I think this is a silly trick, based on the fact that 'orange' is both a fruit and a colour i.e. both a noun and an adjective. "An orange and black bird" is one thing - it's ONE bird which has both orange-coloured feathers and black-coloured feathers. "An orange and a black bird" are two things - a fruit and a bird. However, this sentence is quite contrived, as we don't usually say that fruit sits in trees, but rather that it hangs from them. Another interpretation is that there are two birds - "an orange (bird) and a black bird" - in the second sentence. However, that does not sound natural. We would not normally omit the first 'bird' in this sentence, especially given the ambiguity of the phrase 'an orange'. If you look carefully, this is actually an exercise on articles - it is only the 'a' before 'black' in the second sentence which tells you that this is a plural subject, and only the lack of article in the first which tells you that it is singular. It's still pretty silly and contrived, though.

In the first sentence it is an orange and black bird ---> the two colours of the bird --- hence 1 bird and "is" In the second sentence it describes 1 orange bird, and 1 black bird -- hence 2 birds in total

First sentence means that a bird with black and orange feather ,pay attention to there is one bird ,so chose "is" Second sentence means that a bird with orange feather and a bird with black feather ,pay attention to there are two birds ,so chose 'are'