Swinhoe's Storm-petrel: first for Scilly

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Bob Flood and Ashley Fisher

Introduction

The year 2005 was the sixth year of intensive short-range pelagic trips off the Isles of Scilly involving 50-70 trips each year with an average of around four a week (June to September). We participated in virtually every one and our combined tally prior to the 2005 Swinhoe’s Storm-petrel Oceanodroma monorhis sighting was approaching 500 trips and now is over 700 trips. RLF has undertaken short-range pelagic trips off Scilly annually since 1995.

Our interest in seabirds is not centred purely on rarities in British national waters. However, given the amount of time we spend at sea off Scilly, it was to be anticipated that sooner or later we would encounter something out-of-the-ordinary for the region. So, we identified species that we believed were the ‘more likely’ rarities to occur and read-up on these in detail, including Swinhoe’s Storm-petrel. Thus, if confronted with one of them we would quickly be able to look for the salient features and maximise our chances of a sound identification.

That said, we are well aware that birding at sea poses unique challenges. Sea-birding is different from land-birding. There is often little time with each bird, no opportunity to grill a find through telescopes, whilst there are many unique light and weather conditions that affect the context and impressions of seabirds. In addition, with tubenoses, we are dealing with a complex and often subtle plumage problematique since petrels are, “clad in plumage that is some combination of black, white and shades of grey and brown.” (Brookes, 2004: p.5) These challenges make it all the more important to be prepared with knowledge of the salient features of potential rarities.

Circumstances and General Impression

On 21 July 2005 in the early afternoon we were drifting at sea approximately 9.5 miles (17 kms) south of Scilly. We had been liberal with chum and an excellent slick had formed. Conditions were very good for sea-birding with 100% high cloud cover and a Force 2/3 north-westerly wind. Sea-state was calm to moderate, slightly choppy with little swell. Our tally of birds, however, was limited to good numbers of European Storm-petrels Hydrobates pelagicus.

At 13.07 RLF was scanning the slick sifting through European Storm-petrels looking for a Wilson’s Storm-petrel Oceanites oceanicus when a Leach’s-type Oceanodroma entered his vision at about 250 metres. It swooped over the slick showing its upper-side. RLF’s heart missed a beat because this storm-petrel was darkish brown with buff covert bars on the upperwings and appeared to have an all-dark brown rump. It looked like it could be a Swinhoe’s Storm-petrel!

Inexplicably calmly RLF said to EAF, “You’d better look at this petrel Ash.” EAF didn’t respond immediately thinking it was yet another Wilson’s. So RLF said, “It’s an Oceanodroma,” which sparked an instant response since even Leach’s Storm-petrel Oceanodroma leucorhoa is a genuine scarcity in Scillonian waters. EAF got on it right away as did John Higginson who was standing nearby. EAF helped the remaining four less experienced sea-birders by telling them to look for a Leach’s-type storm-petrel.

The Swinhoe’s actively investigated the slick swooping from one side of it to the other and back again, but its flight overall was more direct (less erratic) than the flight of Leach’s that we have seen. It tilted several times revealing both its upperside and its underside. It bounded forward and one time rose above the sea surface perhaps to three metres and hung there briefly. The slick led the storm-petrel towards the boat and it approached to about 75 metres. It then swooped over a bit of choppy water using wind dynamics to gain lift and tilted towards us presenting a striking view of its upperside. EAF dared to say it, “Swinhoe’s!”

The Swinhoe’s flight action continued in the same manner for all of the three minutes that it was exploring the slick. The flight action was very exciting to watch. However, the entire observation comprised the bird in foraging mode and we did not see it in any other flight mode (i.e., travelling or feeding).

We found size relatively easy to judge with European Storm-petrels plentiful. Experience in Scillonian waters of Europeans alongside the larger Wilson’s greatly assisted our assessment. Roughly speaking you could say our Swinhoe’s was another size-step up from Wilson’s, making it Leach’s size. Each year we see small numbers of Leach’s Storm-petrels amongst European Storm-petrels and the size of our Swinhoe’s looked to be in the region of Leach’s when compared to Europeans.

