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. 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: ; 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 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. Tem- peratures are plotted only for every fourth day, but trict) weather station. every day on which rainfall was recorded is shown. RESULTS NEST MEASUREMENTS AND BREEDING BIOLOGY relative to the nesting saguaro were made later. In 1989 a Ranging 620 rangefinder was used to Timing of breeding was determined from focal estimate distances, but in 1988 a tape measure watches on 16 pairs over both years of the study. was used. The martins were not individually col- Nest building took place in late June and early or banded, so I assumedthat individuals perched July, incubation during early and mid-July, and near or in the nesting cavity were actually the nestlings hatched in late July (Fig. 1). Breeding territory owners. Only rarely was there any con- did not appear to be timed according to daily fusion as to the identity of a in the territory. temperature, but the nestling period did coincide During watches, I recorded all intrusions by with the late summer monsoon season (Fig. l), other martins on the territory of the focal pair. when highly localized thunderstorms produced Although intruders often flew through the ter- much rainfall. ritory, or briefly circled in the territory, I ana- The nesting cavities used by martins were gen- lyzed only those intrusions where the intruder erally at least 7 m above the ground (Table l), perched. If possible, I noted whether the intruder but some nests were as low as 4.7 m. Entrance was an adult male, subadult male, or female. The diameters ranged from 5-9 cm, and cavity depths behavior of the intruder, duration of the intru- from 7-24 cm. Most nesting cavities were within sion, and responseof the focal male and female 3 m of the stem apex (35 of 40 caseswhere the were also noted. saguarowas not broken or bent sharply near the In 1989, in early August when the nestlings top). The mean orientation (Batschelet 1965) of were about 10 days old, I quantified the predator cavities (n = 51) was 233”, and 67% of the cav- mobbing behavior of martins by erecting a plas- ities faced a southerly or westerly direction (136- tic crow near a nesting cavity. The crow was 3 15”). Most nesting cavities (90%) were located mounted on the end of an extensible pole. One in the main stem of the saguaro, as opposed to person quietly walked to the base of the nesting a branch. The density of saguaros(over 3 m tall) saguaro, and erected the pole so the crow was near the nesting cavity ranged from 5-26 per within 2 m and directly below the nesting cavity. 2,500 m2. A secondperson recorded the number of martins Nest building was done primarily by females mobbing the model every 20 set for 5 min. (48/53 of all trips). I observed martins adding From late June through early August, between green leaves to their nest during the incubation 04:45-05:30, I noted the presenceor absence of period on only three occasionsduring 36 hr of male dawnsong (Morton 1988, Morton et al. observation. The leaves were from ocotillo (Fou- 1990) in locations where I was conducting nest quieria splendens)and jojoba (Simmondsia chi- checks. Beginning sometime before 04: 15, over nensis)plants. Clutch sizes obtained from 12-25 MARTINS IN SAGUARO CAVITIES 669

n 1986 q 1989

0 100 200 300 400 500 600 700 600 900 1000 >lOOO

Nearest Neighbor Distance (m) FIGURE 2. Frequency distribution of nearest neighbor distances in 1988 and 1989. Expected frequencies (solid circles) based on a random distribution of nests are shown for 1989.

