AVIFAUNA 



217 



Ashmole (1967) wrote that squid (family Ommastrephidae) 

 and flying fish (Exocoetidae) were of primary importance 

 to a colony of red-footed boobies that nest on the island of 

 Oahu in the Hawaiian Islands. The same authors studied 

 the feeding habits of seabirds on Christmas Island (Pacific 

 Ocean) and concluded that flying fish and squid "are of 

 outstanding importance in the diets of nearly all species of 

 birds typical of the tropical Pacific." These findings were 

 confirmed by Shreibcr and Hensley (1976). Similarly, in his 

 study of the sooty tern in the Hawaiian Islands, Brown 

 (1973) found that the birds ate about half squid and half 

 fish by weight. Four families of fishes were found in the 

 diet, but the Carangidae (genus Decapterus) were the most 

 important. It seems likely that the seabirds of Enewetak 

 have similar feeding habits, even though some species tend 

 to feed in the lagoon and others range far at sea in their 

 search for food. 



Drinking Saltwater 



Fresh water is unavailable on most of the islands where 

 seabirds nest, and the birds are adapted to drinking salt 

 water. Not only do the birds not need fresh water, but 

 Frings and Frings (1959) discovered that captive black- 

 footed and Laysan albatrosses died unless they were fed 

 adequate amounts of salt. All seabirds that have been 

 studied possess special salt glands, located in the orbit, 

 that secrete a fluid that has a higher salt concentration 

 than that in seawater, thus leaving a net gain of water for 

 the birds' physiological needs. The hypertonic solution 

 that drains to the bill tip is discarded by head-shaking 

 movements. 



Guano Production 



The excrement of seabirds, known as guano, is rich in 

 phosphates and ammonium comp>ounds (largely ammonium 

 urate or uric acid). The input of nutrients of such guano to 

 island soils can be great in nesting areas. At Enewetak 

 there are recognizable deposits of bird guano in the Pisonia 

 stands on the southwestern islands, from Igurin to Rigili. 

 Richardson (MS) reported that at Rongelap Atoll there was 

 "a relationship between the greatest concentration of 

 breeding birds, the most extensive stands of large trees, 

 and the best-developed soils." On Kabelle Island he 

 estimated that the 1400 to 1800 birds of three tern 

 species contributed over 40,000 kg yr~' of waste to this 

 island of 2400 square meters. 



Murphy (1936) said that a single Peruvian booby (Sula 

 uahegatus) produces as much as 150 g d~' of guano, and 

 that, if 30 g of this were deposited on an island, a 

 thousand birds would produce more than 10 metric tons a 

 year. He added that Peruvian booby guano is more than 

 33 times as effective as barnyard manure. However, 

 whether the guano accumulates in large quantities or is 

 leached dep>ends on climatic conditions, that is, the amount 

 and pattern of rainfall. Nevertheless, the guano input has a 

 profound effect both on the plant life of the islands and on 

 the lagoon and ocean surrounding them. For example, 



Hutchinson (1950) describes these effects as follows: "If a 

 bird colony is situated on such a coast or island, part of 

 the upwelling nutrients will finally be deposited on the 

 island. Wherever any of this material is washed back by 

 rain or wave action into the sea, there will be locally and 

 momentarily a much greater concentration of nutrients 

 where the guano solution returns to the ocean than at any 

 other place. The result of a very large bird colony on a 

 section of coastline or on an island, whenever climatic con- 

 ditions and the form of the substrate of the colony permit 

 guano to be returned to the ocean, will be to steepen the 

 nutrient gradient. Nutrient elements that, without the 

 birds, would tend to remain in the bodies of the fish in the 

 fseripheral part of the trophophoric field of the colony and 

 on the death of the fish would presumably be distributed 

 widely throughout the general circulation of the ocean are 

 concentrated by the birds on and around the island. The 

 result will be increased littoral productivity and probably 

 increased littoral fish production. If the latter occurs, the 

 birds will not have to move so far out to sea for their 

 food, and a steady-state condition will be set up. . . . It is, 

 in fact, conceivable that large bird colonies, far from reduc- 

 ing the commercial catch of fish by competition, may actu- 

 ally increase the catch by a process of biogeochemical con- 

 centration." Also related to this situation are the excreta 

 voided in flight by the many thousands of seabirds as they 

 fly over the lagoons and the ocean on their way to and 

 from the feeding grounds. 



BREEDING HABITS 



Many of the seabirds or oceanic birds are called pelagic 

 species because they spend most of the nonbreeding sea- 

 son on the open ocean, returning to islands only for the 

 nesting season. Some sf>ecies do not reach sexual maturity 

 for several years (e.g., until about 5 years of age for the 

 frigate bird and black-footed albatross), and they may 

 remain at sea during this pericxl. Certain species (e.g., 

 brown booby, sooty tern) lay their eggs on the ground; 

 some excavate burrows in the sand or other substrate 

 (e.g., wedge-tailed shearwater); and others (e.g., red-footed 

 booby, black noddy) typically build a nest in some kind of 

 vegetation, although this may place the nest only a foot or 

 so off the ground. Most seabirds lay a single egg in a 

 clutch, but the clutch of the brown booby usually contains 

 two eggs. 



On both lava and coral islands, nesting adults are sub- 

 jected to excessive heat and solar radiation that arc coun- 

 teracted by physiological and behavioral adaptations. Birds 

 do not have sweat glands, and both adults and young birds 

 dissipate excess heat by evaporative cooling. This is 

 accomplished either by panting or by gular flGttcring, that 

 is, by rapidly vibrating the throat and floor of the mouth, 

 thus speeding up blood flow and loss of heat through the 

 membranes in the floor of the mouth and throat (gular) 

 areas. Because of the heat stress, the adult bird often must 

 stand over the egg or recently hatched chick to provide 



