It isn't open source but it will remain free. There is a similar Android app called Birds Around Me [1] that is very similar. I have no plans to implement on other devices. This is just a hobby. Ironically found after I built most of this app.

[1]: https://play.google.com/store/apps/details?id=com.sobremesa....

One of my favourite projects to work on was with inaturalist.org . It's open sourced on github (https://github.com/inaturalist) .. we were making the New Zealand port : http://naturewatch.org.nz/

Has Android & iOS ports..