Storm-Petrels (Family Hydrobatidae) are strictly pelagic, and only come to shore to breed. Following breeding, both adults and juveniles disperse and range at sea. The family contains 2 subfamilies: 20 species in 8 genera (depending on taxonomy). Storm-Petrels are generally distributed throughout the World's oceans.
 
Storm-Petrels have well-developed olfactory organs and a highly developed sense of smell. These birds also have a peculiar musky aroma that permeates their feathers. Their aroma can actually be smelled at sea when large numbers of individuals are rafted together on the water.
 
During the late spring and throughout the summer storm-petrels appear in the northern Gulf of Mexico. Three species have been documented in Louisiana waters - converging from very different breeding localities (Fig. 1). We are still in the early stages of learning the status of these species– timing and abundance, and preference for water depth, clarity, or temperature. This article takes a look at the identification of these three species and provides tips on how to see them.

In search of Storm-Petrels

Oberholser (1938, Bird Life of Louisiana) and later Lowery (Louisiana Birds; all editions) reported Wilson's Storm-Petrels in Louisiana waters, but reports were few and many were from "sportsmen and commercial fisherman." Lowery (1974) considered Wilson's Storm-Petrel to be of annual occurrence offshore during the late spring and summer. The species was added to the "first" LBRC Review List in 1979; removed in 1986, re-added in 1994, and, finally removed again in 1999. There are relatively few records submitted to the LBRC, and, compounded by sporadic coverage of offshore waters, even more sporadic reporting to the LBRC, and potential ID confusion with Band-rumped and Leach's storm-petrels, all contributed to the LBRC reversals about whether to include this "probably regular" species on the Review List. Most offshore trips during the summer will encounter small numbers of Wilson's Storm-Petrels.
 
The other two species have been more elusive. A 1956 specimen of Leach's Storm-Petrel (obtained when it landed aboard a ship in the early morning hours of 5 Dec.) is the only accepted record. Apparently observers on GULFCET cruises have logged-in sightings of Band-rumped and Leach's in Louisiana waters, though none of these sightings have been submitted to the LBRC, and these data are apparently not published(?). These two species have also been reported from recent LOS and LSUMNS Pelagic trips, and by observers stationed on offshore oil platforms.
 
To see storm-petrels in Louisiana, it is necessary to go offshore -- WAY offshore. A good opportunity to see these birds might be on a LOS-sponsored pelagic trip (especially one scheduled between late April and mid- September). Another option is to go on a "blue water" fishing charter. The best points of departure are Venice or Grand Isle, which are ports closest to the edge of the continental shelf. If you charter a fishing boat, make sure that the captain knows that you want to go way offshore in search of pelagic birds (and, if the birds don't cooperate, you can always fish). If you plan to tag-along on a fishing charter, find one that fishes for tuna or marlin in deep or blue water. Although you might catch a lot of fish, you are unlikely to see pelagic species if you are tied to a rig in shallow water on the Continental Shelf.
 
All three of the species can be attracted to a "chum slick." A good chum is made of fish, preferably smashed menhaden ("pogeys") mixed with fish oil or other food oil, and any combination of floating food matter (cat or dog food, lard, popcorn, pretzels, etc.). The chum mixture is slowly poured in the water as the boat moves slowly forward. The chum floats on the surface and forms a slick. The oil is easily seen as it glistens on the surface. A good slick generates a strong fishy aroma! After the slick is laid, the boat returns along the slick to check for anything attracted by the smell. If necessary, the slick can be reinforced with more chum. Storm-Petrels are initially attracted by the slick's odor. A "good" slick can draw in birds from quite a distance. As long as there are birds in the vicinity, they will slowly begin to appear at the slick, often when no birds were initially in sight. Storm-Petrels feed on the slick, picking up globlets of oil, fat, fish, and other goodies.