We were confident with our assessment of the bird’s size; however, to make a well-founded claim of a Swinhoe’s Storm-petrel we still had to eliminate the possibility of dark-rumped Leach’s Storm-petrel and consider carefully all aspects of the bird’s field characters.

The showy flight action and the three minute performance enabled us to evaluate structural and plumage features. Overall, the structure of our bird was comparable to Leach’s Storm-petrel, but with notable differences. In general, the wings were held outstretched, unlike Leach’s, making the Swinhoe’s seem longer winged and a bit larger. The hand tapered to blunt wing tips, not pointed ones. The tail often was held closed, reminiscent in side-on profile of a long tailed swift and unlike the open, scooped and more ragged look of Leach’s. The tail fork when visible was relatively shallow, not as deep as Leach’s.

The entire bird was darkish brown, with remiges darker. We were struck by the fact that the tail, rump and remaining body plumage were homogenous in colour. The bird was notably browner than European Storm-petrels that it was amongst, which were just pre-moult and thus worn, bleached and at their brownest. The upperwing displayed a warm buff diagonal covert bar across each wing that extended near to, but not quite up to the leading edge; it was impressive, mainly because it created the only contrasting plumage feature. The warmish plumage tones were totally different from the cooler tones of Leach’s.

On the basis of size, structure, plumage pattern and colour, and flight action, it is our opinion that the only possible identification of the 21 July 2005 storm-petrel is Swinhoe’s Storm-petrel. It is important to say that neither of us or John Higginson thought at any time that the bird could be a Leach’s Storm-petrel (see Illustration 1).

Original Field Sketches by Ashley Fisher

Illustration 1. Swinhoe’s Storm-petrel off Scilly 21 July 2005 - original field sketches by Ashley Fisher.

 

Size The critical first step in identifying our bird as a Swinhoe’s Storm-petrel was accurate judgement of size. Table 1 gives biometrics of all dark-rumped storm-petrels and, for reference, some other seabirds. All measurements are averages and are given in cms. This facilitates a ratio comparison of body length and wing span relative to European Storm-petrel, the commonest storm-petrel off Scilly and our ‘yard stick’. We judged our bird to be roughly the size of Leach’s. Table 1 shows that there is only one dark-rumped storm-petrel closely comparable to Leach’s Storm-petrel in body length and wing span, and that is Swinhoe’s Storm-petrel.

We are aware that there is some biometrical variation in the literature for Leach’s and Swinhoe’s Storm-petrels, but the scale of the variation does not impact significantly on size judgement. For example, Post (1998) measured the wing lengths of 35 Leach’s Storm-petrels from a wreck on beaches in Portugal and found them to be 1-5 mm longer than the range given in the literature. Cubitt (1995: 344) measured the three Swinhoe’s Storm-petrels caught at Tynemouth (UK) in the period 1989-1994 and noted that, “… measurements of wing length appeared to be at or beyond the extreme of those documented for Swinhoe’s … This, though, appears to be due to differences among recorders in experience and technique of measuring maximum-chord wing lengths.”

 

SPECIES

BODY LENGTH

RELATIVE BODY LENGTH

WING SPAN

RELATIVE

WING SPAN

Manx Shearwater

34

2.27

82

2.22

Bulwer’s Petrel

26

1.73

67

1.81

Matsudaira’s Storm-petrel

24

1.60

56

1.51

Tristram’s Storm-petrel

24

1.60

56

1.51

Markham’s Storm-petrel

23

1.53

52

1.41

Black Storm-petrel

23

1.53

48

1.30

Leach’s Storm-petrel

20

1.33

46

1.24

Swinhoe’s Storm-petrel

20

1.33

45

1.22

Ashy Storm-petrel

20

1.33

42

1.14

Wilson’s Storm-petrel

17

1.13

40

1.08

Elliot’s Storm-petrel

15

1.00

38

1.03

European Storm-petrel

15

1.00

37

1.00

Least Storm-petrel

14

0.93

32

0.86

 

Table 1. Average body lengths and wing spans of dark-rumped storm-petrels and other relevant species (averages, all in cms). Measurements mainly are taken from Enticott and Tipling (1997) and are consistent with measurements from other main sources. The ‘relative length’ and ‘relative span’ are ratio comparisons between those of an average sized European Storm-petrel (set at 1) and the average sizes of other species incorporated in the table.

