Bard and his bird-speaking ability
It was not mentioned in the movie; however, in the novel, it was stated that because of Girion, Bard is capable of speaking to the avian species. However, this ability is only extended towards the birds of the Thrush family. This ability is demonstrated through this passage:
( When Bilbo is frustrated he throws a stone at an old thrush (the same one that speaks to Bard) and Thorin admonishes him: )
“Leave him alone!” said Thorin. “The thrushes are good and friendly-this is a very old bird indeed, and is maybe the last left of the ancient breed that used to live about here, tame to the hands of my father and grandfather. They were a long-lived and magical race, and this might even be one of those that were alive then, a couple of hundreds years or more ago. The Men of Dale used to have the trick of understanding their language, and used them for messengers to fly to the Men of the Lake and elsewhere.”
This was to establish that Bard was truly the heir of Girion, for the Men of Dale were capable of speaking to the bird-folks. That being said: Bard has found companions within a family of Thrush that reside near where he gets the barrels near the River Running.
IN CONCLUSION: Bard can talk to thrushes && he loves birds because he can understand them.