I know most folk don't need a reminder but it's more for my own peace of mind to prove I'm not going la-la after having a brief exchange with a bloke on another blog last week who posted photos showing all the features of a migrant labelled as common and then curtly dismissed mine (and another bloke's) polite comments pointing this out 'as he has seen hundreds'.
Afterwards he removed our comments then removed the comment function from his blog altogether; obviously doesn't like people disagreeing with him LoL
But back to the task in hand, if he or anyone needs to go through the pointers then here goes :
|Migrant (top) Common (bottom)|
If you can get them perched up ID should be pretty obvious. The bold yellow T-shape at the base of the abdomen is diagnostic on the Migrant, as is the yellow costa (front of wing) on the Common. Both of these features show well on a static insect. Also if you look at the front of the thorax, Common Hawker has bold yellow braces (antehumeral stripes) whereas the Migrant Hawker just has small pips or dashes there, often difficult to make out at all.
|Migrant (top) Common (bottom)|
The diagnostic abdominal pattern is unlikely to be seen in flight and even the yellow costa is not always clear, and in some cases the light causes the migrant costa to appear yellower than it actually is, so the standard ID rule applies that if you only think it's yellow it probably isn't.
On the side of the thorax both have two yellow stripes, the Common usually more distinctive, being better defined on a slightly darker body, and in between the stripes there is a single yellow dash near the top on the Common, and a line of two to three small yellow spots separate the stripes on the Migrant.
Side-on views of the abdomen also give a couple of clues. At the tail end of the Common from the tip there are three bold blue markings on the top-side before the first blue mark on the underside; the blue mark on the last segment of the Migrant is hardly visible so it appears there are only two bold marks on top before the first blue mark underneath.
Also (but not really apparent from my photos) the large blue patch on the underside of segment 3 of the abdomen is more or less horizontal, forming a rectangular shape on the thicker waist of the Migrant, whereas the blue mark slopes diagonally down forming a triangular shape on the thinner-waisted Common.
The point about the thickness of the waist is a good one actually, in silhouette from below the Migrant abdomen is more-or-less parallel all the way along, the Common abdomen is very thin near the base which is very obvious in an insect flying overhead.
Other more general clues on Jizz :
Away from the breeding areas Common Hawkers are usually solitary and often hunt along woodland rides or between avenues of trees. Migrant Hawkers are more likely found hawking woodland edges next to open land, and may be in small groups.
Common Hawkers are constantly on the move,erratic flight high and low and are nervy of people, usually moving away if approached.
Migrant Hawkers hunt at a much more even height and leisurely flight, often glding as they quarter over open land and will treat the observer as if they didn't exist, not shying away and coming very close if you're surrounded by midges.
But like I said at the start these are mainly generalisations and individuals don't always stick to the rules.
That's about all I can think of, I too have seen hundreds and it's not always clearcut, but should be given a decent enough photo, as the feature differences mentioned are pretty much standard.