XMI X-mini II Mini Speaker
I promptly bought one for use with my iPod Nano and found it to be so successful for attracting Lesser Redpolls and a range of other species, I bought another. I obtained a second iPod Nano cheaply on eBay. There are plenty of first-generation 1 GB Nanos available which have plenty of room for all the bird calls you are ever likely to need.
Both speaker systems pack tidily into the smallest kitchen box I could find