The player plays as a bird and can either play the normal game or the study game. Both game modes are nearly identical except the study game only allows the player to play through one level of their choice before abruptly ending. The player must feed butterflies to the baby birds so that they can grow big and eventually leave the nest. It is suggested that they eventually become the "new mother birds" that take care of their offspring in the subsequent levels. There is an ending to this game. However, reaching the end requires considerable patience because it only comes out at the end of level 999 (approximately 28 repetitions of the game's thirty-six levels).
Each level represents a season in the ecosystem of a bird. The game starts out in early spring. As the virtual year progresses, the season evolves into summer and eventually into autumn. After autumn, the game repeats itself by portraying the following spring. Since harmful influences from sentient beings are not simulated in the game play, the forest that the birds live in is presumed to be a nature preserve. If the proper amount of butterflies are not fed to the babies, then the babies end up starving to death. The player will automatically lose a life if any of the baby bird dies. In addition to this, the player also loses a life when a predator catches the player trying to deliver butterflies to whose baby.
Although the game is titled "Bird Week," the flow of seasons make it seem like virtual years are passing by (as opposed to virtual days of a week). There is no winter in the game because many of the game's enemies are that of the insect, bird, and small mammal species.
Source: Wikipedia, "Bird Week", available under the CC-BY-SA License.