Numbers : In September 1988 we found 189 masked 

 booby pairs (Table 3), including those on territory (with or 

 without nest scrapes) and juveniles (with or without attending 

 adults). We found no "clubs" of nonbreeding birds. We 

 covered most of the habitat favored by this species except the 

 northern 300 m of Nake Island; in 1990 a few scattered pairs 

 nested there. Our population estimate, including pairs we 

 might have missed, was approximately 200 breeding pairs. 

 The only other population estimate was of "ca. 10" birds 

 (Clapp&Sibley, 1971a);in 1965 POBSP biologists (F.Sibley, 

 personal communication) surveyed all locations where we 

 found breeding pairs. Thus, 200 pairs represents a major 

 increase in the population on Caroline Atoll. 



Phenology : In June 1965 only four masked booby nests 

 containing eggs were found ( Clapp & Sibley, 1971a). indicating 

 that nesting began in May or June. We found nests in all stages 

 in September 1988 (Table 3. Fig. 5 1. The large age class in 

 April may include some juveniles that could fly ( i.e.. were older 

 than 1 80 days). We may have undercounted naked chicks, not 

 wishing to expose them to the sun by frightening the brooding 

 adult. Laying began in April or earlier, peaked in June and July 

 (Fig. 5). and continued until our survey in late September. The 

 34 pairs on territory, many with nest scrapes (Table 3 ). indicated 

 that laying was still in progress and would continue into 

 October. 



In March 1 990. 3 1 pairs were on territory or were attending 

 nests, eggs, or older chicks, indicating that a new breeding 

 season was under way as the previous season was ending. By 

 May 1 990 there were 63 nests, mostly with eggs, and there were 

 no older chicks. Thus, the 1990 season augments the 1988 data 

 and suggests an annual cycle with egg laying beginning slowly 

 in lebruary and March, peaking in June and July , and declining 

 to a low ebb from December to February. 



The large number of fledgedjuveniles and nests with older 

 chicks, in both September 1988 and March 1990. indicated that 

 the 1988 and 1989 breeding seasons were very successful. It 

 also suggested that potential predators (rats and coconut crabs ) 

 posed little hazard to this hardy species. 



Brown Benin (Sula leucogaster) (Fig. 2) 



This widely-distributed pantropical species has an 

 estimated population in the Line and Phoenix Islands of about 

 $,200 (Clapp. 1967; Perry. 1980). with over half of them 

 (2.000) recently found on Maiden Island, in the Southern Line 

 Group. 



Distribution and Habitat Preference : Breeding brown 

 boobies on Caroline were restricted to the w mdward edges of 

 Tournefortia scrub and forest, generally within 15-20 m of 

 high water. In 19SS we found nests on four islets (Fig. 2, 

 Table I). Long, with 12 pairs, was the onlv islet supporting 

 more than a single pair. They w ere located on the northern two- 

 thirds of the island: four pairs formed a loose colony near the 

 head of Tr. A ( Subchapter I.I. Fig. Si. All nests were under 

 Tournefortia bushes approximately 3 m tall. In March 1990. 

 we found 20 pairs of brown boobies, all on windward Nake as 

 far as (he islet's northern extremity. There was no evidence of 

 nesting on Long Island. On May 22. 1990. only three nests, all 

 with engs. were found on Nake. 



On 22 September 1988. we saw 2 birds plunge-div ing w ith 

 masked and red-footed boobies approximately 500 in west oi 

 South Island. On the atoll, flying brown boobies were observed 

 soaring only along the windward beaches. Two birds roosted 

 on the south-central beach of South, and another was found 

 roosting on Kota. 



Numbers : We counted 1 5 pairs during perimeter surveys 

 in 1988, yet found none on the transects. Since we covered 

 virtually all the w indward beaches ( Subchapter 1.1. Fig. 9 1. we 

 are confident that fewer than 20 pairs nested on the atoll. Our 

 population estimate for 1990 was 25 pairs. 



The POBSP (Clapp & Sibley, 1 97 1 a) found three nests on 

 Nake in June 1965, estimating a population of 15 birds. Even 

 though our surveys triple the known population, the brown 

 booby remains a rare seabird on Caroline. 



Phenology : With the exception of one recently 

 fledged juvenile, all nests contained eggs in September 1988 

 (Table 3). Clapp & Sibley (1971a) found eggs in June 1965. 

 In March 1990, the 20 pairs were all on nests whose contents 

 ranged from eggs to an older juvenile. However, only 

 2 months later, only three nests, containing eggs, could be 

 found. These data from 3 years suggest that the species may 

 have trouble rearing young on the atoll. More juveniles should 

 have been encountered, especially in May 1990. Predationby 

 Polynesian rats or coconut crabs could limit reproduction on 

 the atoll. 



Red-footed Booby (Sula sula) (Figs. 6.7: Subchapter 1.1. 

 PI. 51) 



This pantropical booby numbers over 55.000 individuals 

 in the Line Group (Clapp. 1967; Perry, 1980), making it one of 

 the most important regions in the world for this species. 

 Caroline holds the fifth largest known red-footed booby colony 

 ( see Nelson. 1978). The largest known colony ( 140.000 pairs) 

 is found on Tower Island (Galapagos): 3 of the 5 biggest 

 colonies occur in the Line Group. 



Distribution and Habitat Preference : In 1988, the red- 

 footed booby bred on 28 islets, ranging in size from Nautonga 

 (0.34 ha) to Nake (107.46 ha) (Fig. 6). On the Windward 

 Islands, red-foots occurred from Nake to Tridacna. absent only 

 from the smallest islets ( Noddy Rock. Skull Islet. Motu Atibu ). 

 The species was also widespread on the leeward islets, extending 

 from Pandanus to Eitei. The tiny islets ( Fishball, Azure, Reef- 

 flat) were not occupied. 



Red-foots are tree nesters w hose distribution on Caroline 

 closely matched that of Tournefortia scrub and forest 

 (Subchapter 1.1. PI. 51). Thev sometimes utilized smaller 

 Pisonia or Cordia trees where thev intermingled with 

 Tournefortia and occasionally built nests in the tallest (>15 mi 

 Pisonia. They nested in smaller Tournefortia patches within 

 the peripheral scrublands, especially those not directly exposed 

 (o the trade winds. They clearly av oided smaller islets because 

 of the lack of suitable Tournefortia in which to breed. Thev 

 nested inward from (he vegetated edges of the islets, generally 

 at 3-6 m in height, and were distributed in broken rings around 

 the smaller motus in areas of moderate winds. A higher 

 percentage of the population occurred on perimeter surveys 

 than on cross-island transects. 



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