That's a very relevant question over here in Catalonia, as every winter large numbers of male Goldfinches are "legally" trapped. Along with 3 other species: Linnet, Greenfinch and Chaffinch. This is contrary to European legislation but is permitted because Catalonia's government makes use of an "exceptionality" clause due to the traditional nature of this activity. These birds are trapped for singing competitions of caged birds (I know, I've lived here for 20 years and I still don't get it), whereby "bird lovers" take their best singers to club meetings and competitions and sit watching their birds at a distance of some 20 metres or so. I've never been to such a meeting (I'd probably get an unpleasant reception) so I don't know if it's the quality or the duration of the song that counts.
The main problem with allowing such a practice (apart from perceptions of cruelty and its impact on the affected species' populations) is actually trying to police it. As I said, only males can be taken, and in the winter months from October onwards, with a permit. For one thing that means that rural protection agents have to spend a lot of their time checking on permits, quotas and if the "only male" rule is being respected. Over the last few years I have delivered an annual course to the rural guards of Catalonia to help them identify the 4 species concerned, to distinguish males from females and to identify other species of finch which could also be caught in the area.
In the case of the other 3 species differentiating males and females rarely presents a problem to the guards, but with Goldfinches it's a different story. Gentoo perfectly summed up the external features that can be used with a bird in the hands of a rural guard but even then there are many times where the sex of the bird in question still remains in doubt. Try it for yourself: download as many different photos of Goldfinch from the Internet as you can find and try to sound confident when you identify each one as a male or a female.
So the rural guards still have quite a headache, which can usually only be solved by applying more personal interpretations, ie. in case of doubt say it's a female and make the bird lover let the bird go, or else agree with the captor that the bird is a male and let him keep it.
Most of those so-called bird lovers rarely delve into the matter beyond the dicatates of popular tradition, often have no idea that the bird in question has a scientific name (like many birders it seems!), and usally employ quite subjective criteria for separating male and female goldfinches. Their attitude however can be very intimidating for a relatively inexperienced rural guard.
And then there is the proliferation of illegal practices associated with this activity, such as the use of bird glue, recorded bird calls, mutilation of released females so they can be recognised if recaptured, trapping without permits, not respecting quotas, etc.
Judge for yourselves.
Steve
http://www.BirdingInSpain.com