These special competitions can last for a day or two, sometimes even a week or more, so it's best to check the left side of the main screen to see all the currently open events you can participate in. Additionally, by tapping the Events button, you will open a list of events you can join, with information on the remaining time, a description of the difficulty, and your progress.
The events in The Birders differ in objectives, variety, and themes, so they are selected for birdwatchers based on their game level. For this reason, sometimes you may not be able to participate in the same events as your friends.