I first heard shortly after publication of the 8th edition of the OSJ check-list (2012) that Japanese research indicated japonica should be elevated to a full species and this was reiterated earlier this year when I was reliably informed it will be reclassified when the next edition is eventually published (in 2020?). Recent research suggests it is closer to Oriental Skylark A.gularis than to Eurasian Skylark (Shiraki et al, 2014). Perhaps once it's formally split in Japan there will be more interest in field identification of the other taxa here. It is possible taxa in addition to the above occur, at least as vagrants. According to Zink et al (2008) deeply divergent eastern and western clades exist with Eurasian Skylark, perhaps warranting a further future split. Nail Moores (birdskorea.org.) has suggested intermedia should be included within Japanese Skylark rather than within the eastern clade of Eurasian.
In October birds are still in fresh plumage and at least on the islands there's a better chance of seeing more than just japonica providing an opportunity to look for possible features that might help to separate them in the field. There isn't a great deal of information available to that end and here I'm just posting some images of typical japonica and what appear to be continental birds. While separation of japonica from continental birds one thing, trying to identify which continental taxa may occur is even more difficult.
There's quite a lot of plumage variation within birds that seem to be japonica, for example the width of the pale inner webs of the scapulars or the predominently grey or brown lesser. Structurally, the bill can be slim and pointed giving a longish appearance or very deep-based and blunt-tipped and thus seeming rather short. I've posted more images of japonica in the post Japanese Skylarks in December
Oriental Skylark A.gulgula has a short primary projection with p6-9 forming the wing tip and the continental taxa reportedly have a longer projection than japonica so this might seem a good starting point. As tertials wear relatively quickly the projection may not be reliable later in the winter but I'd have thought the primary spacing should be unaffected. However, though most japonica are consistent in terms of spacing and projection, there is a degree of variance which may or may not indicate other taxa are involved. Some of the birds below I believe are continental but they differ from typical japonica in other respects.
Starting with japonica, the following six images are of two local birds in Osaka and Kyoto.
These two japonica look typical in all respects. The primary projection comprises of the tips of p8-9 falling almost together (p9 slightly longer) and p7 two thirds to three quarters of the way towards the tip. The whole is about equal in length to the exposed length of the longest tertial when unworn.
The two shots below of a strikingly pale individual show a very different wing tip. The primary projection is much longer than the visible length of the longest tertial, about 150% which is equal in length to the two longest tertials. Further, this is the only bird I've seen where p5 is visible beyond the tertials. The spacings between the other feathers are proportionately similar to japonica but stretched thus there should be a greater difference in relative feather lengths if measured in the hand. The bill is very deep-based and rather conical but so are some japonica. There is variation within japonica, some bills are no deeper than the eye and others significantly more so.
Below is another Hegurajima lark I saw in October 2007. It flew over in a party of larks that were obviously smaller and fortunately came down in the harbour area where the size difference was still clear. Japanese birders who came along called it O-hibari but in those days I think all continental birds were known by that name so it doesn't necessarily mean they thought it was pekinensis.
Presumed japonica (left) with the larger presumed continental taxon (right). Note the larger size and extensive breast and flank streaking. Also the prominent pale crescent behind the ear coverts usually shown by japonica. Hegurajima, 10 October 2007.
Primary projection longer than expected for japonica, flanks heavily streaked and lack of pale crescent surrounding ear coverts.