Leach's Storm Petrel

Maximum longevity: 36 years (wild) Observations: Oldest banded bird was at least 36 years-old (http://bna.birds.cornell.edu/).

Accidental visitor.

Oceanodroma leucorhoa leucorhoa(Vieillot)

LEACH’S STORM PETREL

PELAGIC DISTRIBUTION.—The nominate race of Leach’s Storm Petrel is distributed in its at-sea range over much of the Pacific Ocean with a decided center of abundance in the central Pacific. Measurements of normally used characters of all specimens collected (Figure 95) in this area are compared with known breeding O. l. leucorhoa from the Alaskan area in Appendix Table B. It can easily be seen that central Pacific birds fall well within the range for nominate leucorhoa. As a whole, the specimens are comparable both in measurements and other more subtle morphological characters with nominate leucorhoa from the breeding grounds. However, because of the difficulty involved in separating nominate leucorhoa from O. l. beali, the presence of a small percentage of the latter in the central Pacific is not to be excluded.

Oceanodroma I. leucorhoa attains greatest winter densities in a broad belt along the equator. Numbers drop rapidly to the south, and south of 12°S only an occasional individual is sighted. Waters to the north of this equatorial high-density area present problems of seasonal density specification. High density in small sectors north of the equatorial complex is probably due to several factors, among which are local, temporary, high concentrations of food. Additionally, late fall and early spring migrants may tend to mass at certain points in the northern sector of the wintering grounds before moving on south or north, as the case may be. Birds are found throughout the central Pacific wintering grounds throughout the year, with decided buildups of wintering birds as would be expected (compare Figure 96 “Summer” with Figure 97 “Winter”).

The number of summering birds is considerable, but their age status is uncertain. In the Atlantic population of the nominate race, birds presumably do not breed until three years of age, but two-year old birds may return to the colony during the breeding season (Wilbur, 1969). Wilbur (1969) implies that first-year birds might fly over the colony without entering it; he apparently has no positive evidence for this in the form of banded birds. Huntington (pers. comm.) informed me that of eleven breeding birds banded as nestlings, the youngest were four years old; of thirteen banded birds mist-netted over the colony, breeding status unknown, the youngest were three years old. There may be considerable individual variation among one- and two-year old birds in regard to their return to the breeding colony.

The only other explanation for the large number of birds that summer in the central Pacific is that for some reason, some adults do not return to the breeding colony every year. Both Huntington (1963) and Wilbur (1969) record that approximately 50 percent of birds banded while nesting one year are found nesting the following year. Unless we assume some birds were missed in the study plots or had moved elsewhere, this seems to be an exceptionally high mortality rate. Possibly some adults that successfully raise a chick one year do not return to the breeding grounds the following year.

The migrational pattern of nominate leucorhoa is largely obscured by the high densities of summering birds, and is more readily ascertained by field observations of flocking birds rather than by actual increases or decreases in recorded density. The spring migration north begins in March and probably occurs sporadically throughout April, since April densities in the central Pacific are still far greater than in mid-summer when most breeding birds are on the breeding grounds. The fall migration south is also largely obscured by summering birds, but definite density increases are noted southward to the high-density equatorial center as early as September. At this time of year breeding birds would still be expected on the breeding grounds or at least in more northerly latitudes. These early fall buildups in the equatorial area may either be of birds that have experienced nesting failures, and thus have left the breeding grounds earlier than the successful breeders, or of prebreeders that have spent the summer farther north and have returned to more southerly latitudes for the winter. As it is virtually impossible to distinguish immature prebreeders from adult breeding birds when the latter’s gonads are in a winter quiescent stage, collected specimens from these seasons do not lend information as to their recent breeding activities.

Movements and distribution of the nominate race throughout the eastern Pacific are less clear because of the difficulty of separating individuals from the abundant O. l. beali, which winters prominently along the western American coast. A few specimens from the eastern Pacific are clearly referable to nominate leucorhoa by their greater size; collecting localities of these are shown in Figure 98. Aside from POBSP specimens, Loomis (1918:182) mentions three specimens collected south of the Galapagos Islands whose measurements indicate their allocation to the nominate race. Palmer (1962:227) mentions examples of the nominate form taken off the coast of Ecuador. Comparatively, the numbers of these eastern representatives are low and the equatorial high-density area between 170°W and 175°E is, if not the center of winter abundance, one of the most important of such centers.

TAXONOMY.—Considerable taxonomic work was performed on available POBSP specimens during the course of this study in the hope that the ability to distinguish races of at-sea collected specimens would allow for a clearer picture of their respective at-sea ranges. In some cases this was possible, but many individuals cannot be designated unconditionally to a particular race. Concerning the Pacific races of Leach’s, Austin (1952:400) stated: “On dimensions alone these are all weak races. Despite significant differences between the means of some characters, fewer than half the individuals of many of the recognized subspecies are distinguishable.” Despite the recently collected large series of most of the races concerned, we are no better off in regard to solving these problems. Austin’s (1952) arrangement of the various Pacific subspecies of Oceanodroma leucorhoa is followed here. The unusual situation concerning O. l. socorroensis at Guadalupe Island and the possible need for revision of the taxonomy of that population is discussed beyond.

