One of Britain's best known birds, the cuckoo, has been placed on a list of the most endangered animals in the UK.
It's one of 52 species whose numbers have dropped so far they have been put on the so-called "red list" by a group of conservation groups.
Other new entries on the danger list include the herring gull, the lapwing and the arctic skua.
The last time bird species' numbers were assessed for the list was in 2002, when 40 species were red-listed.
It's not clear what has caused so many birds to become endangered.
Some people think it might be climate change, while others think it could have something to do with habitat loss.
And because lots of birds spend their summers and winters in different countries, they might be twice as at risk as those that only live in one.