Translation:There are six birds in the tree.
Great example of the difference in space representation for English and Chinese. I've talked to Chinese speakers who find it quite curious that English represents this scenario as if the birds were inside (i.e. embedded in) the tree; for English speakers looking at the Chinese, I guess our first guess may be that the birds are at the top/apex of the tree.
Well, this makes it even more difficult for non-native English speakers.
In German, for example, the birds would also be auf dem Baum lit. ‘on the tree‘.
It's quite unintuitive having to translate the Chinese sentence that works much like it would in my native language differently to English.
It might be a different conceptualization of "tree." I.e., when we think of "tree" in English, we're focusing on the whole as one enfolding shape--not just the trunk but also the foliage. To be within the extension of tree branches is to be "in the tree," same as to be "in the bushes." So the birds are "in the tree" because they are within the spatial parameters of tree-ness.
Even more interesting (to me), the birds are "in" the tree and if you threw a shoe or something and it got stuck, it would be "in" the tree too. But if the tree is an apple tree and apples are growing, I'd say there are apples "on" the tree. And of course the leaves are "on" the tree too.
While both can be defined as "tree" in dictionaries, on its own 木 usually means wood(en). When it's used in a compound (with another character) 木 can mean tree though. This page has some helpful info:
Just add a little to the above replies, as a native Chinese(Cantonese) speaker, learning Japanese, I also want to say that it's not a good idea to learn both Chinese and Japanese at the same time, if you have similar levels of them. Only wait until you have at least intermediate level of one of them to start the other. Japanese kanji has their source from (traditional) Chinese, and some of them share the same meanings, but some others has different meanings, usage and characters(how you write them), so beginners can easily get confused(even for me I get confused sometimes). In this example, in Japanese, both 木 and 樹(in its form of traditional Chinese and Japanese kanji, the simplified one looks slightly different ) means trees and use the same hiragana 'ki'. Whereas in Chinese, though 樹 and 樹木 both means trees, 木 standing alone means wood/log.