Status

Wilson's Storm-Petrel (Oceanites oceanicus) breeds around the continent of Antarctica, on sub-Antarctic islands off southern South America and on islands in the southern Indian Ocean. During the non-breeding season, the species primarily ranges at sea. In the western Atlantic, large movements can be tracked along the coast, moving north towards Labrador. Birds also range throughout the Gulf of Mexico. On the other side of the Atlantic, birds range north to the British Isles. Smaller numbers move east into the Mediterranean as far as Sardinia. Birds are also found throughout the Indian Ocean north to the Red Sea and the Persian Gulf. Wilson's are also found off Australia and New Zealand, north to Indonesia and New Guinea. In the South Pacific, it ranges north along the west coast of South America to Peru, and casually as far north as California. It is a relatively common visitor to Louisiana's offshore waters between mid-April and early September (extreme dates 8 April and 16 September). Wilson's Storm-Petrel does not seem as dependent on "deep water" as the following two species, and may move closer to shore in association with blue-green water boundaries or "rip" lines. It has been observed as close as 15-20 mi. from South Pass, and presumably could be even closer when blue water penetrates to within a few miles of South Pass. It has also been observed in deep green water, as well as in green water 15 mi. off Belle Pass with a depth of 97 ft. Probably it is most easily found at and beyond the shelf drop-off.
 
Band-rumped Storm-Petrel (Oceanodroma castro) also has a widespread breeding distribution. In the Pacific, it breeds on islands off Japan and in the Galápagos Islands, and is suspected to breed in the Hawaiian Islands and Cocos Island off Costa Rica. In the Atlantic, it breeds on the Azores (probably), Salvage, Madeira, Cape Verde, Ascension, and St. Helena islands. During the non-breeding season, some apparently range at sea near the breeding islands, whereas others undertake long distance migrations. The non-breeding movements are largely unknown. It is a regular visitor to the Gulf Stream along the mid-Atlantic coast of North America (May-August), with smaller numbers entering the Gulf of Mexico. There are only a few well-documented records for Louisiana waters. With continued exploration of offshore waters, this species will likely be regularly encountered in deep water. Timing of occurrence in the northern Gulf of Mexico is probably similar to occurrence in the western Atlantic. This species is probably a "blue-water species," preferring blue water beyond the continental shelf drop-off, probably in depths in excess of 1000 feet. It has been observed in 3000 ft. deep greenish-blue water. It is unknown whether this species will move into shallower water as blue water nears shore or attend blue-green rips close to shore. At writing, this species has not yet been officially added to the State List, but recent well-documented (specimen) records will be reviewed at the upcoming LBRC Meeting in September.
 
Leach's Storm-Petrel (Oceanodroma leucorhoa) is a more northerly breeding species. In the Pacific, it breeds along the coast of Japan and on the west coast of North America from islands off Alaska south to Baja California. Following breeding, the birds move south to winter in the equatorial and subtropical Central and Eastern Pacific. In the Atlantic, it breeds from southern Labrador south to Newfoundland, Maine, and Massachusetts, and from southern Iceland, the Faroe Islands, and Norway south to Scotland. Spring migration is primarily north through the western Atlantic; fall migration primarily, but not exclusively, south through the eastern Atlantic. Birds winter in the tropical Atlantic, many off the coast of w. Africa. The species' status in the Gulf of Mexico is not clear, though recent Louisiana records indicate that it may be regular in small numbers. To date, there is one accepted record; a specimen collected 41 mi. S South Pass on 5 December 1956. There are three well-documented (two specimens, and in-hand photos of a bird captured and released at an oil platform) records pending review at the LBRC Annual Meeting. Louisiana records of this species may result from individuals that wander into the Gulf of Mexico, or are "trapped" during northward migration. It is a deepwater species (particularly in the western Atlantic), and should be looked for beyond the Continental Shelf.

Identification basics and pitfalls

The main "fieldguide" sources for storm-petrel identification are: Harrison, P., 1983, Seabirds, an Identification guide; Harrison, P., 1987, Seabirds of the world: a photographic guide; and the ID series in Birding vols. XIII, No. 6 and XIV, No. 1-4 (two numbers are combined in one issue), Naveen, R. 1982, Storm-Petrels of the world: an introductory guide to their field identification Parts 1-IV. These are all good places to start for basic information. The last two are of particular interest, if you would like to see photos of some of the species. These sources primarily stress size and shape, rump pattern, and flight behavior as the best distinguishing characters. The National Geographic Fieldguide to the Birds of North America has adequate illustrations, but the text exaggerates the difference in flight styles between the species.
 