July, when females were incubating, ranged from about 500 m of the focal nest. Mobs consisted 3-5 eggs(mean = 3.9, SD = 0.70, n = 11). Of of birds circling the model and alarm calling, 16 nests where focal watches were done, 94% sometimes repeatedly diving at the model (e.g., had at least one nestling hatch and survive to an Brown and Hoogland 1986). In 7 of 13 trials, age of 10 days, as indicated by parental feeding one or two birds (likely the owners of the nest) behavior. Both males and females fed the nest- dove repeatedly at the model. lings, with females making 115 of 205 (56%) Only 2 of 30 breeding males (7%) were sub- feeding trips. adults in 1988, and only 8 of 52 (15%) in 1989. A similar proportion (4/23; 17%) of breeding COLONIALITY AND SOCIAL BEHAVIOR males were subadults in the Rincon Mountain I located 30 breeding pairs in 1988 and 52 pairs Unit population about 50 km east of the main in 1989. I never found more than one pair breed- study site. In both years, five subadult males de- ing in a single saguaro. The median nearest fended cavities but did not have mates. Although neighbor distance was 325 m in 1988, and 238 these males were not color banded, their dis- m in 1989 (Fig. 2), and no pairs nested less than tinctive subadult plumage allowed reliable iden- 100 m apart. However, on a larger scale,breeding tification of individuals over successivedays. The pairs formed distinct groups (Fig. 3). In 1989, amount of time these males were known to de- areas within about 500 m of roads between fend a cavity alone ranged from 4-37 days. breeding groupswere searchedfor nests,and had Subadult males bred later in the summer than similar densities of saguaros.The expected fre- adult males. In 1988, I found 14 mated adult quency of nearest neighbor distances was cal- males between 8-28 June, but found no subadult culated for a randomly distributed population males with territories and mates. Despite inten- based on a nest density of 1.9/km2 (total number sive searchesfor nests in 1989, only one mated of nests/total area searched) using the methods subadult male was found before the end of June, of Clark and Evans (1954). The observed distri- but the seven other mated subadult males were bution of nests in 1989 was significantly more found from 7-27 July. Three of these latter pairs clumped than the expected random distribution were seen nest building from 7-l 1 July, which (Fig. 2; G-test, G = 34.1, df = 5, P -c 0.001). is when most adult pairs have already begun in- The predator mobbing response of martins cubation (Fig. 1). gives a measure of the extent to which neigh- Of 11 territorial subadult males that were boring pairs function as a social unit. The num- sketchedin detail, the average of blue feathering ber of martins mobbing the model crow ranged on the underside was 1.4 f 0.8 cm*. An addi- from 1 to 10 (Fig. 4), and was significantly cor- tional 10 territorial subadults for whom detailed related (r, = 0.71, n = 10, P -c 0.05) with the sketcheswere not made were classifiedas having number of birds known to be breeding within a dull plumage score, falling within the range 670 BRIDGET J. STUTCHBURY

FIGURE 3. Location of martin nests in 1989. Adult male pairs (solid circles) and subadult male pairs (open circles) are indicated. Roads are shown with solid lines, park boundaries with dashed lines.

