So on Friday September 20th, I received a call from Curtis Dykstra that a darkish juvenile jaeger was sitting on the beach at Holland SP, and that it had rounded central rectrices. I even received this photo (copyright Judy Manning) confirming this, in addition to the testimonies of multiple people standing at close range with scopes:
This photo shows clearly rounded central rectrices (=rectrix 1, aka. 'R1'). [Aside: the bird actually only has one of its two R1s, (see below)]. I mean, does anyone looking at this photo think the R1s are pointed? (but seen the detailed analysis below). So, according to my understanding, this eliminated a juvenile Parasitic Jaeger (PAJA) from the picture, securing the bird as either Long-tailed (LTJA) or Pomarine (POJA), both of which are state birds for me. So the 1.5 hour one way chase was on. Here are my photos from my 2 hour visit, with my full ID analysis embedded in no particular order:
Here is Pyle's treatment of uppertail coverts in juvenile jaegers, this bird clearly matching PAJA and being wrong for LTJA or POJA.
Compare to this juv LTJA from Oregon, which shows the typical pattern of a thinner breast band not extending this far down.
BIRD 1, BIRD 2, BIRD 3.
and Kaufman's illustration of these traits:
This trait really is notibeable and very useful for ID if seen well, at least on many individuals. Another way of putting this is that the Holland bird's wings are wider than the amount of bird behind them, as visible here:
I really believe that given good views, that these two traits can be accurately judged on most or all individuals, given sufficient looks. And both of these heavily favor PAJA on the Holland bird. As of the time of my field observation I was not aware of how to apply these traits.
And Kaufman's illustration of all three juveniles:
Finally, here is the R1 of the Holland jaeger after it was found dead, taken in-hand:
Here is my best interpretation of this field mark for the Holland bird. First, if you were to judge this bird by Kaufman's illustration, you would have to say it best matches the shape of LTJA. If you judge it by Pyle's illustration, it is close to choice C in Fig. 519 (PAJA), but not as pointy. It more closely matches choices E or F in Fig. 519, which are LTJA. Also note how Kaufman illustrates juv. POJA as having slight pointed tips at the rachis, but in that instance the feather appears rectangular and flat-tipped, but with the slight point. I don't think that is a good match for our bird.
Another thing which hasn't yet been mentioned, is that there is no guarantee this feather is juvenal. If in fact the juvenal feather was lost (perhaps with the other R1 in whatever incident caused it to lose the first one), and this is in fact formative (vernacular: "first winter"), then it is a very close match to choice C in Fig. 517 (POJA!). Since Pyle doesn't illustrate R1s in LTJA across ages, I am not sure whether this bird also matches that species in formative plumage. Clearly, this field mark is greatly oversimplified by field guides, and it needs to be applied extremely gingerly!
The take home for me is as follows (though as stated earlier, all of this is subject to correction and I welcome everyone's thoughts on where I am getting it wrong):
1) jaeger R1 tip shapes are a morass of variability which are oversimplified by the literature.
2) obviously pointed R1s, as long as one is CERTAIN that they are juvenal feathers and not formative feathers, do appear to heavily favor PAJA (presumably diagnostic, but who knows?)
3) slightly pointed R1s like in the Holland bird are not distinguishable from truly rounded ones under normal field conditions, even in photographs (!!)
4) b/c of #3, rounded-appearing juv. R1s are **NOT USEFUL** for identification of juvenile jaegers!
Item 4 is a total revelation for me. I have always thought that a good look is plenty to establish a rounded R1, and that this was diagnostic of either LTJA or POJA, but I no longer believe it. I even wonder if this field mark has any value? If anyone takes issue with this conclusion, please post your comment below. I love being proved wrong, as I think it is where the most progress in birding is made. All I care about here is getting the ID right, not being right up front.
Now let's get to the issue of bill proportions. Here is how Kaufman illustrates this (apparently diagnostic?) trait:
The ratio appears to be about 44-45%, as calculated by using the pixel measuring tool in Photoshop. Here is an overlay of the Kaufman figure with both shots.
My judgment of this trait is that the Holland bird is closest to LTJA, but that it may be inermediate and not helpful for ID. But at the very least I would argue that this trait does not favor PAJA. Some have also mentioned the position of the gonys (the angle on the lower mandible) is supportive of PAJA, but compare it to Kaufman's illustration. If anything this also seems to either favor LTJA or perhaps be intermediate. Since I believe the Holland bird to be a PAJA, the take home would be:
1) nail to bill ratios are variable, not static, as illustrated (there is apparently some age-related variation as well, see Pyle)
2) this field mark is supporting, never diagnostic (except perhaps at the extremes?)
3) the Holland bird's bill ratio is indeterminate, but not inconsistent with LTJA
I think that's enough for a first volley! I am comfortable calling this bird a juv. PAJA on the LTJA end of the spectrum. So far, no measurements or DNA analysis have been reported from the specimen. Hopefully we will get this confirmation at some point. I look forward to alternate views (corrections?) to those I've put forth, and I want to thank Curtis Dykstra, Carl and Judy Manning, Mike and Jeremy Overway for finding the bird and spreading the word, to Tim Baerwald for his quick correction and detailed laundry list of field marks supporting PAJA (extremely helpful), Josh Kamp and Marc North for asking why this wasn't a PAJA, and Adam Byrne, Brad Murphy, Scott Terry, and Joe Kaplan for their detailed analyses of this very instructive bird.
PS. here are the in-hand shots I later received of the specimen