The storm-petrels that have occurred in Louisiana are all relatively small, uniformly blackish-brown to brown with a lighter brown or gray panel in the upper wing, and have a white rump patch. The three species are very similar in overall appearance (see Fig. 2). It is difficult to imagine that two genera (and subfamilies) are involved. Band-rumped and Leach's belong to the genus Oceanodroma (Subfamily Hydrobatinae); Wilson's belongs to the genus Oceanites (Subfamily Oceanitinae). The sexes are similar in size, with females averaging slightly larger.
 
Close views on a calm sea can provide ample opportunity for careful inspection of distinguishing field characters. As mentioned above, storm-petrels are easily attracted to a chum slick, and on calm seas can be approached to within several feet. Wilson's is the most frequently encountered species and, therefore, the one that provides the best learning opportunity for Louisiana observers. The identification of other species can usually be made in comparison to Wilson's.

Size and shape

Most references indicate that Wilson's is the smallest of the three species. In general, Wilson's are proportionately smallest, Band-rumpeds intermediate, and Leach's the largest species, though there is some overlap in measurements. In the field, the overall size difference between Wilson's and Band-rumped is not very obvious. Wilson's do appear smaller than Leach's. Wing shape is frequently cited as a good distinguishing character. Wing tracings (Fig. 3), however, illustrate how similar the three species actually are. Note that there is essentially no appreciable difference between species in length of the wing from the bend (carpal) to the tip of the outer primary. Compare wing length measurements (in millimeters) from the Birds of the Western Palearctic (BWP): Wilson's 133-163; Band-rumped 142-162; and Leach's 148-166. Wing length (= wing chord, not total wingspan; Fig. 3 "B") overlaps extensively between the three species. The more obvious difference between the two genera is the length of the forearm (ulna-radius), or inner wing, which is the distance between the body and the bend of the wing. Wilson's has a very short inner wing. This can create the illusion of a more "rounded" wing, but the wing is actually just as pointed as the other two species (Fig. 3). Compare Wilson's to the longer "forearms" of Band-rumped and Leach's. The longer "inner wing" of these species makes the carpal joint more pronounced. Forearm length is the best character to distinguish these two genera flying alongside or towards the observer. Also note that Leach's has a "broader" wing throughout the secondaries.

Bill size and tarsus length

Two other excellent distinguishing characters between the two genera are bill size and tarsus length. Compared to the other two species, Wilson's Storm-Petrel has a relatively small and delicate bill (Fig. 4). Bill size can only be seen under the best field conditions, but is obvious when a bird is observed at close range. There is only a minimum of overlap (BWP) in length of culmen between Wilsons's (10-14) and Band-rumped - Leach's (combined range of 13.0-16.9). Tarsus length is also an important character. The legs of Wilson's are long. Band-rumped and Leach's have much shorter legs (Fig. 8), and, when feeding, the difference between the two genera is obvious (Figs. 5 and 6); there is no overlap in tarsus length. In flight, the long legs of Wilson's typically extend beyond the tail tip (but see Molt below).

Plumage coloration and pattern

As previously noted, all species are very similar in overall color and pattern. Leach's tend to be browner than Wilson's and Band-rumped. This is primarily a by-product of plumage wear (see Molt) and the difference of timing of molt. Most ID sources suggest that rump pattern is a helpful ID mark. Good views are necessary to determine shape, pattern, and extent of the rump patch. The white rump can be inconspicuous in poor viewing conditions – a distant Storm-Petrel can initially appear all dark. Also, because Storm-Petrels are in various stages of body molt, relying on this feature alone could steer you towards a misidentification.

Flight and feeding behavior

Flight style has been one of the most discussed field identification characters. All three generally have the same flight pattern: a few to several flaps followed by a glide. The flight style of each species is a little different as a result of wing length, depth or altitude of the wing beats, how the wings are held during the glide, molt, type of activity (direct flight from point A to point B versus foraging over chum), and wind and sea conditions. The flight style of each of the species has been compared to other types of birds or insects to describe the "characteristic" style: butterflies, swallows, nighthawks, terns, shearwaters, bats, and so forth. It is always best to form your own interpretation of flight style, rather than trying to fit a species into someone else's description. Flight style can be a helpful characteristic under certain conditions: observer is familiar with all three species; the observation takes place during good field conditions; and birds can be compared under the same field conditions (see also Molt).
 