observed for the 11 detailed drawings. All sub- Of the 30 cavities used in 1988,40% were used adult males had tan colored foreheadsand napes, by martins again in 1989, 14% had a pair nesting as did all females. in a nearby cavity, 7% were occupied by Gila Although breeding pairs were spacedover 100 Woodpeckers or Northern Flickers, 10% were m apart, there were cavities within this area that not available becausethe saguarohad fallen down, were within the range of cavity depth and height and 29% were apparently vacant. above ground of cavities actually used for nesting Breeding pairs frequently defended cavities (Table 1). Of 15 territories where I searchedfor other than their actual nesting cavity, by perching extra cavities within a 2,500 mz area centered on or in the cavity, usually during intrusions by around the nesting cavity, six territories had no other martins. Both males and females were seen other cavities, four had one other cavity, two had participating in multiple nest site defense. Of 16 two other cavities, and three had three other cav- focal pairs where watches were conducted, nine ities. In 6 of 15 territories, there were one or two pairs defended one or more additional cavities. other cavities in the same saguaroas the breeding Extra cavities ranged from O-l 15 m from the pair. Cavities used by other specieswere not in- nesting saguaro (mean = 29.1 m, SD = 33.4 n cluded in this analysis, but martins were found = 12). nesting in the same saguaro as Brown-crested Territorial boundaries were difficult to define, Flycatchers (Myiarchus tyrannulus) and Gila so I used the location of perch sites observed Woodpeckers (Melanerpes uropygielis), and during half hour watchesas a measure of territory within 15 m of Northern Flickers (Colaptes au- size. The distance of perch sites (including the ratus). nesting saguaro, but not the cavity itself) from MARTINS IN SAGUARO CAVITIES 671 the nesting saguaro was weighted for each bird l2 1 according to the amount of time spent perching al l l at that location, over all watches. The weighted .g l0i average perching distance for males was 27.5 m 8 (SD = 14.1, n = 16), and 17.8 m (SD = 10.6, n E = 16) for females. Although someterritorial pairs had greaterthan three intrusions during a 30 min watch, there 2- l a were no intrusions during 133 of 183 watches . l (72.6%). Since subadult males had so little blue 0 I feathering, it was sometimes not possible for me 0 5 10 15 20 25 to distinguish them from females during brief No. of birds breeding within 500 m intrusions. For all intrusions by males, 32 of 39 FIGURE 4. Maximum number of martins mobbing were subadult as opposed to adult males. Ter- the model crow, in relation to the number of birds ritory owners were often not aggressivetoward breeding within about 500 m. intruders, but in 17.7% of the intrusions one of the territory owners displaced, chased,or fought lows native to southern Arizona, $yr- with the intruder. All three fights observed in- rhonota melanogaster, also breeds later in the volved birds grappling in the air; I never ob- summer than more northern races(Phillips et al. served birds fighting inside a cavity or on the 1964; C. R. Brown, pers. comm.). ground. Martins are dependent on Gila Woodpecker From 29 June-5 July, I observed five instances and Northern Flicker cavities for nesting sites. of pairs copulating outside the nesting cavity. I Purple Martins nested in cavities with entrance never observed forced copulations by males. No diameters covering the range ofboth woodpecker males were seenclosely guarding their mates dur- species(McAuliffe and Hendricks 1988). Flickers ing the presumed fertile period. During the pe- almost always excavate cavities within 3 m of riod of 27 June-5 July, when most females would the stem apex, but over half of Gila Woodpecker be fertile (Fig. l), females were guarded (male cavities are closer to the ground than this (Mc- departed with female) on only 7% (3/45) of nest Auliffe and Hendricks 1988). Most cavities used building trips, not guarded (male present but did by Purple Martins were within 3 m of the stem not follow) on 40% of trips, and females were apex, suggesting that martins were avoiding alone (male absent from area) on 53% of trips. available lower cavities. Over both years, I heard males dawnsinging Females made the majority of nest building on all 37 days that I was in the study area before trips and about 50% of the feeding trips, as is 05:30 from 30 June-8 August. Many different found in nest-housepopulations (Allen and Nice males participated in dawnsong,as I heard dawn- 1952). Although males and females were seen song at 12 different breeding groups in the study adding green leaves to their nests during incu- area. The minimum number of males estimated bation, it occurred at a much lower frequency to be singing at one location on a given morning than in easternpopulations (Allen and Nice 1952). ranged from l-7. This is likely because most plants in the study area did not have fresh green leaves during early DISCUSSION July. NEST SITE CHARACTERISTICS AND Several studies on secondary cavity-nesting BREEDING BIOLOGY specieshave shown that populations using nat- Purple Martins in this study area bred fairly late ural cavities have smaller clutch sizesthan those in the summer. Other cavity-nesting speciesin- using nest boxes (Korpimaki 1984, Nilsson cluding Gila Woodpeckers, Northern Flickers, 1984a, Robertson and Rendell 1990), a pattern and Brown-crested Flycatchers were feeding which may be due to the smaller floor area in young when the martins began nest building. The natural cavities (Karlsson and Nilsson 1977, nestling period coincided with the rainy season Robertson and Rendelll990). The averageclutch (Fig. l), suggestingthat martins may delay breed- size in this natural population (mean = 3.9 eggs, ing so that aerial insects are abundant during the SD = 0.70, n = 11) was significantly (t-test; t = time they feed nestlings. The race of Cliff Swal- 5.88, df = 273, P < 0.001) smaller than the av- 672 BRIDGET J. STUTCHBURY

TABLE 2. Reports in the literature of colony size (number of pairs) of Purple Martins breeding in natural cavities for eastern and western populations (not including the Sonoran Desert). The number of reports is a measure of the frequency of occurrenceof that colony size.

