Lack of DNA differences mean free DNA flow between three bar-less crossbills (common, scottish and parrot) and they are one species.
Did you really just say that ? Believe me they are NOT acting as 'one species' ! They have completely different morphology that allow for ecological specialisation on different food sources - think Darwin's finches on the same islands never mind across the whole of the Western Palearctic !
"Difference" in calls and "assortative mating" observed can be easily explained:
- differences in both bill size and calls are minute and prone to error (we talk about milimeter-two differences measured by hand on living bird. And call differences imperceptible by ear and judged by eye on sonograms).
Difference in bill size between Common, Scottish and Parror Crossbills is anything but minute. Between the curvirostra 'complex' maybe. Regarding error in measuring Marquiss and Rae solved this by only one ringer doing all the measurements in a 10 year study " The mean difference between consecutive measurements for 23 full-grown birds was 0.10mm ie. 0.9% of mean bill depths". That is not "prone to error". I would also add that having caught and handled all three species (!) they are different not just in bill morphology but also wing length and body size - these birds are not the same species. Regarding sonograms I have gone into this on other threads - I use my ears as well and don't just "judge by eye", that would be lazy.
- sample size was small (my apologies to field researchers, I know that crossbills are very difficult to follow),
The Dutch apparently have a paper coming out with a sample of over 500 curvirostra, biometrics and calls ( I think it is available to preview ? ). Marquiss and Rae caught 437 Crossbill in Deeside. Small sample size - maybe, given there three (4?) types, but still valid don't you think ?
- "assortative mating" is well known within species (lots of ecological races like tree-breeding and cliff-breeding peregrines, even city pigeons tend to pair with lookalikes to some extent).
Well if they are all one species as you say, in my patch there sure as heck is a helluva lot of really big billed ones that tend to stay on one pinewood territory and breed in the same area with birds that give the same calls and have the same bill morphology. Oh, and their progeny look and sound the same. This is all as a result of an ecological factor.
- there is no indication that observed mating pattern is constant (what happens e.g. in years of poor seed crop or during long waderings)?
Common Crossbills tend to form flocks with birds that give the same call. There may be two main call types present in a large group of birds, but they are in groups with individuals that share the same characteristics (pers. obs). I can validate this by having spent time catching them at drinking sites. You catch 10 birds and they tend to be one type. An hour later you catch another 8, a different type. The same BTW happens with Scottish and Parrots. They are like women - they all go for a piss together, or in the crossbills case a drink ! They are acting as groups suggesting they have bonds within those groups.
- if they were species, we should observe pattern NOT strictly following bill size: large-billed and small-billed individuals of one species would prefer each other against other species apporaching it them in bill size.
Bill size seems to be important, though an optimal bill is not necessarily the biggest one. Your argument only holds any water IMO where there is a 'clinal ' or intermediate form such as Scottish Crossbill, which is somewhere between Common and Parrot in morphology, the latter two isolated by niche preference. Also, a small billed Parrot at say 11.9mm is a very different looking bird from a big Common at 11.5mm so can't see them pairing.
- calls were not studied to be inherited and constant. In fact, many finches are known call learners.
Calls can be learned, no one is denying that. But juvenile crossbills learn the calls of their parents who mostly happen to share a distinct 'call type'. I have seen call convergence several times in crossbill pairs and never has it IMO been a Parrot trying to sound Scottish or vice versa. Rather, it is a pair bonding and trying to sychronize their own particular calls within the parameters of their own 'type'.
Not meaning to single you out, it's just you had said a lot of stuff I felt was , in my opinion and experience, misinformed.
Lindsay