Hi Sejer, I would collect the entire wing- all the primaries and all the secondaries; just incase you need them. Or... if someone else needs them in the future.

Not sure about the timing a flight feather replacement in Sharp-tailed Grouse, but I would read the work Pyle cites in his sharp-tailed account. For the adults, you may need to learn how to spot moult limits, or the newly replaced feather along side older feathers. If you don't have access to a ornithological museum collection, this site maybe useful:http://digitalcollections.pugetsound.edu/cdm/landingpage/collection/slaterwing just keep looking at wings until you start to see moult limits.

If you can separate the young of the year form the adults, all of the young bird's primaries all grew in at the same time, so P1's isotope signature will be the same as P5 and S4. But it takes a few weeks for the feathers to become full size, and the bird diet may change during the feather's development. So you could collect a primary and cut the feathers perpendicular to the shaft in 2 mm strips, analysis each strip for isotopes. Compare isotope signatures through the growth of the feather, and see if the diet changes in diet- if you need a plan "B"

Hope that helps,

Paul

Thanks Paul, great tips, and that site looks like a very useful resource!

One part of Paul's answer should be clarified: Most grouse will moult primaries 1-8 in the first fall, but retain primaries 9 and 10 until the following summer. This is usually visible by comparing wear on the feather tips (P9-10 usually more worn and sometimes faded) and P9-10 will usually have smaller calamus diameter than P8.