Location Colony size No. reports References

Eastern states 1 4 4, 11,20,29 2-6 5 3, 8, 16,20, 23 “colony” 1 1 50: 3OP 2 20 Western states 1 11 6, 7, 10, 13, 14, 17, 18, 26,28, 30 2-7 5 5, 12, 14, 15,22 “numerous” 1 2 12-20,25’ 4 9, 25, 27, 29 40 1 21

= Colonies nesting under boulders or in rock crevices. References: (1) Scott 1872, (2) Cows 1878, (3) Roberts and Benner 1880, (4) Scott 1881, (5) Means 1890, (6) Sharp 1907, (7) Edwards 1914, (8) Phelps1914,(9)VanRossem 1914,(10)Dawson1916,(11)Eve~annandClark1920,(12)Dawson1923,(13)Bryant1924,(14)FinleyandF~nley 1924, (15) Bailey 1928, (16) Howell 1932, (17) Lumley 1932, (18) Kitchen 1934, (19) Huey 1936, (20) Roberts 1936, (21) Huey 1939, (22) Hayward 1941, (23) Bent 1942, (24) Grinnell and Miller 1944, (25) Richmond 1953, (26) Bunch 1964, (27) Banks and Orr 1965, (28) Svoboda et al. 1980, (29) Wade 1987, (30) C. R. Brown, pen. comm. erage clutch size (mean = 4.97 eggs,SD = 0.58, tane western race, Prognesubis subis. Reports of n = 214) in a nest-house population in Texas colonies (2 or more pairs) are as common as (Brown 1978a). The smaller clutch sizes in this reports of solitary nests, and large colonies have natural population could be due to the use of been reported in both eastern and western pop- natural cavities, the smaller size and weight of ulations (Table 2). Colony size ranged from l- females (Brandt 195 1, Johnston 1966, Behle 300 pairs, with an average of 6.6 pairs (SD = 1968) or some other factor such as food avail- 11.6, IZ = 29) excluding the extremely large col- ability. ony. Allen and Nice ( 195 2) report a similar range in colony size for house-nesting martins in east- COLONIALITY AND SOCIAL BEHAVIOR em North America, and an average colony size Although martins were not highly colonial in this of 8.7 pairs in Michigan (n = 22 nest-house col- natural population compared with martin colo- onies) and 7.9 pairs in Illinois (n = 29-40 nest- nies in nest-houses,nests were grouped on a larg- housecolonies). In ancestral natural populations, er geographic scale (Fig. 3). This clumping of solitary nesting likely occurred more frequently nests was not due solely to physical factors since than it doestoday in nest-houses.However, nest- areas along roads without martin nests had sim- house colonies clearly fall within the range of ilar densities of mature saguaros,and appeared breeding densities to which martins were ex- to have an abundance of nesting cavities. Despite posed historically. the relatively large nearest neighbor distancesin Brown and Hoogland (1986) suggestthat div- these groups (100-400 m), martins quickly ing at predatorsis a high risk behavior associated formed mobs of 6-l 0 birds in responseto a mod- with solitary (colonial) nesting in . Pur- el crow predator at the nest (Fig. 4). These breed- ple Martins in this study dove at the predator ing groups were focal points for many social in- model in 54% of trials, and similar predator pre- teractions, including dawnsinging, intruder sentation trials at martin houses in Oklahoma activity, and predator mobbing. resulted in diving behavior in 9 of 10 trials at The distribution of nesting cavities in the So- different colonies (Stutchbury, unpubl. data). noran Desert is not necessarily similar to the However, this does not indicate that historically distribution of natural cavities in other parts of Purple Martins were solitary-nesters. The di- the Purple Martin’s range. Thus, the Sonoran chotomy between solitary and colonial nesting Desert population cannot be used as a direct overlooks those specieswhose nests are loosely model of ancestral eastern populations. A review clumped and have a high degree of social inter- of reports in the literature of colony sizesof mar- actions. Tree (Tuchycinetu bicolor),also tins nesting in natural cavities in tree snags or secondary cavity-nesters, are not colonial (Mul- rock crevices (Table 2) gives an indication of da1 et al. 1985) but often nest at densities of 4- common colony sizes for the eastern and mon- 12 pr/ha (Robertson and Rendell 1990). Similar MARTINS IN SAGUARO CAVITIES 613 to the Purple Martin, this speciescommonly dives fending cavities without a mate (for 24 and 37 at predators (Stutchbury, pers. obs.). days, in two cases),and not all cavities were re- Many breeding pairs in this study had extra used in subsequentyears, there may be a lack of nesting cavities within 25 m of their own cavity, females, rather than nesting cavities. Non-breed- even in the same saguaro, thus the availability ing