When foraging, all three species pick food items from the surface of the water. They often hold themselves motionless on stationary extended wings (in a breeze) or flap as they patter or drag their feet on the water. "Petrel" was derived from the Bible; the pattering behavior of storm-petrels was likened to Peter (who could walk on water). Storm-petrels patter their feet on the water's surface to help hold them in place when they search for or pick up food. Each of the three species may employ slightly different foraging maneuvers. Again, the differences may be subtle and misinterpreted by an inexperienced observer relying on fieldguide descriptions.

Molt

Like many other birds, storm-petrels molt from natal down into Juvenal Plumage. This plumage is similar to Basic plumage. Juvenal Plumage is replaced during the first Pre-Basic Molt. This molt occurs sometime around the first "breeding" season (through most first year birds probably do not breed) or is skipped until the following "second" breeding season (Leach's BNA account; April of the second calendar year, BWP). The delay of a complete molt during the "first" year could result in VERY worn birds the following spring and summer. Older birds of all species undergo a complete (Pre-Basic) molt to Basic Plumage following the breeding cycle (primarily on the "wintering" grounds).
 
Most storm-petrels observed in Louisiana will be in something other than "fresh" plumage. Most birds will be worn or in active molt. The overall coloration of the bird's plumage is dependent on the condition of the body feathers. Fresh body feathers are blackish-brown, often with a blue sheen; worn feathers become progressively browner.
 
Wing (primary) molt can dramatically affect the way a bird flies. Worn birds with tattered wings do not have the same aerodynamic prowess of a bird with fresh primaries. Primary molt can dramatically affect the overall "shape" of the wing (Fig. 6). Birds with active wing molt, or with missing or growing primaries may show more atypical flight behavior. It is important to be aware of these possibilities when assessing "flight style." Individual Wilson's Storm-Petrels in the same flock may show very different wing shapes and flight styles due to the condition of their flight feathers (Fig. 6). From observations off Louisiana, Wilson's begin primary molt in June; Band-rumpeds slightly earlier. Leach's are worn and not molting (during the summer months).
 
Tail molt can also influence the appearance of the bird. Missing or growing feathers can make the tail appear shorter than normal. Feet may extend through the tail of a molting Band-rumped or Leach's, causing possible confusion with Wilson's. Very worn feathers can change the shape of the tail. For example, the fork of the Leach's may not be as obvious if the outer feathers are very worn or missing. Wilson's and Band- rumpeds may appear forked if the central feathers are missing or growing in.
 
Molt and wear can also influence the appearance of the white rump. Fresh feathers will appear more dazzling white against fresh body plumage. The rump will not be "as white" if feathers are worn or missing and the body plumage browner. The "dividing line", a characteristic of Leach's, may be inconspicuous due to worn or missing feathers, or conversely, worn or missing feathers on a Wilson's or Band-rumped may suggest a dividing line.

Our three Storm-Petrels

Wilson's Storm-Petrel
This species is relatively small, with a proportionately short forearm portion of the wing (Figs. 2 and 3). The plumage is overall blackish-brown; juveniles with paler-edged belly feathers (often appear browner in the field). Wilson's has a broad band of white wrapping around the rump patch onto the undertail coverts. The inner web near the base of the outer tail feather is white (Fig. 8), the next feather in also has white at the base (probably impossible to see in the field). The outer tail feathers are only barely longer than the inners, and the tail appears square or barely forked in the field (Figs. 2 and 5). The legs are very long and slender (Fig. 8). The center of the toe webbing is pale yellow to yellow. The bill is small and delicate (Fig. 5). All of these features can be observed under good field conditions. The gray greater-covert upperwing patch is variable depending on the stage of molt (fresh adults the greatest and worn juveniles show the least amount of contrast). Fresh feathers are medium gray and are edged with white and form a distinctive silvery-gray wedge on the upper wing in flight. These same feathers, when worn or molting, contrast little against the rest of the wing.
 
