Gisborne pelagic (Oct 21) addendum: Manx Shearwater

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Re: Gisborne pelagic (Oct 21) addendum: Manx Shearwater

Postby Matthias » Sat Nov 11, 2017 9:25 pm

Hi Sav,

the bird approached the boat from from 5 o'clock and went out of sight only a few seconds later at about 2 o'clock. When I first spotted it without binoculars, it looked clearly different from Fluttering or Hutton's Shearwater (which we had also seen in small numbers earlier on the day). It had stronger wing beats and because of the bright underside and the contrast between upper and underparts my first thought was that it could maybe be a gadfly petrel (maybe because I was specifically on the lookout for these). When I took the photos, it was clear that it was a shearwater. Nevertheless, the quick glimpse on the camera screen afterwards left me puzzled because it showed a bird that looked superficially like a Fluttering Shearwater, but I had expected to see something different. Regrettably, none of us had Manx on their radars, so we left the ID as "Fluttering Shearwater". I did not look at the photos again until I returned home. As soon as saw the photos on the computer screen, I noticed that this bird looked different from Fluttering and Hutton's Shearwater.

Ian Southey
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Re: Gisborne pelagic (Oct 21) addendum: Manx Shearwater

Postby Ian Southey » Sun Nov 12, 2017 7:44 am

I don't know what this bird is but have a few general remarks.

Normal variation in common species - There are tens, if not hundreds of thousands of Fluttering Shearwaters in New Zealand every year so it is perhaps more likely that an unusual bird is an extreme part of the normal variation or some rare colour variant than such an extreme vagrant. The quickest check of variation is to look at the pictures in ... shearwater You can quickly see that there is variation on how white or dusky they are underneath and there is a photo of a bird from Burgess Island with an underwing pattern that approaches the bird in these photographs.

Regarding moult - this bird does appear to be in early primary moult. The holes in the outer primaries indicate that it is either in need of a good preen or these are juvenile flight feathers (not as strong) that need replacing. Looking up HANZAB (from nzbirdsonline) I see that Fluttering Shearwaters have been recorded with post juvenile moult of primaries from November and this isn't very much earlier. These pictures may be of a northern hemisphere adult, or a local first year bird.

Regarding lighting - If you look at the shadow cast by the leading edge of the wing it seems that the sun was almost directly overhead and slightly in front of this bird with light just glancing off the face so any surface irregularities will shade large areas. Thinking of the contours of the face in general - the crown bulges over the eyes a bit, the eyes are set a little deeper back, and the cheeks begin to puff out a little. I think the dark spot behind the eye is only shadow and I wonder if the light area on the cheek may be due to stronger reflection of light hitting more directly and angling it back toward the observer. I'm not sure that it is distinct enough to mean a great deal but if people pay attention to how these Fluttering Shearwaters look in similar positions it may be possible to come back to this bird.

I did not see this bird but I have seen Fluttering Shearwaters in a variety of situations and they can be over the shelf edge. The way they fly depends on the wind and maybe what they are doing at the time.

But I still can't say for sure just what this bird is.

Graeme T
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Re: Gisborne pelagic (Oct 21) addendum: Manx Shearwater

Postby Graeme T » Mon Nov 13, 2017 8:05 pm

Hi Matthias and Russ
Thanks for posting this interesting bird. Shows how difficult it is to identify birds at sea even when you have good photos to admire. The bird in question is not a Little shearwater. They have much shorter wings. The identification of it being a Manx seems based mainly around the white axillaries and the moulting of the primaries. There are some problems for the moult theory. Petrels and shearwaters have 10 primaries and they moult sequentially from the inner primary P1 to outer primary P10. Often in groups of 2-3 feathers. But a key point is primary moult occurs evenly on both wings. This bird does not have matching wing moult. There is no evidence of moult on the right wing (all secondaries are present and so are the primaries). The left wing appears to have primary gaps that are not sequential. Either because its lost some feathers randomly or they are tucked in some how (less likely). So I don't consider this to be a bird moulting in the non-breeding season. Secondly the extent of white in the underwing is quite variable in fluttering shearwaters. I have seen photos of flocks of both Hutton's and fluttering shearwaters where some birds seem to have totally white underwings. It may be related to bright sunshine "bleaching" the feathers in the photos or they are genuinely white. Either way I don't think its overly reliable. Even in the hand its fairly variable. Manx shearwaters in October would be much browner than birds in fresh plumage as the dark feathers fade to brown with wear (happens in all Procellariiformes). Manx shearwaters have long bills like Hutton's and also long wings as they are migrants. This bird does seem to have quite long wings. I can't totally eliminate this bird being a Manx but I would not be surprised either if it was a fluttering shearwater based on variability in plumage in that species. Cheers Graeme
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Re: Gisborne pelagic (Oct 21) addendum: Manx Shearwater

Postby RussCannings » Mon Nov 13, 2017 10:19 pm

Thanks so much for this great feedback-- some very valid points coming forward. Graeme--It's tough to say for sure but I'd have to disagree with your moult assessment. To me the inner primary moult appears to be symmetrical (See first photo) with those ragged outer primaries on the left wing a feature of heavy wear, not moult (i.e. This bird is not in the relatively fresh plumage of resident Fluttering).

I absolutely take the point that a 'weird Fluttering' would be much more likely than a vagrant. But it's the combination of all features that has me still leaning toward Manx (i.e. Facial pattern, underwing, moult). That said I won't lose any sleep if enough doubt is cast. Sav is absolutely right that some birds just can't be IDed to satisfaction.

Cheers all!