subadults are typical of many passerineswith of cavities did not prevent martins from nesting delayed plumage maturation (e.g., Flood 1984, closer together. However, breeding pairs occu- Stutchbury and Robertson 1985, Hill 1988, pied an area around their nesting site with an Bjorklund 1989). Although some subadult males average radius of 20-30 m, and defended extra in nest-house populations are floaters (Brown cavities up to 115 m away. In nest-houses,mar- 1978b) or remain unmated after obtaining a ter- tins defend a much smaller area (usually lessthan ritory (Rohwer and Niles 1979), a high propor- 1 m*), but they also defend multiple cavities tion (36-55%) of breeding males are subadults (Brown 1979). The potential benefits of defend- (Brown 1978a, Morton 1987) in contrast with ing multiple nesting cavities include having an this natural population. alternate nesting site in case the original site is Subadult males had little blue feathering on destroyed, reducing interference from neighbor- their undersides; all 21 breeding subadults had ing pairs (Robertson and Gibbs 1982) increasing an area of O-2.7 cm2of blue feathering(dull rank). the potential for polygyny (Brown 1979, Quinney This contrasts sharply with eastern populations 198 5), and gaining opportunities for extra-pair of martins, where subadults can have extensive copulations if neighbors are allowed to settle in blue featheringon their undersides,ranging above those cavities (Morton et al. 1990). In early July 5 cm2 (Rohwer and Niles 1979, Stutchbury, in 1988, I discovered one adult male with two fe- pressb). In addition, many subadult males from males nest building in cavities only 0.5 m apart eastern populations have extensive blue feath- in the same saguaro. This saguaroblew over in ering on the forehead (Rohwer and Niles 1979) a storm a few days later, and a pair (presumably whereas all subadults from this population had the male and one female) immediately renested tan-colored foreheads (as do all females). Roh- in a third cavity only 15 m away. wer and Niles (1979) found that subadult males In this study, intrusions occurred during only from western populations, which also use natural 30% of the half hour observation periods, but cavities, tended to have less blue on their throat owners repelled intruders in 18% of all intru- and breast than eastern populations. If the dull sions. Intense fights, where birds peck and grap- subadult plumage plays some role in reducing ple with each other on the ground or inside a the costs to floaters of searching for breeding cavity, were not observed. In an Oklahoma nest- opportunities (Studd and Robertson 1985) then housepopulation early in the season(Stutchbury the selection pressureto maintain a dull plumage 1990) the intrusion rate was much higher (60% may be stronger in natural populations where of all watches) but the attack rate of male in- fewer subadult males are able to breed in their truders was similar (20%). The relatively low fre- first year. However, since females are also paler quency of intrusions may be representative of in coloration in thesewestern populations (John- low-density martin populations, and corre- ston 1966, Behle 1968), this geographicvariation spondingly high availability of cavities. in coloration may be due to ecological or non- Basedon an annual post-juvenile survival rate adaptive factors, rather than the use of natural of 0.45 from returns of banded adults (Stutch- cavities. bury, unpubl. data), and assuming a similar sur- The close mate guarding and high frequency vival rate for the Sonoran Desert population, the of forced copulations by males documented in expectedratio of subadults to adults in the pop- nest-house populations (Morton 1987) were not ulation (Robertson and Rendell 1990) is about observed in this population. The low density of 1 subadult to 0.82 adults, or 55% subadults in breeding pairs, and the defenseof relatively large the population. Since only 5-17% of breeding territories around the nest cavity, likely limits males were subadults, many subadults were not the opportunities for extra-pair copulations, and breeding in their first summer. Subadult males hence reduces the benefits of mate guarding. were often seen intruding on territories, whereas Morton et al. (1990) proposed that male dawn- adult male intruders were rarely observed. Since song functions to attract subadult males to the there were five instances of subadult males de- colony, so that older males can gain forced extra- 674 BRIDGET J. STUTCHBURY