In flight, Wilson's Storm-Petrel is characterized by a very short body to "shoulder" distance, giving the bird a short-winged appearance. We would not characterize this species as possessing a more rounded wing, because this could lead an inexperienced observer astray. The wing of the Wilson's is just as pointed as the other two species (Fig. 3), at least comparing the actual shape of the wing or primaries. Like the other species, Wilson's flies with a few flaps followed by a glide. This pattern could be interpreted as "shearwater-like," especially by the inexperienced observer. The flight style of Wilson's looks very different under different circumstances and is much more "shearwater-like" when a bird is traveling fast or flying in windy conditions (especially with a tail wind). Birds completing primary molt (Fig. 7; middle) exhibit the most "butterfly"-like flight (perhaps the origin for that description?). The legs are long, and in flight usually extend beyond the tail creating a "point" (see Fig. 2). If forced to make comparisons, the flight is more reminiscent of a swallow.
 
When feeding, Wilson's "walks on water." A bird may stay stationary, pattering its feet on the surface, or move in one direction or another. A bird holds its wings out, bowed and horizontal with its body, or in a shallow "V" above the body, with occasional flaps to hold the bird above the water. Close inspection may reveal the yellow centers to the toe webs as a bird "patters" on the surface.
 
Voice. Generally silent at sea, but has been reported to make almost inaudible high peeping notes, given most frequently at feeding aggregations. It is probably difficult to hear Wilson's Storm-Petrels above the sound of a boat engine.
 
Band-rumped Storm-Petrel
This species is slightly larger on average than a Wilson's Storm-Petrel. Band-rumped's larger size is usually not apparent in the field. Band-rumpeds also have a white rump patch, though, generally, the white is not as extensive as Wilson's. There is less white that "wraps" around the tail and although the white extends onto the outer webs (outer web dark in Wilson's), there is less white on the outer two tail feathers. Band- rumpeds may appear blacker than Wilson's (and especially Leach's), but this is a reflection of molt/wear rather than a real field mark. Band-rumpeds are easily identified in the hand by black-tipped white upper tail coverts (lacking in Wilson's), but this character unfortunately cannot be seen in the field (maybe in really good photographs). The tail length and tail shape of Band-rumped is similar to Wilson's, square or shallowly forked (Fig. 8). If a bird holds outer and inner feathers at different levels, the tail may appear more forked (as sometimes frozen in photographs). When close to the boat, the larger bill of Band-rumped (Fig. 4) is very different from the small bill of Wilson's. The legs (tarsus) are noticeably shorter than Wilson's, and this species lacks the yellow centers to the foot webbing. In flight, the feet do not extend beyond the tail. As in Wilson's, the appearance of the upper wing panel is dependent on plumage wear. In fresh plumage the panel is brownish- gray, edged with white. When worn, it is browner and barely contrasts with the rest of the upper wing.
 
The easiest way to distinguish Band-rumped from Wilson's is by forearm length. Band-rumpeds have proportionately longer wings (between the body and the "shoulder"; Fig. 3 and 4). Band-rumped's flight is more shearwater-like, with distinct glides on bowed wings, followed by a few flaps, compared to Wilson's. Band- rumpeds will also patter their feet on the surface when feeding, but generally do not "walk." When foraging, the wings are held horizontal to the body, and often stationary.
 
Voice. Apparently silent at sea.
 
Leach's Storm-Petrel
This species is very similar in size and proportions to Band-rumped (Fig. 2), though generally appears larger. The best feature to distinguish Leach's from either Band-rumped or Wilson's is the relatively long and deeply forked tail (Fig. 8). The outer tail feathers are noticeably longer than the inners. The white rump is separated by a darker partition, though not always obvious under field conditions. There is very little white wrapping around the side of the rump and no visible (hidden by tail coverts) white in the bases of the outer tail feathers. Leach's wings are broader through the secondaries than Band-rumped (and especially Wilson's). The broader wing and longer tail makes Leach's appear larger in the field than either of the other two species. One of the best field ID clues is the "long-winged look." In coloration, Leach's tends to be browner. This is due to the different timing of molt between Leach's and the other two species; Leach's would normally begin Pre- Basic Molt several months later.
 