 

On numerous occasions off Lima, Peru RLF has seen for extended periods many Elliot’s Storm-petrels (roughly the same body length and wing span as European Storm-petrels) alongside Markham’s Storm-petrels, these two having a body ratio of 1.53 and a wing span ratio of 1.41. The same ratios for European compared to Swinhoe’s are 1.33 and 1.22 respectively. It is highly relevant that RLF found the difference in size between Markham’s and Elliot’s in the field considerably greater than the differences between our Swinhoe’s and nearby Europeans. Thus, RLF was confident that our bird was not as large as a Markham’s and thus Black Storm-petrel and, therefore, certainly not as large as the even larger Matsudaira’s Storm-petrel Oceanodroma matsudairae or Tristram’s Storm-petrel Oceanodroma tristrami.

In summary, our Swinhoe’s Storm-petrel bounded over, swooped over, and traversed the slick amongst about 30 European Storm-petrels. We found it relatively easy to establish the size of the bird in terms of personal experience and the relative dimensions given above. It was about the size of a Leach’s.

Structure While the size of our Swinhoe’s was comparable to that of a Leach’s Storm-petrel, there were notable structural differences (see Illustrations 1 and 2).The Swinhoe’s appeared particularly long-winged. The leading edge of the arm from body to carpal joint appeared slightly concave. The hands were long and tapering (as in photographs of foraging Swinhoe’s Storm-petrel on Brian Patteson’s web site www.seabirding.com and others in our possession). However, they were not pointed at their tips, unlike all Leach’s Storm-petrels that we have seen. The leading edges of the hands were slightly convex. The trailing edge of the hands appeared at times slightly swollen and convex suggesting a different ratio formula for the lengths of P1-P10 than Leach’s that has a straighter trailing edge to the hands; also suggested, for example, in Peter Hayman’s comparative illustration of Swinhoe’s and Leach’s in Cubitt (1995: 348). Most of the time the Swinhoe’s wings were held outstretched, unlike Leach’s, making the trailing edge as a whole look straighter than that of Leach’s and yielding a different longer-winged feel to the overall profile.

Comparison: Swinhoe's vs Leach's

Illustration 2. Swinhoe’s Storm-petrel off Scilly 21 July 2005 (left) and Leach’s Storm-petrel off Scilly 27 September 2005 (right) illustrating structural differences in the wings and tail covered in the text, although the ragged look of Leach’s tail is not depicted here (Bob Flood). Note that the wings of the Swinhoe’s are held outstretched making it appear longer winged than the Leach’s (the body length including the tail are illustrated as the same size for the Swinhoe’s and the Leach’s).

Comparing these observations to the illustrations of Swinhoe’s and Leach’s in BWPi, we find there is agreement except that BWPi does not illustrate Swinhoe’s with outstretched wings and thus does not capture the seemingly longer-winged look of Swinhoe’s (see Illustration 2). There is a similar discrepancy between our observations and the Swinhoe’s illustration in the Collin’s Guide (Mullarney et al, 1999). In this respect the two guides seem to be inaccurate.

The tail was quite different from Leach’s. First, the tail was forked, but did not have the depth of fork typical of Leach’s. In step with our observations, BWPi describes Swinhoe’s as having a slightly forked tail and Leach’s a fairly forked tail. Indeed, this feature on our Swinhoe’s might be better described as a notable notch rather than a shallow fork. This feature was seen on several occasions as the bird manoeuvred when it opened its tail. Otherwise the tail mainly was held closed. Second, the side-on profile simply saw the tail narrow to its end. There seemed to be no concave curvature across the width of the upper-tail (i.e., it was not scooped) or raggedness to the tail-end, something always apparent on Leach’s that we have seen. The tail thus was reminiscent in side-on profile of one of the longer tailed swifts (see Illustration 1).

