gor two birdy bikes, one from the 60ies one from the 80ies, a good old fashioned mountainbike i don't ride that often
and an old atala racer i turned into a singlespeed.
to that guy who said he ain't got enough money for a singlespeed, just do everything by yourself. a bike isn't that complicated, just check some youtube tutorials. i found the race frame in the dumpster, just start from there. the entire bike didn't cost more than 150 euros.
i highly recommend DIY, otherwise you could buy a heavy prebuilt chinese complete bike for that same money, but your bike will weight 25kilos. and look like it's from wallmart.
and to herculessrockefeller: that's not a singlespeed, that's omas r?cktrittkraxn, auch wenn puch oben steht.