Leach's has a somewhat more tern-like flight under calm conditions, flying with deeper wing strokes than either of the other two species. Unlike the other two species, Leach's holds its wings up when pattering on the surface, also very tern-like. When foraging, Leach's may fly upwards several feet, then suddenly dip to the water's surface on raised wings.
 
Voice. Reported to make vocalizations at sea similar to those uttered on the breeding grounds, including staccato ticking notes ending in a slurred trill-- probably difficult to hear over a boat's engine noise.
 
Other species possible?
White-faced Storm-Petrel (Pelagodroma maritima), like Band-rumped, breeds on the Salvage and Cape Verde islands. In addition, it breeds in the South Atlantic (Tristan da Cunha and Gough Island) and on islands off Australia and New Zealand. It is probably the most likely long shot possibility to stray into the Gulf of Mexico. There is a record from the Caribbean (17°N, 65°W) and it is casual to Gulf Stream waters of the mid- Atlantic from Aug.-Oct. It's larger size (as large or larger than Leach's, much larger than Wilson's), light gray upperparts, white underparts, pale rump, and prominent white eyebrow make it dramatically different from any of the species discussed above. Overall size and color combined with its bounding kangaroo-like flight and long legs and feet should allow an immediate identification.
 
Another possibility, and similar in general appearance to storm-petrels, is Bulwer's Petrel (Bulweria bulwerii). Bulwer's Petrel is in the family Procellaridae but bears a superficial resemblance to Storm-Petrels. Though significantly larger than the Storm-Petrels, Bulwer's is all dark brown and has a pale upperwing covert panel. Bulwer's lacks a white rump and is proportionately longer-winged and longer-tailed. Compare also with noddies (Anous ssp.) Like Band-rumped Storm-Petrel, Bulwer's breeds in the Atlantic Ocean on the Azores, Madeira, and Cape Verde islands, as well as the Canary Islands and islands in the Pacific Ocean. In the Atlantic, it ranges at sea primarily in the eastern Atlantic north to England and near the breeding islands, but ranges as far west as the western Atlantic off Trinidad. There are a few records of Bulwer's from the western mid-Atlantic off North America.
 
Both of these species would be predicted to stray to Louisiana waters during the summer to late fall based on Atlantic Coast records.
 
***
Specimens at LSU Museum of Natural Science provided an invaluable resource. Distributional information from The AOU Check-list of North American Birds, Sixth Edition (and sources listed below); LSUMNS card file, and LSUMNS Pelagic trips.
 
Other recommended references (and sources of information for this article):
Cramp, Stanley, et al. eds. 1977. Handbook of the Birds of Europe and the Middle East and Africa. Birds of the Western Palearctic. Vol. 1. Oxford University Press, Oxford, London, New York.
 
Del Hoyo, J. Elliot, A. & Sagatal, J., eds. 1992. Handbook of the Birds of the World. Vol. 1, Lynx Ed., Barcelona.
 
Huntington, C. E., R. G. Butler, and R. A. Mauck. 1996. Leach's Storm-Petrel (Oceanodroma leucorhoa). In The Birds of North America, No. 233 (A. Poole, F. Gill, eds.) The Academy of Natural Sciences, Philadelphia, PA, and the American Ornithologists' Union, Washington, DC.
 
Palmer, R. S., ed. 1962. Handbook of North American Birds. Yale University Press, New Haven and London.
 
Watson, G. E., D. S. Lee, and E. S. Backus. 1986. Status & subspecific identity of White-faced Storm-Petrels in the western North Atlantic Ocean. American Birds 40 (3): 401-408.
 
Donna L. Dittmann & Steven W. Cardiff
435 Pecan Drive
St. Gabriel, LA 70776
 
Figures by Donna Dittmann: Fig. 1 || Fig. 2 || Fig. 3 || Fig. 4 || Fig. 5 || Fig. 6 || Fig. 7 || Fig. 8