In summary, the body of our Swinhoe’s was about the size of Leach’s, but the wings were held outstretched rather than swept back making the bird look longer winged than Leach’s, and the Swinhoe’s tail mainly was held closed rather than open, but when opened was seen to be notched rather than forked.

Plumage The entire head, body, rump, uppertail-coverts and undertail-coverts, the tail, upperwing-coverts (except for the buff diagonal covert bars on each wing) and, as far as we could see, the underwing-coverts were all darkish brown. The bird was notably browner than the European Storm-petrels, which were just pre-moult and thus in worn and faded plumage (i.e., at their brownest; moult of adult Europeans starts during the second half of incubation BWPi). Chris Mead (2000) in The State of the Nation’s Birds describes Swinhoe’s as, “This browner cousin of the Leach’s Petrel.” To be more precise, the colour of the Swinhoe’s Storm-petrels off North Carolina on Brian Patteson’s web site (www.patteson.com) is how we remember the colour of our bird and this is what we mean by darkish brown.

We recollect being struck by the fact that the tail, rump and body plumage were homogenous in colour. We witnessed the Swinhoe’s in many different flight profiles where the rump was scrutinised and seen to be wholly and unequivocally darkish brown. Given the excellent light conditions and multiple angles of view we feel confident that we would have seen pale in the rump if the bird were a partially dark-rumped Leach’s. The remiges were darker brown than the body, but not very much darker. The upperwing displayed warm buff diagonal covert bars across each wing that extended near to, but not up to the leading edge. The buff upperwing covert bars stood out being the only contrasting plumage feature. This characteristic is evident in photographs of the exhausted Swinhoe’s captured at sea off Israel on 19 April 2003 (www.birdingisrael.com) as well as the North Carolina birds mentioned above.

In addition, there was no obvious sign of moult or wear and certainly the bird was not in primary moult. The bill was not seen well. The eye was not seen well, but we presume it was black or blackish. The legs were not seen.

We did not see white shafts at the base of the primaries. The literature suggests that these are difficult if not impossible to observe in the field unless at close range and this has been confirmed in discussion with observers experienced with Swinhoe’s. For example, Carter and Hobcroft (1997) noted the 15 Swinhoe’s that they located, “lacked any visible white at the base of the primaries.” Force (1997) did not see white primary shafts on 120+ Swinhoe’s Storm-petrels observed at sea in the Indian Ocean. Experienced sea-birder colleagues from Australasia rate size, structure, plumage colour, and the distinctive flight action (all of which we saw) as the important diagnostic features (M. Carter pers. comm.). This is a view shared by Hadoram Shirihai (pers. comm.).

Why Swinhoe’s Storm-petrel and not …?
Size is the critical factor that excludes most dark-rumped storm-petrel taxa. Structure, plumage, and behaviour lead to the conclusion that our bird was a Swinhoe’s. Accordingly, key points are set out below that differentiate our Swinhoe’s from like taxa. I list my at sea experience of each form. Here I do not mean ‘ticked’, but extended views often for many hours spread over numerous days, thus facilitating thorough critical examination. To start, I deal with the most obvious identification problematique concerning the similar sized Leach’s Storm-petrel.

