Our Feeding Stations :):)

Blackbirds (and thrushes, and all other songbirds) *have* to learn their songs from other individuals. While starlings, among others, are famous for mimicking other species, occasional mimicry is recorded from many species. One article I read a while ago (Helb et al, 1985, Zeit. Tierpsychol.) did a review and found really that most species learn their songs from the wrong species occasionally (up to 1% of the time, mostly). Blackbirds have been recorded as mimicking articifial sounds like car alarms, so it is not implausible that they could mimic thrushes.

Why do birds do this? For species like blackbirds, nobody really knows, to be honest: because it is a rare phenomenon, it is difficult to collect much data. Regular mimics like starlings tend to pick up surprising sounds in their environment. In this case, the theory is that females are also attracted by weird sounds, and the males that sing them get laid more frequently. On the other hand, if you sing a song that just sounds like a car alarm, well the birds aren't trying to attract car alarms! Mimicking birds still have to make sure that females of their own species recognize them, particularly since song is often the first point of contact between male and female. So there is this balance between incorporating weird new sounds, but making sure that you are still recognized. Regular mimics have tricks to manage this: starlings put their mimicked sounds in between segments that are very stereotypical and starling-like. Perhaps your blackbird decided that a thrush song was blackbirdy enough to suit his needs.

Blackbird songs normally consist of a combination of loud fluty whistled notes at the beginning, followed by a quieter twitter at the end. The current thinking is that the quiet twitter part at the end is actually the really aggressive part. Current research is looking at the idea that blackbirds sing different songs that are more or less aggressive by varying how much twitter there is in their songs. So maybe the songs you were hearing, which presumably lacked the twitter part were part of the bird's less aggressive arsenal of songs, which he had picked up from a song thrush.

Or not...