(04°03'N, 163°31'E). Huber ( 1971 ) has shown that Wilson's 

 storm-petrels move through the Marshall Islands from April 

 through September, sometimes in considerable numbers. 



White-throated or Polynesian Storm-Petrel (Ncsotrcuetta 

 titliaiiiosa) : The white-throated storm-petrel is an uncommon 

 central Pacific resident. We saw two (0. 1 6 birds/ 1 knr ) flying 

 north on 5 October in the Phoenix Islands, 200 km northwest 

 of the nearest land. McKean Island, where the world's largest 

 population! 1.000 birds) breeds (King. 1973). Approximately 

 500 birds also breed on nearby Phoenix Island, remaining in 

 adjacent waters throughout the year, with limited dispersal 

 eastward along the South Equatorial Current ( Harrison, 1 985 ). 



Leach's Storm-Petrel (Oceanodroma leucorhoa) : This 

 species breeds at subarctic and temperate latitudes in the North 

 Pacific, wintering primarily north of the equator. Its at-sea 

 range is centered in the central Pacific, with greatest winter 

 densities in a broad belt along the equator (Crossin, 1974). Our 

 seven observations, all between and 3°S latitude, fell within 

 the known range of the species (i.e., the Line, Phoenix, and 

 Gilbert Groups). Five were flying north, and two were flying 

 southeast. 



Family Phaethontidae 



Tropicbirds were represented by two species observed in 

 small numbers (N = 46) from the Line Islands west to the 

 Carolines, including the Gilbert Islands. Most sightings were 

 of solitary birds during the morning hours, up to 300 km 

 from the nearest landfall. The highest density was that of the 

 white-tailed tropicbird {Phaethon leptums) in Micronesia 

 ( 0.40 birds/ 1 km : ), six times higher than elsewhere. Tropicbirds 

 accounted for 1.3% of the total birds seen (Fig. 3, Table 3). 



Red-tailed Tropicbird (Phaethon rubricatuhi) : The red- 

 tailed tropicbird ranges widely in the tropical and subtropical 

 Pacific, breeding on many islands. Typically solitary, it is 

 highly pelagic and is often observed many hundreds of 

 kilometers from the nearest landfall (Harrison, 1985). Long- 

 term studies have found that it is observed in roughly the same 

 density regardless of distance from land (King, 1970). 



We observed five individuals in Regions I and II between 

 1 7 September and 5 October. In the Line Islands, two occurred 

 close to islands (Christmas. Maiden). The remainder were 

 approximately 300 km equidistant from Caroline, Maiden, and 

 Starbuck. all of which harbor small breeding colonies (Clapp, 

 1967; Gould el al., 1974). although that on Caroline is the 

 largest ( Kepler et al. . Subchapter 1 .2, this vol.). All birds were 

 characteristically solitary; one was resting on the water. This 

 was also found by POBSP, whose number of sightings exceeded 

 one thousand: 87% of sightings were of lone birds, and 14% 

 were sitting on the water (Gould et al.. 1974). Pacific Ocean 

 Biological Survey Program found that birds were most 

 commonly observed during morning hours: 80% of our birds 

 were seen before 0820 h. All flying birds were adults flying 

 south. One bird was observed in the Phoenix Islands 

 approximately 300 km northwest of McKean Island, where 

 some 500 breed (Gould et al.. 1974). 



Densities were highest in the Line Islands 

 (0.07 birds/10 knr). Such at-sea abundance is low, probably 

 because the species was breeding. On Caroline Atoll, for 

 example, eggs and chicks were present in late September 

 (Kepler et al., Subchapter 1 .2. this vol.). 



White-tailed Tropicbird (Phaethon leptums) : Like 

 P. rubricauda, the white-tailed tropicbird is a resident breeder 

 throughout the tropical Pacific. It is. however, less pelagic. Of 

 the 41 individuals we observed, 38 were seen on 13 October, 

 with Pohnpei in sight most of the day: most were alone or in 

 groups of up to four or fewer. Maximum numbers were seen 

 in the morning. Those seen with directed flight were generally 

 traveling toward or away from Pohnpei. Ten (26%) were 

 sitting on the water, and four had joined a large feeding flock. 

 White-tailed tropicbirds favor high islands for nesting and are 

 known to nest in trees and rocky cliffs on Pohnpei (Baker, 

 1951). 



A single white-tailed tropicbird was sighted 400 km north 

 of Belau. where it breeds on several islands (Baker, 1951). Our 

 last sighting was of a lone bird on 21 October in Region V. on 

 the far northwestern edge of the species' range (Harrison. 

 1985). It was flying east approximately 200 km east of Samar 

 Island (Philippines) at 10°49'N. 127°53'E. 



Family Sulidae 



Boobies were seen throughout the study area except in 

 Region V, the Philippine Sea and Basin. All three pantropical 

 species were seen in the Line Islands only. Although we saw 

 only 87 boobies, this family ranked third in total numbers, 

 contributing 1.3% of all individuals observed on the cruise 

 (Fig. 4, Table 3). It was most common in the Line Islands and 

 Micronesia (Figs. 5,6; Table 3). Many sightings were within 

 80 km of their nearest breeding islands, but there were several 

 exceptions, one being a possible new pelagic record of a 

 masked booby (S. dactylatra) in the central South China Sea 

 (see below). 



All three species of boobies commonly participate in 

 feeding flocks. This tendency is greatest with the red-footed 

 booby (5. sula)but is much less overall than for more gregarious 

 species such as sooty terns or wedge-tailed shearwaters (King, 

 1970). Our data ( 14 flocks) indicate that boobies participated 

 in 21.4% of the flocks (Table 7), accounting for 1.6% of the 

 total number of flocking birds (Fig. 7). Of 893 feeding flocks 

 in a large study area centered on the Hawaiian Islands, boobies 

 participated in 12.3%. although their numbers only accounted 

 for 1.8% of the total (King. 1970). 



Masked Booby (Sula dactylatra) : Masked boobies, resident 

 breeders in the tropical Pacific, were seen sporadically across 

 the entire cruise track (N = 37). They were most common in 

 the Line Islands, where densities reached 0.49 birds/10 knr. 

 Small numbers were present in the Phoenix and Gilbert Groups 

 and South China Sea (Table 2). Although wide-ranging 

 throughout the Pacific, this species was not mapped as occurring 

 west of around 143°E (i.e., the Marianas chain) (Harrison, 

 1985) but is recorded elsewhere from the Philippines, coastal 



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