Natural History of the Guam Rail
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Condor, 81:4041108 0 The Cooper OrnithologicalSociety 1979 NATURAL HISTORY OF THE GUAM RAIL J. MARK JENKINS The Guam Rail (RaZZus owstoni) is a flight- tats. Since 1961, counts have been conducted at dawn less species, endemic to Guam of the Mar- from vehicles moving between 20-30 kph, first weekly iana Islands. Little has been published on and later bimonthly. In addition, roadside counts dur- ing 1968 recorded the number of rails seen in four dif- this rail except Bakers’ (1951) brief notes, ferent habitat types: savanna, mowed grass with brush, Perezs’ (1968) note on breeding seasons, mature mixed forest, and mixed woodland with brush. Kiblers’ (1950) description of the call note, Total numbers of rails and rail broods observed month- and Carpenter and Staffords’ (1970) work on ly and yearly were converted to rail and rail broods per 160 km of travel. its salt glands. Lint (1968) published a pop- Food habits were determined by examining gizzards ular article on the Guam Rail containing lit- and proventriculi, primarily from road-killed individ- tle biological information. A study of the uals (N = 34). The degree of digestion of stomach con- biology of the species appears especially tents often limited the specific identification of food appropriate in light of numerous recent ex- items. Weights and measurements were obtained from specimens collected during the early 1960s’ and from tinctions of flightless rails from oceanic is- road-killed birds in good condition. lands (Ripley 1977), and current declines in Guam Rail populations. RESULTS AND DISCUSSION Systematists have variously interpreted GENERAL DESCRIPTION phylogenetic affinities of R. owstoni. Olsen RaZZus owstoni is a large rail with dark (1973) reclassified R. owstoni into the ge- brown head, neck, back, rump, tail, legs, nus GaZZiruZZus. Ripley (1977) questioned feet, and bill. The wings, lower breast, ab- this classification and continued to place the domen and under tail coverts are barred Guam Rail in the genus RaZZus; that no- black and white, while extensive areas of menclature is used herein. Generally, R. ash gray occur on the neck, upper breast, owstoni is believed to have colonized and in a superciliary stripe (Fig. 1). Plum- Guam through the Philippines from stock ages of the sexes are similar (Mayr 1945, probably resembling the Banded Land-Rail Baker 1951). As in other RaZZus, the body is (R. philippensis; Baker 1951, Olsen 1973) or elongated and laterally compressed, partic- the Barred Rail (R. torquatus; Ripley 1977). ularly in the neck and breast regions, allow- Since 1960, staff biologists of the Guam ing the birds to move rapidly through dense Division of Aquatic and Wildlife Resources vegetation. Males are larger than females have collected field notes on the Guam Rail (Table 1); the sexes often are distinguish- and monitored population sizes through able by their sizes when seen together in roadside counts. These data form the basis the field. for the present paper. DAILY ACTIVITIES STUDY AREA The Guam Rail is a monogamous territorial Guam is the largest and southernmost island of the species. The sizes of daily or seasonal Marianas Archipelago. Lying at approximately 13” 13N,’ 145”E, Guam is 45 km long, 6 to 13 km wide, ranges are unknown. Recorded incidents of and has a uniformly warm and humid climate through- fighting, presumably between males, prob- out the year. Rainfall is heaviest from July through ably are related to territoriality. November, with an average annual rainfall of 219.6 cm. These birds are often seen during the ear- Guam is divided into a northern half, a limestone pla- teau or “mesa” with coralline limestone soils predom- ly morning and late evening when they ven- inating, and a southern half which is primarily volcanic ture from cover to forage and bathe along in origin, with laterite soils. Stone (1970) described the field edges and roadsides. They also forage predominant vegetation of the northern plateau as “ty- at night, a trait considered common among phoon forest” and recognized seven subtypes, includ- RaZZus species (Ripley 1977). The Guam ing Pandanus forest and holophytic-xerophytic scrub forest, widely used by the Guam Rail. Dominant vege- Rail is wary, seldom wandering far from the tation on the southern volcanic soils includes ravine shrub growth to which it retreats when forest and savanna. alarmed. Though capable of short bursts of flight of l-2 m, the bird seldom flies. METHODS Nothing is known of the rails’ behavior Roadside counts of 40.8 km and 39 km were conducted when under cover, but in open areas it de- over northern and southern Guam, respectively. The votes much time to plumage care. Bathing northern route encompassed