In working with fresh series of recently molted birds of all populations, I find that little significance can be attached to the oft-mentioned character of a lead bloom to the plumage of the northern races, which is supposedly most prominent in nominate leucorhoa. All races in very fresh plumage show this particular bloom but with considerable individual variation, and this particular cast is lost quite rapidly, both in living birds and in museum specimens. Freshly molted series of either O. l. chapmani or O. l. socorroensis show this character as prominently as newly molted nominate leucorhoa from the Alaskan area, although the overall plumage is admittedly considerably darker in the southern races. Juvenile socorroensis from Guadalupe are especially prone to show this particular character, but drastic differences exist between new and old plumage. When the plumage is several months old the entire feather coat in all races is of a washed-out brown hue in addition to its ragged condition, with hardly a trace of the rich charcoal present in new featheration. Body contour feathers, as well as flight feathers, are badly worn at the end of a plumage sequence. Of the flight feathers, primaries appear to take the most wear. In contour feathers, those about the head and neck appear most subject to wear and at the beginning of a new molt sequence these are often no more than ragged portions of the original appendages.

Only in the case of exceptionally large individuals of O. l. leucorhoa, and exceptionally small individuals of O. l. socorroensis, can birds of the leucorhoa complex be segregated with any degree of certainty. As is evident from the measurements in Appendix Table B, nominate leucorhoa from the Alaskan area is the largest of all races. While the average individual of the nominate race has a generally more massive bill, this subtle difference is useful only in direct comparison of specimens, and does not lend itself meaningfully to mensural application.

MOLT.—Members of the nominate race of Leach’s that enter the central and eastern Pacific from northern breeding grounds, as well as year-round semipermanent residents there (nonbreeders), are already undergoing a pronounced primary molt by July. This continues in some individuals throughout the fall and winter (to December or even later). In a large percentage of birds the entire primary molt has been accomplished by November, and in exceptional individuals by October (Figure 99). For many others this molt may not terminate until April. It cannot be determined whether individuals finishing primary molt from, say, February through April, are birds that have a very prolonged cycle or whether they are individuals that began molt very late in the year. Primary molt can be clearly ascertained and follows a set pattern of replacement of new feathers from the innermost to the outermost primaries with corresponding feather pair members of each wing coincident or very closely attuned. Most individuals of chapmani and socorroensis, returning to the breeding islands, were in terminal stages or had very recently completed a body and primary molt. Some individuals were still growing the outermost (tenth) primary as defeatheration of the brood patch and gonadal enlargement were occurring. We took no specimens of either beali or leucorhoa on any breeding grounds except for willetti (= beali) on the Coronados Islands.

Rectrix molt, like the preceding, is already well along in our July sample of O. l. leucorhoa, and is for all practical purposes, finished by November. Only a few isolated individuals show any evidence of rectrix molt during the period from January through May. There seems to be no set pattern of rectrix molt except that opposite members of a pair of tail feathers are generally molted at the same, or at nearly the same time.

Investigation of the molt pattern of the secondary feathers was less satisfactory because of the difficulty of working with study skins in these small, long-winged birds. In general, it appears that the outer secondaries begin to molt at about the time the first primaries shed, and molt then proceeds progressively toward the body. Secondary molt is closely parallel to primary molt and is usually terminated before the last primary has finished growing.

Extensive body molt of the contour feathers in the nominate race begins in November and is apparent in a decreasingly smaller number of individuals through April. Body molt is generally distributed over most of the body; the crown and throat regions and the primary and greater coverts are the last areas to replace feathers. The various races of Leach’s show certain variations in molt patterns.

No doubt much of the variation in molt between individuals of the nominate race can be attributed to differences between juvenile, immature, and adult breeding birds. However, I cannot satisfactorily separate immature birds from adults on the wintering grounds after the latter’s gonads have regressed, except for truly adult females, which form only a small part of our sample. The more extensive lighter margins of the coverts and the scapulars, and the wrinkling of the nostril area in study skins, which are supposed to be good characters for separating juvenile birds from older individuals, are not positive determinants and at any rate these differences disappear within the first year of life. Hence, the monthly samples shown in the molt graphs are unquestionably made up of a variety of age classes.

Leach's storm petrel or Leach's petrel (Hydrobates leucorhous) is a small seabird of the tubenose order. It is named after the British zoologist William Elford Leach. The scientific name is derived from Ancient Greek. Hydrobates is from hydōr "water", and batēs "walker", and leucorhous is from leukos, "white" and orrhos, "rump". It was formerly defined in the genus Oceanodroma before that genus was synonymized with Hydrobates.

It breeds on inaccessible islands in the colder northern areas of the Atlantic and Pacific Oceans. It nests in colonies close to the sea in well concealed areas such as rock crevices, shallow burrows, or even logs. It lays a single white egg, which often has a faint ring of purple spots at the large end. This storm petrel is strictly nocturnal at the breeding sites to avoid predation by gulls and skuas, and even avoids coming to land on clear, moonlit nights. The largest colony of Leach's storm petrels can be found on Baccalieu Island of eastern Canada, an ecological reserve with ~1.95 million pairs of the birds at last estimate in 2013.

Vagrant

Palearctic migrant

circum-global

North America; range extends from Labrador to the Gulf of Maine

Least Concern