The endangered Hawaiian Petrel (Pterodroma sandwichensis) breeds among the main Hawaiian Islands. Geographic variation of Hawai'i’s avifauna is well known among evolutionary biologists. However, there is no evidence of variation in highly mobile species of Hawaiian seabirds. Each colony of Hawaiian Petrels may be distinct because of low degrees of gene flow and dispersal between islands. Geographic variation in characters can be expected if each colony is isolated. Behavioral and morphological traits of individuals from colonies on four islands were analyzed to determine the extent of population differentiation in these traits. The breeding cycle of Hawaiian Petrels within Hawai'i Volcanoes National Park was established and fledge dates were compared to those of Maui, Lana'i, and Kaua'i. Breeding cycle stages of colonies on Hawai'i Island, Lana'i and Kaua'i were nearly identical; with fledge dates all occurring in late November. However, the cycles of colonies on Maui are temporally segregated, each stage occurring an entire month earlier. Dates of fledge were highly synchronous within colonies, and there was no change over time in fledge dates, even over 30 years of data collection on Maui and Kaua'i. The fixed timing of fledge suggest that breeding cycles of Hawaiian Petrels do not vary in a changing environment. Cues that initiate the breeding season are not understood, but could involve endogenous mechanisms that operate independently of environmental factors such as sea surface temperatures or availability of food resources.
A number of morphological characteristics of petrels from Hawai'i Island, Maui, and Kaua'i were measured and compared. Individuals from the large colony on Maui had significantly longer wing chord, culmen, and tarsus measurements than petrels from Hawai'i Island and Kaua'i. Size differences may be evidence of ecological segregation, as petrels from Maui may be foraging in more distant areas of the North Pacific.
Hawaiian Petrel vocalizations were analyzed to investigate acoustic traits and dialect, which may be affected by the degree of dispersal among colonies. Calls of petrels in flight were recorded on Hawai'i Island, Maui, Lana'i, and Kaua'i. The vocal repertoire was consistent with other species of the genus, having only two major calls and 3–5 minor calls. Unique characteristics of major calls were detected in each colony. The general structure of major calls was similar among islands, each having the characteristic “Ooooo ahh ooooo ahh ooooo” structure. However, significant differences in the temporal spacing of call syllables, the fundamental frequency of syllables, and syntactic structure of calls were found. Acoustic traits of petrels of Lana'i were consistently different from each of the other island colonies. Major calls of the petrels on Hawai'i Island were also different; syllables of calls there repeat at a faster rate. The majority of acoustic characteristics of Kaua'i petrel vocalizations were similar to those from Maui and Hawai'i Island. Geographic distance of colonies did not affect results, as the two closest colonies investigated had many differences in acoustic variables (Lana'i and Maui). Conversely, two of the most distant colonies on Kaua'i and Hawai'i Island shared many similarities in acoustic variables. Even small differences in acoustic traits may convey important biological information. Vocalizations are not learned in petrels; thus, acoustic characters and observed differences are likely genetic.
Differentiation of characters among colonies of Hawaiian Petrels on the islands supports the hypothesis that each colony is distinct. Recent feather isotope analysis as well as molecular research strengthens the argument. Geographic differentiation is not unusual among many species of island nesting petrels. The condition exemplifies the “seabird paradox”, where highly mobile seabirds with no apparent physical barriers to dispersal display unique character and genetic traits in a relatively small geographic area. However, the species may have a behavioral barrier because of a strong instinct to return to its natal colony site to breed, also known as philopatry. Regardless, each colony may be considered a separate genetic management unit, which must be considered in future discussions of down listing and when prioritizing conservation actions.