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Re: Gisborne pelagic (Oct 21) addendum: Manx Shearwater

Postby Matthias » Sun Nov 19, 2017 1:35 am

Thanks for the feedback! Given the rarity of the sighting, it is important to discuss other species. I also appreciate that some people apparently take the position of "Devil's Advocate" proposing some bolder hypotheses. It seems, however, that in the discussion there is a mix of important facts, less important facts, and also a dash of personal opinion. In the following, I will try to sort the information brought forward so far.

1) Neutral traits

- width of trailing edge
- thickness of neck

These traits are not relevant for the ID of the Gisborne bird as either Manx or Fluttering Shearwater. Most importantly: the Gisborne bird is well within the range for Manx regarding these traits.

2) Important traits

- white underwing
- moult

Manx Shearwater has very bright underwings, especially the wingpit is supposed to be clean white. The Gisborne bird shows such bright underwings. It is an important trait for the ID (a prerequisite) and maybe one of the first traits to catch someone's attention. If the Gisborne had dark wingpits, it could not be a Manx. If it has a clean white underwing, it can be a Manx. That does not mean that Manx Shearwater is the only bird species that has bright underwings nor that the presence of a white underwing in another species proves that the Gisborne bird is not a Manx. It is known that in some Fluttering Shearwaters the underwings can appear white in strong sunlight (or if photographed with a flashlight, like the bird from Burgess Island on nzbirdsonline). However, the white underwing alone is not relevant for the identification of the Gisborne bird. Manx Shearwater cannot be ruled out only because some Fluttering Shearwaters can also (appear to) have bright underwings. There are more traits in the wing of the Gisborne bird that are relevant for the ID: the relative length and the dark leading edge. Fluttering Shearwaters with a bright underwing do not show a dark leading edge. They also do not show the fine lining along the wingpit shown by the Gisborne bird. The wing of the Fluttering Shearwater from Burgess Island does not resemble the wing of the Gisborne bird at all. The wing of the Gisborne bird fits perfectly for Manx. It is not in accord with Fluttering Shearwater.

Moult can be another potential important factor for the ID. The Gisborne bird is moulting the primaries. Manx is supposed to moult its primaries on the wintering grounds (from September/October onwards). The timing of moult is therefore perfect for Manx.
Adult Fluttering Shearwaters moult their primaries in the wintering grounds from February onwards, juvenile birds moult their primaries from November onwards. The Gisborne bird would therefore be slightly outside the known range. However, moulting juvenile birds of Fluttering Shearwater have never been recorded in New Zealand before December. The Gisborne bird hence would be outside the known range. Based on moult alone (i.e. ignoring the important traits!), it is not impossible but very unlikely that the Gisborne is a juvenile Fluttering Shearwater.

The ID of the Gisborne bird as a Manx is not based mainly around the white underwing and the moult. These are just characteristics that are in accord with Manx. The ID of the Gisborne bird as a Manx is based around the traits that are known to differ between Manx and Fluttering (discussed in HANZAB) and that I stated in my original post:

3) Critical traits to distinguish Manx Shearwater from Fluttering Shearwater.

- in Manx, the black of the cap extends only just below eye level and is sharply delimited from white cheeks.
The Gisborne bird has such a dark cap that is sharply delimited from white cheeks and throat.

- in Manx, the dark patches on the sides of the upper neck and breast do not join in midline of the throat or foreneck. They do in most (but not all) Fluttering Shearwaters.
The Gisborne bird has a very clean white throat and lower neck.

- Manx has a white crescent behind the ear-coverts. It is absent in Fluttering and Hutton's.
The Gisborne bird has a white crescent behind the ear-coverts.

- the dark leading edge of the underwing in Manx is prominent and more sharply defined than in Fluttering Shearwater
The Gisborne bird has a prominent and sharply-defined dark leading edge.

- Manx appears more distinctly black-and-white than Fluttering Shearwater in fresh plumage
This is not considered to be a definite trait because moulting birds can appear brown. The plumage of moulting Manx Shearwaters can be much browner than the plumage of the Gisborne bird.

- Manx has, typically, less white on side of flanks.
The Gisborne bird has white flanks but it is not clear how far they extend onto the back. The white flanks of the Gisborne bird are within the range for Manx.

- Manx has relatively longer wings than Fluttering Shearwater
The Gisborne bird has very long wings.

In conclusion, there is not a single characteristic in the Gisborne bird that would exclude Manx. All the field marks of the Gisborne bird are within the range, most of them are perfect for Manx. In contrast, most characteristics--especially all the critical ones that are used to distinguish Manx from Fluttering Shearwater--exclude Fluttering Shearwater. The combination of field marks of the Gisborne bird is outside of the range for Fluttering Shearwater. It would make sense to discuss the possibility of the Gisborne bird being a "weird" Fluttering Shearwater if some of the traits were in favour of Manx, and some were in favour of Fluttering. It does not make sense if all the important traits are clearly in favour of Manx.

The only issue with this record is that Manx is supposed to be "rare". However, there are three documented records of Manx Shearwater in New Zealand. Hence, the species is known to reach New Zealand waters, and it was only a matter of time until we would observe the first live bird. Sightings are becoming even more likely now that there is a growing population in the Northern Pacific. In addition, it is very likely that Manx Shearwater has been overlooked in the past. In the past year alone, I have provided the first documented live records (outside of wreck events) from NZ mainland waters for Antarctic Prion, Salvin's Prion, and Blue Petrel, and the second live record for NZ for Tasmanian Mollymawk. All these species have in common that they are difficult to distinguish from similar but more common species in the field, and are therefore probably mostly ignored. Manx Shearwater is another such a species. I am convinced that some of these species are more common in NZ waters than we think, and that there will be more records of these species if people (1) familiarize themselves with these species and, most importantly, (2) go out more often and look specifically for them.


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