Distribution and Habitat Preference : Great frigatebirds 

 nested on 25 islets, including Nake, Long, and most of the 

 larger islets (Fig. 8. Table 1 1, ranging in size from Azure 

 (0.20 ha) to Nake ( 107.46 ha). Every occupied islet had some 

 Pisonia forest, even if only a single tree (Azure). The larger 

 islets lacking Pisonia forest (Arundel. 7.34 ha; Tridacna. 

 9.08 ha) lacked frigatebirds in 1988. although frigatebird 

 chicks were present on Arundel in early 1989 (Anne Falconer, 

 personal communication). 



Although great frigatebirds were similar in nest 

 requirements to red-footed boobies, there were significant 

 differences: the frigates tended to nest higher in, and closer to, 

 the outer edge of the canopy (although nests were found as low 

 as 1.3 m). Nest sites were more sheltered from the wind than 

 those of red-foots and in locations where the birds could take 

 flight easily. Such site preferences may explain the association 

 with Pisonia. Pisonia reaches 25 m on Caroline, taller than 

 other tree species, providing a windbreak on most islets. The 

 largest colonies (Nake. Long, Pig. Mannikiba) were found 

 leeward of these stands. We found nests in Tournefortia, 

 Pisonia. and Cordia. They were often in the Tournefortia- 

 Pisonia interface, generally in the taller Tournefortia. One 

 colony on south Long overhung the lagoon in a dense Pisonia 

 stand. Frigates were not found in any anthropogenic forests 

 and were absent from then-inhabited Ana-Ana. 



Numbers : The previous population estimate for great 

 frigatebirds on Caroline was 10. 000 birds (Clapp & Sibley, 

 1971a: Perry, 1980). We calculated that 2.427 pairs bred or 

 attended territories. An additional 617 birds roosted, thus the 

 entire population was approximately 5,471 individuals. A 

 large but undetermined number of birds soared over the atoll 

 throughout the day, and an uncountable number of birds, 

 including Hedged juveniles that would ultimately return to the 

 island to nest (Diamond. 1971). were undoubtedly at sea. 

 Because this species is difficult to count accurately, it is unclear 

 if the population has changed since 1965. 



Phenology : In frigatebirds. the scapulars, which first 

 appear at 81 days in Fregata magnificent (Diamond. 1973). 

 erupt before the primaries. Because we lack chick stage data 

 for F. minor and F. arid, we have modified ages from Diamond 

 ( 1 973 ) for F. magnificens, using the hatching times for F. arid 

 and F. minor from Nelson (1976) and fledgling ages from 

 Diamond! 1 975b) to construct approximate development stages 

 for the species on Caroline. Since (hey Hedged at an earlier age 

 than /•'. magnificens, we have reduced the ages for chicks with 

 erupting primaries for F. arid and F. minor, kept the duration 

 of the earlier stages approximately the same, and reduced the 

 period in juvenile plumage. 



We found 214 nests in 1988. Of the 144 in which we 

 determined contents, 49 contained eggs or young chicks. 

 27 held chicks with developing scapular feathers, and 

 68 contained older chicks (fable 5 1. The additional 70 adults 

 occupied nests oi unknown contents; mans probably held eggs 

 or young chicks or were empty. We saw fewer than 

 10 displaying males, so the breeding season was winding 

 dow n. This was also indicated by the high proportion 1 87' i |oi 

 nests with chicks, mam ol them old. A major laving effort had 



begun in March-April (Fig. 9) and continued into September. 

 In March 1 990. an abundance of fly ing juveniles and occasional 

 larger chicks down to the downy stage indicated that the 

 previous year's breeding season was ending. A small number 

 of males were beginning another courtship cycle. By May 

 1990, courtship and egg-laying were still under way, and nests 

 contained eggs or small chicks up to the "remiges" stage. Peak 

 laying on Christmas Island ( Pacific Ocean) occurs from March 

 to May (Schreiber & Ashmole. 1970), the same laving cycle 

 observed on Caroline in 1988 and 1990. 



Lesser Frigatebird (Fregata ariel) (Figs. 8.10) 



The lesser frigatebird is. along with F. minor, one of the 

 true pantropical species. It breeds and disperses widely within 

 the tropical Pacific (Sibley & Clapp. 1967). One of the largest 

 populations in the world (30.000-85.000) breeds on McKean 

 Island, in the Phoenix Group (Garnett. 1983). Lesser frigatebirds 

 breed on four of the Line Islands, with the population on 

 Maiden (7.000) the largest in the archipelago (Perry. 1980). 



Distribution and Habitat Preference : In June 1965. lesser 

 frigatebirds were found nesting in one compact colony on the 

 leeward north end of Long (Clapp & Sibley. 1971a). We found 

 a single colony in leeward Pisonia forest on western Nake 

 (Fig. 8). both in September 1988 and May 1990. The birds 

 nested high (to 18 m) in the Pisonia and Pisonia-Conlia edge 

 facing an open Tournefortia savannah. Although primarily 

 composed of F. ariel. a few F. minor were scattered along all 

 but the eastern edge of the colony. West of the birds, across the 

 open forest. F. minor and Sula sula nested in a mixed colony in 

 a denser stand of Tournefortia. Birds were seen soaring over 

 Nake. Long, and the leeward islets but were not found roosting 

 or nesting away from the colony on Nake. How ever, in March 

 1990. approximately 650 lesser frigatebirds were swarming 

 above, and roosting on. Motu Nautonga in a light cluster. 

 possibly preparing for nesting. 



Numbers and Phenology : Pacific Ocean Biological Sun ev 

 Program biologists estimated a population of 1.000 lesser 

 frigatebirds on Caroline in June 1965. with 400 ± \0% breeding: 

 only eggs were found (Clapp & Sibley. 197 la). On Christmas 

 Island. F. ariel laid in May and June in 1959. 1963. 1964. and 

 1 967 ( Schreiber & Ashmole. 1970). Of 46 nests found in 1988. 

 we could inspect the contents of only 26: all contained 

 feathered chicks (Table 5). Laying dates ranged from March 

 through July (Fig. 10). with a peak from April to June. Caroline's 

 lesser frigates, therefore, appeared to be synchronous with 

 those on Christmas. 



Because w e did not determine the colony limits, we cannot 

 provide a population estimate. There was a minimum of 

 20( ) birds in 1 988 ( 46 nests, plus roosting and flying indiv iduals) 

 and 500 pairs in 1990. 



Sooty Tern (Sterna fuscata) (Figs. 11.12: Subchapter 1.1. 

 PI. 59) 



This tern is the most widespread and abundant tropical 

 seabird in the world. Under favorable conditions it forms 

 immense colonies numbering into the millions. It is known to 

 breed on 7 of the Line Islands: the largest population in the 



144 



