<div class="container">
  
  <h1>SVG Animated Birds</h1>
  
  <div class="bird-container bird-container--one">
    <div class="bird bird--one"></div>
  </div>
  
  <div class="bird-container bird-container--two">
    <div class="bird bird--two"></div>
  </div>
  
  <div class="bird-container bird-container--three">
    <div class="bird bird--three"></div>
  </div>
  
  <div class="bird-container bird-container--four">
    <div class="bird bird--four"></div>
  </div>
  
</div>

@import url('https://fonts.googleapis.com/css?family=Arima+Madurai:300');

*,
*::before,
*::after {
  box-sizing: border-box;
}

h1 {
  font-family: 'Arima Madurai', cursive;
  color: black;
  font-size: 4rem;
  letter-spacing: -3px;
  text-shadow: 0px 1px 1px rgba(255,255,255,0.6);
  position: relative;
  z-index: 3;
}

.container {
  z-index: 1;
  position: relative;
  overflow: hidden;
  display: flex;
  align-items: center;
  justify-content: center;
  // min-height: 100vh;
  min-height: 35rem;
  background-image: linear-gradient(to bottom,  rgba(255,168,76,0.6) 0%,rgba(255,123,13,0.6) 100%), url('https://images.unsplash.com/photo-1446824505046-e43605ffb17f');
  background-blend-mode: soft-light;
  background-size: cover;
  background-position: center center;
  padding: 2rem;
}

.bird {
  background-image: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/174479/bird-cells-new.svg);
  background-size: auto 100%;
  width: 88px;
  height: 125px;
  will-change: background-position;
  
  animation-name: fly-cycle;
  animation-timing-function: steps(10);
  animation-iteration-count: infinite;

  &--one {
    animation-duration: 1s;
    animation-delay: -0.5s;   
  }
  
  &--two {
    animation-duration: 0.9s;
    animation-delay: -0.75s;
  }
  
  &--three {
    animation-duration: 1.25s;
    animation-delay: -0.25s;
  }
  
  &--four {
    animation-duration: 1.1s;
    animation-delay: -0.5s;
  }

}

.bird-container {
  position: absolute;
  top: 20%;
  left: -10%;
  transform: scale(0) translateX(-10vw);
  will-change: transform;
  
  animation-name: fly-right-one;
  animation-timing-function: linear;
  animation-iteration-count: infinite;
  
  &--one {
    animation-duration: 15s;
    animation-delay: 0;
  }
  
  &--two {
    animation-duration: 16s;
    animation-delay: 1s;
  }
  
  &--three {
    animation-duration: 14.6s;
    animation-delay: 9.5s;
  }
  
  &--four {
    animation-duration: 16s;
    animation-delay: 10.25s;
  }
  
}

@keyframes fly-cycle {
  
  100% {
    background-position: -900px 0;
  }
  
}

@keyframes fly-right-one {
  
  0% {
    transform: scale(0.3) translateX(-10vw);
  }
  
  10% {
    transform: translateY(2vh) translateX(10vw) scale(0.4);
  }
  
  20% {
    transform: translateY(0vh) translateX(30vw) scale(0.5);
  }
  
  30% {
    transform: translateY(4vh) translateX(50vw) scale(0.6);
  }
  
  40% {
    transform: translateY(2vh) translateX(70vw) scale(0.6);
  }
  
  50% {
    transform: translateY(0vh) translateX(90vw) scale(0.6);
  }
  
  60% {
    transform: translateY(0vh) translateX(110vw) scale(0.6);
  }
  
  100% {
    transform: translateY(0vh) translateX(110vw) scale(0.6);
  }
  
}

@keyframes fly-right-two {
  
  0% {
    transform: translateY(-2vh) translateX(-10vw) scale(0.5);
  }
  
  10% {
    transform: translateY(0vh) translateX(10vw) scale(0.4);
  }
  
  20% {
    transform: translateY(-4vh) translateX(30vw) scale(0.6);
  }
  
  30% {
    transform: translateY(1vh) translateX(50vw) scale(0.45);
  }
  
  40% {
    transform: translateY(-2.5vh) translateX(70vw) scale(0.5);
  }
  
  50% {
    transform: translateY(0vh) translateX(90vw) scale(0.45);
  }
  
  51% {
    transform: translateY(0vh) translateX(110vw) scale(0.45);
  }
  
  100% {
    transform: translateY(0vh) translateX(110vw) scale(0.45);
  }
  
}