Wilson’s storm petrel (Oceanites oceanicus) and Leach’s storm petrel (Oceanodroma leucorhoa) are two similar looking species of small seabirds in the storm petrel family Hydrobatidae. They can be challenging to tell apart in the field due to their overall similarity in appearance and behavior. However, upon close inspection, there are some key differences between these two species in terms of size, plumage, breeding habitat, migration patterns, and vocalizations. Understanding these differences is important for properly identifying and distinguishing between Wilson’s and Leach’s storm petrels.
What are storm petrels?
Storm petrels are a group of small seabirds in the order Procellariiformes. There are around 20 species of storm petrels found worldwide, mostly in temperate and polar regions. They are characterized by their small size, erratic fluttering flight, ability to walk on water, and tube-nosed beaks that exudeexcess salt. Most species nest colonially in burrows on remote islands and return to land only to breed. They feed on small fish, crustaceans and other marine invertebrates, often picking food items from the ocean surface while fluttering low over the water. The name ‘storm petrel’ refers to their habit of appearing during stormy weather conditions at sea when they are able to feed more actively on organisms driven to the surface by winds and waves.
Size Difference
One of the main differences between Wilson’s and Leach’s storm petrels is their overall size. Wilson’s storm petrel is a small bird, measuring 14-16 cm (5.5-6.3 inches) in length with a wingspan around 33 cm (13 inches). They weigh just 18-33 grams (0.6-1.2 ounces). In comparison, Leach’s storm petrel is noticeably larger, measuring 21-23 cm (8.3-9.1 inches) in length with a 38 cm (15 inch) wingspan. Leach’s storm petrels weigh around 40-60 grams (1.4-2.1 ounces). So while not a huge bird by any means, Leach’s is significantly larger overall than Wilson’s storm petrel. This size difference can be used as a helpful clue when observing these birds in the field.
Plumage Differences
In addition to their size, Wilson’s and Leach’s storm petrels can be told apart by subtle differences in their plumage.
Wilson’s storm petrel adults have uniformly dark brown to blackish plumage on their entire body, except for paler brown undertail coverts. Their legs and feet are black and they have a black bill with pale greenish base to the lower mandible. In flight, Wilson’s storm petrels show a distinct white rump patch and pale bar along the upperwing coverts that forms a weak ‘M’ pattern.
Leach’s storm petrel adults are dark greyish-brown overall. Their upperparts are dusky brown while the underparts are lighter brown to grey-brown. Leach’s storm petrels lack any distinct paler rump patch or upperwing pattern that Wilson’s shows. Leach’s also has a longer, more forked tail and squarer wings than Wilson’s. The legs and feet of Leach’s storm petrel are pink with a blackish webbed toe and a darker grey-brown bill.
Breeding Habitat Differences
Wilson’s and Leach’s storm petrels show some preference for different breeding habitats and nesting sites.
Wilson’s storm petrels nest colonially on remote oceanic islands across a wide range of latitudes, from the subtropics to subantarctic islands. They nest in crevices among rocky scree slopes, cinder cones, lava flows, and cliff faces. Wilson’s storm petrels show high nest site fidelity, returning to the same burrow site year after year.
In comparison, Leach’s storm petrels typically breed on cool, damp islands with extensive soil burrows, turf, or peat banks suitable for excavating nesting burrows. In North America, they mainly nest on islands off the northeast coast of the United States and Canada. Leach’s storm petrels nest in long burrows up to 1-2 meters deep dug into soft soil or turf. Their colonies are often in dense forest or scrub habitat.
Migration Differences
The migration routes and wintering areas used by Wilson’s and Leach’s storm petrels also show some differences.
Wilson’s storm petrels undertake a transequatorial migration in the nonbreeding season. Northern hemisphere populations migrate south to spend the boreal winter off the coasts of Central and South America, Africa, and Australia. Meanwhile, southern hemisphere breeding populations of Wilson’s storm petrel migrate north to spend the austral winter off North America, South Africa, and New Zealand.
Leach’s storm petrels have a more restricted migration pattern, remaining largely within the North Atlantic. Northern breeding populations migrate south to winter off the southeastern United States and Gulf of Mexico. Meanwhile, populations that breed off western Europe migrate southwest to winter off the coast of tropical Africa. Unlike Wilson’s, Leach’s storm petrel does not cross the equator during migration.
Vocalization Differences
The calls and vocalizations of Wilson’s and Leach’s storm petrels also show some distinctions. However, storm petrel vocalizations can be highly variable and are most useful for distinguishing the species when heard within a known breeding colony.
Wilson’s storm petrel’s primary call is a loud “tsit-tsit-tsit” chatter or squeaking sound given both in flight and while sitting in colonies. Their calls are faster, shorter, and higher-pitched compared to Leach’s storm petrel.
Leach’s storm petrel vocalizations include a “chook-chook” call sounded while sitting at the nest burrow, and a wheezing “chirr” call given in flight. The Leach’s storm petrel flight call is lower-pitched and more drawn-out compared to the short “tsitting” of Wilson’s storm petrel.
Identification Tips in the Field
Applying knowledge of the differences in size, plumage, breeding sites, migration routes, and vocalizations makes identifying Wilson’s versus Leach’s storm petrels much more straightforward in the field. Here are some quick identification tips:
– Size – If it’s a small storm petrel around 13-16 cm in length, it’s likely a Wilson’s. If it’s a larger storm petrel around 21-23 cm long, it’s likely a Leach’s.
– Upperwing pattern – Look for a distinct white rump patch and pale “M” pattern along the upperwing coverts which indicates Wilson’s storm petrel. Leach’s lacks these markings.
– Underparts – Wilson’s shows uniformly dark underparts while Leach’s has lighter grey-brown underparts contrasting with a dark grey-brown back.
– Bill – Wilson’s has a black bill with some pale at base. Leach’s has a darker grey-brown bill.
– Legs – Wilson’s black, Leach’s pink.
– Flight style – Wilson’s has more rapid, stiff-winged flight. Leach’s flight is lighter and more bouyant.
– Voice – Listen for the faster, higher-pitched “tsitting” of Wilson’s versus slower, lower “choooking” or “chirring” of Leach’s.
Conclusion
In summary, Wilson’s and Leach’s storm petrels overlap significantly in their overall appearance and habits as small, pelagic seabirds. However, paying close attention to differences in their size, plumage details, breeding sites, migration patterns, and vocalizations allows even experienced birders to confidently distinguish these two species in the field. Remembering key identification points like upperwing patterns, underpart colors, bill and leg color, and flight calls are essential to properly differentiating Wilson’s storm petrel from the similar but distinctly larger and differently-marked Leach’s storm petrel.