Leach’s Storm-petrel Oceanodroma leuchoroa
Experience RLF Nominate, about 10 off Scilly, small numbers elsewhere England over many years, 100s in Atlantic from pelagic trips and cruises since 1990, c50 western Pacific from cruise April 2008, c50 eastern Pacific from on each pelagic trip mid-1990s, September 2007, and September 2008; O. l. chapmani 100+ on five day pelagic trips from San Diego, California September 2007 and September 2008; O. l. socorroensis 20+ believed to have been this form on five day pelagic trips from San Diego September 2007 and September 2008.
Elimination a) When observing dark-rumped chapmani Leach’s off California at similar range to our Swinhoe’s RLF was struck by the fact, as trite as it may sound, that they looked like normal Leach’s but with a dark rump! The flight behaviour of birds considered to be socorroensis Leach’s off California was completely different from either Leach’s or Swinhoe’s. In light winds socorroensis is fast and direct, rises and falls, and tilts and flicks to either side. It has moderately deep wing beats where the arc extends c40°-60° above and below the horizontal, probably not as deep as Leach’s. It glides and shears for short periods. In strong winds, socorroensis increasingly rises and falls, flicks and tilts, glides and shears. Returning to comparison with chapmani, our Swinhoe’s was wholly and unequivocally dark rumped but did not look like a Leach’s. Furthermore, a Leach’s in the Atlantic with a wholly and unequivocally dark rump is yet to be proven. With Leach’s, state of wear and moult is important since white in the rump is likely to be least obvious with worn rump feathers. Thus, naturally occurring partially dark-rumped Leach’s in moult not seen well could appear wholly dark-rumped. Such an individual might be termed a ‘pseudo dark-rumped Leach’s’. A ‘psuedo’ in British waters might be expected September-January, probably not as early as July when we saw our Swinhoe’s. b) Our Swinhoe’s primary foraging flight behaviour was direct and strident, with several strong wing beats, then a bounding glide low over the sea surface. Leach’s equivalent flight is irregular, punctuated with unpredictable speed and/or direction changes, involving darting, vertical leaping, and bounding ahead with frequent long shearing glides. This difference immediately stood out on 21 July 2005. c) Our Swinhoe’s held its wings outstretched when foraging, whilst Leach’s wings mainly are held swept back. As a result our Swinhoe’s appeared to be slightly longer-winged than Leach’s (see Illustration 2 above). d) Our Swinhoe’s outstretched wings revealed the caudal projection more clearly than in Leach’s, therefore the caudal projection appeared longer in Swinhoe’s even though Swinhoe’s actually has a shorter tail (see Illustration 2 above). e) Our Swinhoe’s had blunt wing tips, whilst Leach’s wing tips are pointed. f) The tail in our Swinhoe’s was held closed in foraging flight when its shallow fork was hardly visible, unless the bird manoeuvred fanning its tail; whilst the tail in Leach’s seems always to be open, is scooped, and has a much deeper fork that is more readily visible (see Illustration 2 above). g) Our Swinhoe’s plumage overall was warm-toned black-brown, whilst Leach’s is cool-toned brown-black, though Leach’s bleaches browner. Relevant here is that Europeans also bleach browner by late July, but our Swinhoe’s was browner still than local Europeans. h) The secondary upperwing-covert bars were warm-toned buff in our Swinhoe’s, whilst are cool-toned grey in Leach’s. Covert bars were less prominent in our Swinhoe’s compared to a typical Leach’s, though would become less distinct in a worn Leach’s later in autumn. i) Vagrancy of eastern Pacific dark-rumped forms of Leach’s into the Atlantic is highly improbable (see Appendix 2). Conclusion Our bird was not a Leach’s Storm-petrel.

Bulwer’s Petrel Bulweria bulwerii
Experience RLF About 10 in Canary Current 1999, c20 Atlantic cruise April 2006, c25 western Pacific cruise April 2008, 100+ Madeira to Selvagens June 2008.
Elimination Size alone excludes this species with Bulwer’s very large compared to our Swinhoe’s (see Table 1).  Also, in contrast with our Swinhoe’s, Bulwer’s jizz is like a giant storm-petrel but with unique flight action where it zigzags, swings body from side-to-side, often flip-changes direction, all with fairly shallow elastic-like wing beats. It has very long wings, a long wedge-shape tail, a long slim and lanky body, is overall sooty-brown with indistinct upperwing-covert bars. Our bird was not a Bulwer’s Petrel.

Matsudaira’s Storm-petrel Oceanodroma matsudairae
Experience RLF 100s with small numbers permanently on view often at point blank range over five days during western Pacific cruise April 2008.
Elimination Size alone excludes this species with Matsudaira’s very large compared to our Swinhoe’s (see Table 1).  Also, in contrast with our Swinhoe’s, Matsudaira’s has the jizz of a large long-winged falcon, has long slim wings, a long deeply forked tail, is overall dark-brown with broad distinct upperwing-covert bars. Our bird was not a Matsudaira’s Storm-petrel.

