I always try to use seasonal produce because birds are all seasonal eaters and, although the plant material we can offer is, of course, not the same as what they would get in their natural habitat, eating seasonally is part of their circannual cycle and I do believe it helps -somehow- their internal clock to correctly set to the season. But, please, taken into consideration that this is a list for humans and not for birds so, for example, avocado is a no-no, same as onions and green onions and there is stuff there that should be given VERY sparingly because of their high oxalic acid or iron (like spinach, collard greens, apricots, etc)
https://www.thespruceeats.com/summer-fr ... es-2217706