body {
  background-color: #ffffff;
}

.bird {
  position: relative;
  width: 100px;
  height: 100px;
  background-color: midnightblue;
  border-radius: 50%;
  animation: fly 2s infinite;
}

@keyframes fly {
  0% {
    transform: translate(0, 0);
  }
  25% {
    transform: translate(0, -20px);
  }
  50% {
    transform: translate(0, 0);
  }
  75% {
    transform: translate(0, 20px);
  }
  100% {
    transform: translate(0, 0);
  }
}