Tristram’s Storm-petrel Oceanodroma tristrami
Experience RLF About 1000 during western Pacific cruise April 2008, with large numbers permanently on view one afternoon at close range, small numbers next day, and numerous birds again on the following day.
Elimination Size alone excludes this species with Tristram’s very large compared to our Swinhoe’s (see Table 1).  Also, in contrast with our Swinhoe’s, Tristram’s jizz is somewhat like a very small Pterodroma, has long wings, a long deeply forked tail, is thick-set with a long and solid body, is overall sooty-black with bold upperwing-covert bars. Our bird was not a Tristram’s Storm-petrel.

Markham’s Storm-petrel Oceanodroma markhami
Experience RLF Many 100s seen from pelagic trips out of Lima, Peru almost annually since 2000.
Elimination Size alone excludes this species with Markham’s large compared to our Swinhoe’s (see Table 1).  Also, in contrast with our Swinhoe’s, Markham’s jizz is like a large kestrel, has very long wings, a very long deeply forked tail, and a long slim and lanky body. Our bird was not a Markham’s Storm-petrel.
    
Black Storm-petrel Oceanodroma melania
Experience RLF  Many 100s seen from pelagic trips on 13 days out of San Diego and Monterey, California September 2007 and September 2008; storm-petrels thought to be Black observed amongst Markham’s on a number of pelagic trips out of Lima, Peru since 2000.
Elimination Size alone excludes this species with Black large compared to our Swinhoe’s (see Table 1).  Also, in contrast with our Swinhoe’s, Black has a Black Tern-like jizz, long wings, a long deeply forked tail, a long and lanky body, is overall rich black-brown. Our bird was not a Black Storm-petrel.

Ashy Storm-petrel Oceanodroma homochroa
Experience RLF 1000s seen from pelagic trips on eight days out of Monterey, California September 2007 and September 2008.
Elimination In contrast with our Swinhoe’s, Ashy jizz is rather like a small kestrel, has much shorter wings that it tends to hold more swept back, has a long deeply forked and scooped tail, is quite chunky bodied, is overall ashy-black with distinct but greyish upperwing-covert bars. Our bird was not an Ashy Storm-petrel.

Least Storm-petrel Oceanodroma microsoma
Experience RLF About 20 over three days on five day pelagic trip San Diego, California September 2007 and September 2008.
Elimination Size alone excludes this species with Least very small compared to our Swinhoe’s (see Table 1).  Also, in contrast with our Swinhoe’s, Least has a rather bat-like jizz, tends to hold its wings more swept back, has a medium-short rounded wedge-shape tail, is overall sooty-black with indistinct upperwing-covert bars. Our bird was not a Least Storm-petrel.

Conclusion
In this note we have reported a sighting off Scilly on 21 July 2005 of what we are certain was a Swinhoe’s Storm-petrel. Swinhoe’s Storm-petrel breeds in the Northwest Pacific Ocean, but since 1983 records of Swinhoe’s in our region have come from Britain, France, Madeira, Norway and the Mediterranean coasts of Spain and Italy. There are four records for North Carolina, USA. Indeed, to date in the Atlantic and neighbouring seas 23 Swinhoe’s have been recorded most of which were trapped (several DNAd) with 18 accepted formally by national panels, plus five other well documented at sea records Also, Swinhoe’s has been recorded five times off Eilat, Israel. There have been several recaptures including the original 1983 bird on Selvagens 24 years later at the same sight! Swinhoe’s Storm-petrel routinely frequents the Atlantic and neighbouring seas, albeit in small numbers. Our record, then, is far from extraordinary, whereas a record in these seas of a wholly and unequivocally dark-rumped Leach’s or any other candidate all-dark storm-petrel would be unprecedented (save for one Matsudaira’s Storm-petrel off Cape Town, South Africa on 25 March 2002).

We believe that our identification of the Scilly Swinhoe’s Storm-petrel is sound based on our observations, our experience, our preparedness, and that there is adequate knowledge about the differences between Swinhoe’s Storm-petrel, Leach’s Storm-petrel, and other dark-rumped storm-petrels to facilitate at sea identification given adequate views (see Howell & Patteson, 2008).

References
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