I stayed on Skopelos in May 2002 and it wasn't bad at all. Also made an overnight trip to Alonnisos.
There's a small creek on Skopelos town beach worth visiting , i went daily and saw Broad billed,Marsh and Curlew sandpiper, Little stint . Walking around the villages brought Black headed bunting, RF Falcon, Lesser Kestrel, Bonellis & Golden Eagle. Also Eleonoras Falcon, a couple of Audouins gulls, Yelkouan Shearwaters, Orphean, Bonellis, Subalpine, Sardinian warbler, and a fair few others.
Ah, good holiday that, the local ouzo was good stuff, music (Rembetika) and the walking and swimming were superb. Not sure how Mamma Mia may have affected things but the locals reckoned April/ May were the best time to go before the summer influx from Italy and Athens.