Prognesubis hesperia) breeding in natural cavities in saguaro cacti did not form dense colonies, unlike martins (Progne subissubis) that breed in nest- housesin easternNorth America. However, nestswere clumped on a largergeographic scale. Birds in these groupsof nests quickly formed mobs of up to 10 birds in responseto a model crow at the nest. Males and females defended an area around their nest cavity with a radius of about 20-30 m, and often defended cavities other than the nesting cavity. Only 5-l 5% of breeding males were subadults and some subadult males defended cavities without a mate, indicating that many subadult males did not breed as one-year-olds. A literature review of martin colony sizesin natural cavities indicatesthat nest-housecolonies fall within the range of breeding densities to which martins were exposed historically. The many similarities in breeding biology and social behavior between this desert subspeciesand nest- house populations suggestthat nest-housesare appropriate models of natural conditions. Key words: Purple Martin; Progne subis; saguaro;coloniality; cavity-nester. INTRODUCTION slightly smaller bodies (Brandt 195 1, Johnston Purple Martins (Progne subis) in eastern North 1966, Behle 1968) and females are paler in color America are highly colonial and have bred al- (Johnston 1966) than Progne subis subis. Apart most exclusively in man-made nesting housesfor from their roosting habits (Cater 1944, Anderson at least a century (Allen and Nice 1952, Morton and Anderson 1946), there is little information 1988). Many studies of the breeding biology and on the breeding biology of this subspecies(Brandt behavior of martins in nest-houses have been 195 1, Phillips et al. 1964). The purpose of this conducted (e.g., Allen and Nice 1952; Johnston study was to describe the breeding biology and and Hardy 1962; Finlay 1971; Brown 1978a, social behavior of Progne subis hesperia and to 1979, 1980, 1984a; Morton 1987; Morton et al. compare this natural population with martins 1990; Stutchbury, in pressa, b), but there is very that nest in artificial houses. little known about natural populations. Recent Colony sizesin martin housesrange from sev- studieson other secondarycavity-nesting species eral to over 40 pairs (Allen and Nice 1952). In have revealed important differencesin breeding nest-houses, young males in subadult plumage biology between nest box and natural popula- arrive relatively late in the spring (Morton and tions (Korpimaki 1984; Nilsson 1984a, 1984b; Derrickson 1990), and compete with older males Rendell and Robertson 1989; Robertson and for a nesting cavity (Rohwer and Niles 1979; Rendell 1990). Although montane western pop- Stutchbury, in pressa, b). Extra-pair copulations ulations of Purple Martins still breed primarily and intraspecific brood parasitism are common in natural cavities, they are widely scattered (Morton 1987, Morton et al. 1990). Dawnsong (Grinnell and Miller 1944, Gabrielson and Jewitt by adult males may function to attract subadult 1970). A subspeciesof the Purple Martin (Progne males to the colony, so that adult males can gain subishesperia) breeds in abandoned woodpecker opportunities to cuckold their subadult neigh- cavities in saguarocacti (Cereusgiganteus) in the bors (Morton et al. 1990). To assessthe evolu- Lower Sonoran Desert (Cater 1944, Brandt 195 1, tionary significance of such behavior, it is im- Phillips et al. 1964). Birds of this subspecieshave portant to know to what extent natural populations of martins were colonial, because colony size can affect the nature and intensity of social interactions such as mate guarding (Hoog- * Received26 October 1990. Final acceptance 11 March 1991. land and Sherman 1976, Moller 1987), cuckold- * Present address:National Zoological Park, Smith- ry (Brown and Brown 1988), intraspecific brood sonian Institution, Washington, DC 20008. parasitism (Brown 1984b), and predator mob- w51 MARTINS IN SAGUARO CAVITIES 667 bing (Brown and Brown 1987). To evaluate the attached to fishing line running through two pul- extent to which ancestral populations of martins leys and down the length of the pole, into the were colonial in natural cavities other than sa- nesting cavity. A ruler attached to the pole was guaros, I also conducted a literature review of used to measure how far the weight descended colony sizes reported for naturally nesting mar- into the cavity. Depth measurements were taken tins. only after the eggshad hatched. The density of saguarosnear the nest cavity METHODS was measured by counting the number of sagua- This study was conducted in the summers of ros greater than 3 m tall within a 50 m x 50 m 1988 and 1989 in the Saguaro National Monu- square. The grid was centered on the nest site, ment (Tucson Mountain District) and Tucson and ran on a north-south axis. For 15 breeding Mountain Park, about 25 km west of Tucson, pairs in 1988, I identified all possible nesting Arizona. Purple Martin nests were located by cavities within this 2,500 mz area. Woodpeckers driving slowly, or walking, along roads in the often abandon excavation of a cavity, leaving study site. Martins were very conspicuousas they only a dead-end hole in the side of the saguaro foraged or perched on saguaros 8-10 m above (Kerpez and Smith 1990). If a cavity did not the ground. Alarm calls and male song were also measure at least 5 cm in depth, the cavity was usedto identify areasin which martins were nest- not included as a potential nesting cavity. ing. In 1989, each location where martins were Since saguarosare unstable, ladders were not nesting was searchedfor l-2 hours for additional used to inspect nest contents. Instead, I used nestsduring late July, when most pairs were feed- lighted mirrors attached to extensible poles to ing young. The open landscape allowed for easy reflect the image of the nest contents inside the viewing of large areas of potential nesting habi- cavity toward the ground. A 5 cm round convex tat. Areas along roads between known locations mirror, with two miniature flashlight bulbs at- of breeding martins were also searched thor- tached in front, was mounted at the distal end oughly. In July 1989, I searched for breeding of a narrow, thin aluminum bar. A larger (10 x pairs over four days in a second population, lo- 14 cm) flat mirror was mounted diagonally at cated in the Rincon Mountain District of the the intersection of the bar and the extensiblepoles, SaguaroNational Monument, about 50 km east so as to reflect the image from the lighted convex of the main study area. mirror inside the cavity toward the ground. An For each breeding pair, I noted whether the observer on the ground viewing the large mirror male had adult or subadult plumage coloration. with 10 x binoculars could see the image of the One-year-old male Purple Martins are sexually nest contents.Nest checkswere done before dawn mature, but have a distinctive subadult, female- to obtain best visibility of nest contents. Many like plumage (Niles 1972). Adult males have an cavities were not oriented vertically inside the entirely glossy dark blue plumage, whereas sub- saguaro, so that the bottom of the cavity could adult males are mostly white on their undersides, not be seen. However, I obtained clutch size in- with varying amounts of blue feathering. Where formation on 1 l/23 nests attempted. Incubating possible I made detailed sketches of subadult females remain on their nests overnight, and in males to document the amount of blue feathering some casesclutches could not be viewed because on their undersides. These drawings were trans- the female would not leave the nest cavity. Al- ferred to a grid to estimate the actual area of blue though eggswere usually clearly visible, nestlings feathering. When detailed sketcheswere not pos- could not be counted. sible, I classifiedsubadult males as dull or bright, I recorded breeding behavior during half-hour based on the range in blue feathering found in observations on 16 focal adult pairs over two easternpopulations of martins (Rohwer and Niles years. Watches were done on each pair every 2- 1979). 4 days, between 06:00-09:00, from mid-June The height of nesting cavities was measured through early August. Nesting behavior and the with a Suunto Clinometer (PM-5/100P). The movements of the focal pair were noted, includ- width of cavity entrances was estimated by plac- ing their exact perch sites. Saguarosvary greatly ing a ruler, attached to an extensible aluminum in the number and shape of their branches, so I pole, over the nest hole. The depth of the nesting used sketchesto identify perch sites. Measure- cavity was estimated by lowering a 20 g weight, ments of distance and orientation of perch sites 668 BRIDGET J. STUTCHBURY TABLE 1. Measurements(mean, standarddeviation, sample size, and range) of martin nesting cavities in saguaros. Cavity characteristics MWll SD n RalX$Z Cavity height (m) 1.4 1.4 49 4.7-10.4 Saguaroheight (m) 9.0 1.6 49 5.1-12.2 Entrance diameter (cm) 1.4 2.0 17 5.0-9.0 Cavity depth (cm) 15.5 8.4 17 7.0-24.0 FIGURE 1. Timing of the settlement and nest build- ing (ST/NB), copulation (CP), incubation (IN), and an hour before dawn, males flew high in the air nestling (NL) stage of Purple Martins in relation to giving the distinctive dawnsong. maximum daily temperature (open circles) and daily Weather data were obtained from the Saguaro rainfall (closedcircles) in 1988. The dashedline for the National Monument (Tucson Mountain Dis- nestling period indicates the probable duration.