pair copulations with the mates of the younger successof adult and subadult Purple Martins. males. However, in this natural population where SouthwesternNat. 23:597-604. BROWN,C. R. 1978b. On early spring arrival of Pur- mate guarding and forced copulations were not ple Martins. Bird-Banding 49:130-133. observed, dawnsong was widespread. Dawnsong BROWN,C. R. 1979. Territoriality in the Purple Mar- in this population must have some benefit other tin. Wilson Bull. 91:583-591. than increasing opportunities for extra-pair cop- BROWN,C. R. 1980. Sleepingbehavior of Purple Mar- tins. Condor 82:170-175. ulations. The benefits of grouping nests (as op- BROWN,C. R. 1984a. Vocalizations of the Purple posed to nesting solitarily) are not known for Martin. Condor 86:433442. Purple Martins, but dawnsong could function to BROWN,C. R. 1984b. Laying eggsin a neighbor’s nest: increase local colony size, possibly increasing an benefit and cost of colonial nesting in swallows. individual’s ability to detect predatorsand locate Science224~5 18-5 19. BROWN,C. R. 1988. Enhanced foraging efficiency ephemeral food resources(e.g., Brown and Brown through information centers: a benefit of coloni- 1987, Brown 1988). ality in Cliff Swallows. Ecology 69:602-6 13. BROWN,~.R.,ANDM. B.BROWN. 1987. Groupliving ACKNOWLEDGMENTS in Cliff Swallowsas an advantagein avoiding pred- ators. Behav. Ecol. Sociobiol. 21:97-107. I thank S. Beissinger,C. R. Brown, E. H. Burn, L. Buss, BROWN,C. R., AND M. B. BROWN. 1988. Genetic J. Finlay, E. S. Morton, R. J. Robertson, and S. Zack evidence of multiple parentagein broods of Cliff for comments on this manuscript. The Department of Swallows. Behav. Ecol. Sociobiol. 23:379-387. the Interior and Pima County gave me permission to BROWN,C. R., AND J. L. HOOGLANIZ 1986. Risk in conduct my researchon the SaguaroNational Monu- mobbing for solitary and colonial swallows.Anim. ment and Tucson Mountain Park, respectively. Mardi Behav. 34:1319-1323. Gieseler, Donald Lamm, and Steve Russell provided BRYANT,H. C. 1924. Sacramento’s Western Martin much advice, and I thank Mardi Gieseler. Rhonda colony. Condor 26: 195. Swineford and Lisa Judkins for their assistancein the BUNCH,C. H. 1964. Nesting of the western Purple field. This researchwas supported in part by an NSF Martin. Murrelet 45:10-l 1. Doctoral Dissertation Improvement Grant (BSR CATER,M. B. 1944. Roosting habits of Martins at 8800902), North American Bluebird Society Student Tucson, Arizona. Condor 46: 15-l 6. Research Grant, Sigma Xi Grant-in-aid of Research, CLARK,P. J., AND F. C. EVANS. 1954. Distance to and Behavior Society Research Grant. I was nearest neighbor as a measure of spatial relation- supported by scholarshipsfrom the Natural Sciences ships in populations. Ecology 35:445453. and EngineeringResearch Council of Canada and Yale COVES, E. 1878. Birds of the Colorado Valley. U.S. University. This study is part of a dissertation sub- Geol. Survey of the Territories Misc. Publ. 11, mitted to fulfill in part the requirements for the degree Washington, DC. of Doctor of Philosophy in Yale University. DAWSON,W. L. 19 16. A personal supplement to the distributional list of the birds of California. Con- LITERATURE CITED dor 18:22-30. ALLEN, R. W., AND M. M. NICE. 1952. A studv of DAWSON,W. L. 1923. The birds of California. South the breeding biology of the Purple Martin (Pv&re Molton Co., San Diego, CA. sub& Am. Midl. Nat. 41~606-665. EDWARDS,H. A. 1914. Birdnotesfrom the Sierra Ma- ANDERSON,A. H., AND A. ANDERSON.1946. Notes dre mountains, southern California. Condor 16: on the Purple Martin roost at Tucson, Arizona. 207-2 10. Condor 48:140-141. EVERMANN,B. W., AND H. W. CLARK. 1920. Lake BAILEY,F. M. 1928. Birds ofNew Mexico. New Mex- Maxinkuchee. 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