primarily mixed wood- land and second growth habitats, while the southern and preening occupied 35% of 113 min of route covered savanna and mature ravine forest habi- observation of 26 rails. Birds frequently 14041 THE GUAM RAIL 405 sects, particularly butterflies. They first lo- cate a prey item while standing still and then attempt capture with a quick burst of speed. Baldwin (1947) observed similar be- havior in the now extinct Laysan Rail (Por- xanula palmeri). R. owstoni obtain seeds and flowers from low grasses and shrubs. They can stretch their bodies nearly per- pendicularly, reaching food 40 cm or more above ground. Adult rails may locate specific foraging spots and allow their chicks to peck there, often moving away and permitting the ju- FIGURE 1. The head of a Guam Rail. veniles to forage independently. Alternate- ly, adults may secure food (usually insects) and then allow the chicks to peck the items preen or bathe in small puddles along se- from their bills, or they may lay them in cluded roadsides following early morning front of the chicks. rain showers. FOOD HABITAT PREFERENCES Rallus owstoni apparently prefer animal The Guam Rail formerly occurred in most over vegetable foods (Table 3). The exten- habitats on Guam. Table 2 shows the mean sive use of gastropods, particularly snails, is number of rails observed per 160 km of trav- probably related to the accidental introduc- el in four habitat types in 1968. Mowed tion in 1945 of the giant African snail (Acha- grass with brush habitats included main- tina fulica). Believed introduced from a tained areas of mowed grass along road- ship at commercial port, the African snail sides, telephone lines and antenna fields, rapidly expanded its range into most habi- bordering scrub communities. The number tats on the island and has become an im- of rails recorded in the mature mixed forest portant food for the rail. The birds also have habitat is likely an overestimate as a result been observed eating carrion and geckos of roadside censuses being conducted along (Hemidactylus frenatus), although these the edge of these habitats, where rails were items were not found in the 34 stomachs abundant. Rails are seldom seen or heard in examined. Insects, representing at least the interior of the mature limestone forest, three orders (Othoptera, Dermaptera, Lep- and this habitat is best considered marginal idoptera), are important in the Guam Rail for the species, as are savanna habitats oc- diet. curring in southern Guam (Table 2). The Vegetable foods included unidentified species does not occur in the freshwater seeds and palm leaves. During the dry sea- wetland habitats of Guam. son rails have been reported to damage crops such as cucumbers, tomatoes, and var- FORAGING BEHAVIOR ious melons, though such damage probably Guam Rails most frequently peck food results from the birds ’ securing moisture items directly from the ground. This tech- rather than food. nique is of primary importance in securing Snail shells, or their pieces, may be in- snails and slugs, especially following rain gested in the course of feeding, but also ap- showers. Birds also “hawk” low flying in- pear to function as grit. Coral chips ranging TABLE 1. Body measurements (mm) and weights (g) of male and female Guam Rails. Wing Tail Culmen Tarsus Weight Male Mean 124.8 51.7 39.0 52.1 241.4 Range 120-133 50-53 33-43 50-56 174.0-303.0 N 6 3 6 4 27 Female Mean 119.2 47.6 38.1 47.0 211.9 Range 122-125 45-52 3442 43-54 170.0-274.2 N 7 5 7 6 20 406 J. MARK JENKINS TABLE 2. Number of Guam Rails recorded in four TABLE 3. Stomach contents of 34 Guam Rails. habitat types during 1968 on roadside counts totaling over 800 km. Number of times Percent occurring in occurrence in P&y; 34 stomachs 34 stomachs Habitat type Number (%) Snails 22 64.7 Savanna 2(1.0) 1.6 Slugs 4 11.8 65.3 Mowed grass with brush 83 (40.1) Insects 13 38.2 43.8 Mature mixed forest 54 (26.1) Vegetable matter 4 11.8 56.2 Mixed woodland with brush 68 (32.8) Coral chips (grit) 14 41.2 Total 207 Snail shells (grit) 12 35.3 in diameter from I-9 mm are eaten specif- YOUNG ically as grit; eight chips were the maximum The eggs hatch asynchronously and the number found in one gizzard. young are highly precocial, leaving the nest within 24 h after hatching. The eggshells NESTING are consumed by an adult, presumably the Guam Rails nest on the ground, both sexes female. Brood nests, which have been ob- sharing in the construction of a shallow nest served in other rails, have not been reported of interwoven loose and rooted grasses. One for R. owstoni. nest measured 13 cm in diameter and 3 cm Guam Rail broods range from one to four deep. The eggs are white to pinkish with chicks, with two being the mean and me- small spots of pink or blue concentrated at dian size.