Like Kanchii said, it doesn't necessarily apply just to cold weather. Here is a definition:
har·dy
1. Being in robust and sturdy good health.
2. Courageous; intrepid.
3. Brazenly daring; audacious.
4. Capable of surviving unfavorable conditions, such as cold weather or lack of moisture. Used especially of cultivated plants.
So it basically means that a hardy bird will do will in lots of different conditions and won't die at any little unfavorable thing. A non-hardy bird will be more likely to die in poor conditions whether it's cold, heat, wet, etc.
har·dy
1. Being in robust and sturdy good health.
2. Courageous; intrepid.
3. Brazenly daring; audacious.
4. Capable of surviving unfavorable conditions, such as cold weather or lack of moisture. Used especially of cultivated plants.
So it basically means that a hardy bird will do will in lots of different conditions and won't die at any little unfavorable thing. A non-hardy bird will be more likely to die in poor conditions whether it's cold, heat, wet, etc.
