The form of the verb होना should be हैं not है since the subject, पक्षी, is plural.
Non-native broken hindi speaker here. Totally agree it should be a हैं.
In addition, why is it not पक्षियों ? This would make the sentence: "आसमान में पक्षियों हैं". Is there anything wrong with that? Is पक्षी also a plural?
Agree, hain. (Psst DL team... there's no button for us to tell you about this - it's missing on many sentences. That's why people are writing about sentence mistakes here instead of telling you straight up, just in case you didn't know.)