Hi Mike! The status, phenology and abundance of the different subalpine forms I've registered while monitoring migration during the 5 Spring seasons I've spent in Cabrera is resumed below:
Sylvia cantillans cantillans is a frequent and regular migrant, first scarce sightings at the end of March, fairly common through April, especially on the first 3 weeks of the month. Scarce again in May.
Sylvia (cantillans) moltonii. Fairly scarce breeder and moderate migrant. First birds around mid April, much commoner on the last week of April and especially first week of May.
Sylvia cantillans albistriata. Very rare migrant. Four records in the last 5 years: 1 on March 27th, 1 on April 10th, and 2 on April 17th.
I guess phenology should be about the same in mainland Mallorca, with the main arrival of Moltoni's at the end of April and through most of May, thus more or less replacing the earlier migrant ssp cantillans. Moltoni's are not easy to find as breeders in Mallorca, they are fairly scarce and localised, breeding mainly in the Serra de Tramuntana and in the Llevant mountains, but can turn up as (scarce) migrants almost anywhere.
Cheers,
Eduardo