ABSTRACT

Residing in waters just west of the Americas, in a roughly triangular area from about 50°N to 50°S and out along the equator, are 26 distinct taxa of storm-petrels (Hydrobatidae), a diversity far greater for this group than in any equivalent stretch of the world ocean. We sought to understand how so many forms can co-occur within this very poorly known family.

We describe the ranges, at-sea behavior, and marine habitat affinities of 22 (of the 26) distinct forms included within 5 (of 6) genera and 16 (of 20) species of storm-petrel, all of which occur in that portion of the Pacific Ocean that stretches from the California Current south through the Humboldt Current and out to 170°W along the equator. We base our analysis on data collected during 23 cruises conducted in the eastern Pacific during the period 1980-1995. We also provide, for the first time for most forms, information on the annual cycle, as well as abundance estimates based on at-sea censusing during both the breeding and nonbreeding periods. Such information is unknown for almost all populations of storm-petrels, unless their at-sea range has been thoroughly censused. We include the following storm-petrels in our analysis: Leach's (Oceanodroma leucorhoa; represented by a light-rumped form, O. l. leucorhoa, and several dark-rumped forms: O. l. socorroensis, O. l. cheimomnestes, and O. l. chapmani), Band-rumped (O. castro), Ashy (O. homochroa), Least (O. microsoma), Wedge-rumped (O. tethys; represented by two races: O. t. tethys and O. t. kelsalli), Black (O. melania), Markham's (O. markhami), Ringed (O. hornbyi), Wilson's (Oceanites oceanicus; represented by two races: O. o. oceanicus and O. o. chilensis), White-vented (Oceanites gracilis; represented by two races: O. g. gracilis and O. g. gala- pagoensis), White-bellied (Fregetta grallaria; represented by three races: F. g. grallaria, F. g. segethi, and F. g. titan), Black-bellied (F. tropica), White-throated (Nesofregetta fuliginosa), and Whitefaced (Pelagodroma marina; represented by two races: P. m. dulciae and P m. maoriana).

Information was gathered by strip censuses (400-600 m wide), observations of storm-petrel behavior along cruise tracks, and collection of specimens. Within the entire study area, we made 9,308.1 h of observation and surveyed 111,029 km2 of ocean, including 61,131 km2 in boreal spring-austral autumn and 49,898 km2 in boreal autumn-austral spring. Surveys included 768.3 h within 1,000 km of the South American coast; >11,203.7 km2 of ocean was surveyed, 7,382.1 km2 in austral autumn and 3,821.6 km2 in austral spring. Surveys within 500 km of North America included 2,557.2 h over 12,473 km2 in boreal spring and 3,061.3 km2 in boreal autumn. We also collected specimens during numerous stops where oceanographic studies were being conducted by other researchers who were also aboard the ship.

For the majority of taxa, our surveys covered the entire at-sea range of the taxon. We had complete coverage for the following storm-petrels: Ringed, White-vented, Markham's, O. t. segethi race of White-bellied, both races of Wedge-rumped, Galápagos race of Band-rumped (O. c. bangsi), races of dark-rumped Leach's (O. l. socorroensis, O. l. cheimomnestes, and O. l. chapmani), and Ashy. During boreal autumn, we also had nearly complete coverage for Black and Least storm-petrels, both of which vacate the Gulf of California after the breeding season. We also had nearly complete coverage of the Pacific range of the White-throated Storm-Petrel. Using generalized additive models, population estimation was quite satisfactory for these taxa.

Our results indicate that most storm-petrel taxa in the study area have robust populations, this report presenting the first estimates ever for most of the taxa treated. On the other hand, meager populations are indicated for Ashy and White-throated storm-petrels, for two races of White-bellied Storm-petrel (F. g. grallaria and F. g. titan), and for Band-rumped Storm-Petrels in Hawaii. All appear to have populations of <10,000 birds, especially in the case of the newly rediscovered (present study) but apparently nearly extinct population of F. g. titan on Rapa Island.

The eastern Pacific is oceanographically heterogeneous at the middle to large scale, and such heterogeneity with strong environmental gradients apparently contributes in a major way to the diversity of storm-petrels in the area. The occurrence patterns of all forms sorted along gradients, such as those for sea-surface temperature and salinity and thermocline depth and strength, all of which separate the major current systems and water masses in the region. Except for the three endemic storm-petrels of the Humboldt Current (White-vented, Ringed, and Markham's storm-petrels), species were further sorted at a smaller scale in accord with a habitat gradient from shelf to slope to pelagic waters. Gradients in ocean productivity correspond with, and are affected by, gradients in the above physical habitat features. Flight and foraging behavior also differentiated species ecologically. Finally, intense competition for nesting space is indicated for many taxa by clear evidence of "floating populations" (i.e., surpluses of breeding adults denied the chance to breed owing to lack of nesting space). In part, these surpluses are the result of intense competition for limited nesting space and are resolved by body size, with a size differential dictating what taxa can breed sympatrically (on the same island). At sea, especially in areas of high ocean productivity where habitats are more finely demarcated than elsewhere, storm-petrels sort by foraging habitat likely derived from trophic competition that was far more intense during glacial periods. At that time, continental shelves were much narrower and, therefore, depth-defined foraging zones were more closely packed than at present or throughout the Holocene. Where species co-occur at sea, the two factors that explain co-occurrence among most storm-petrel species are foraging habitat and body size, with competition for food being resolved by the latter, which affects prey size, as well as by foraging behavior. Overall, the high degree of speciation among storm-petrels of the eastern Pacific is likely a product of relatively sparse nesting islands sprinkled among a confluence of distinct water types separated by intense environmental and productivity gradients. Received 15 June 2005, accepted 19 September 2006.