Will use it mainly as a sound amplifier (not as a recorder) together with a Zoom Hno or Android phone. Alternatives that are good and worth the price?
The Birdmic is just a small parabola with an interface for a phone, which you use as the recorder - on this basis it would work with your Android phone. I think it would also work with any other recorder with a 3.5mm minijack input, but I am not sure if the device has it's own battery or you can use PiP from the recorder.
Another option is the Dodotronics Hi-Sound Compact, although they seem to have withdrawn the old phone interface, so you can only use it with a recorder with a stereo PiP 3.5mm input socket. I think they are bringing out a new interface that allows you to split the signal, so you can monitor the sound through headphones at the same time as using Merlin on your phone - I believe it was previously either, or.
Getting technical, but the gain of a parabola is related to (Dish Diameter/Wavelength)^2. This means that there is no gain where the wavelength exceeds the dish diameter - the bigger the dish, the lower this lower frequency threshold is. Also, as gain is related to D^2, a relatively small increase in dish size makes a big difference in gain (a dish with a diameter 1.4x as big as the Birdmic will create circa 2x the gain - all things being equal).
This is why there are larger parabolas (see Telinga and Dodotronics), although they tend to be expensive.
I have a Hi-Sound Compact, that I use for holidays as it is easy to carry, and think it is pretty good - the mics are super sensitive in any case, and for higher pitched birds the dish creates a nice bit of extra gain.