.bird {
    background-image: url('https://s3-us-west-2.amazonaws.com/s.cdpn.io/174479/bird-cells-new.svg');
    filter: invert(34%) sepia(55%) saturate(427%) hue-rotate(141deg) brightness(93%) contrast(91%);
    background-size: auto 100%;
    width: 44px;
    height: 62px;
    will-change: background-position;
    animation-name: fly-cycle;
    animation-timing-function: steps(10);
    animation-iteration-count: infinite;
}

.bird-one {
    animation-duration: 1s;
    animation-delay: -0.5s;
}

.bird-two {
    animation-duration: 0.9s;
    animation-delay: -0.75s;
}

.bird-three {
    animation-duration: 1.25s;
    animation-delay: -0.25s;
}

.bird-four {
    animation-duration: 1.1s;
    animation-delay: -0.5s;
}

.bird-container {
    position: fixed;
    top: 0;
    left: -3%;
    transform: scale(0) translateX(-10vw);
    will-change: transform;
    animation-name: fly-right-one;
    animation-timing-function: linear;
    animation-iteration-count: infinite;
    z-index: 99999;
    pointer-events: none;
    height: 64px;
}

/* Random delays using CSS custom properties */
.bird-container-one {
    animation-duration: 20s;
    animation-delay: calc(var(--random-delay-1, 0) * 1s);
}

.bird-container-two {
    animation-duration: 20s;
    animation-delay: calc(var(--random-delay-2, 3) * 1s);
}

.bird-container-three {
    animation-duration: 20s;
    animation-delay: calc(var(--random-delay-3, 7) * 1s);
}

.bird-container-four {
    animation-duration: 20s;
    animation-delay: calc(var(--random-delay-4, 12) * 1s);
}

/* Alternative random variations */
.bird-container-one.variant-1 { animation-delay: 0s; }
.bird-container-one.variant-2 { animation-delay: 2s; }
.bird-container-one.variant-3 { animation-delay: 4s; }
.bird-container-one.variant-4 { animation-delay: 6s; }
.bird-container-one.variant-5 { animation-delay: 8s; }

.bird-container-two.variant-1 { animation-delay: 1s; }
.bird-container-two.variant-2 { animation-delay: 3s; }
.bird-container-two.variant-3 { animation-delay: 5s; }
.bird-container-two.variant-4 { animation-delay: 7s; }
.bird-container-two.variant-5 { animation-delay: 9s; }

.bird-container-three.variant-1 { animation-delay: 2s; }
.bird-container-three.variant-2 { animation-delay: 4s; }
.bird-container-three.variant-3 { animation-delay: 6s; }
.bird-container-three.variant-4 { animation-delay: 8s; }
.bird-container-three.variant-5 { animation-delay: 10s; }

.bird-container-four.variant-1 { animation-delay: 3s; }
.bird-container-four.variant-2 { animation-delay: 5s; }
.bird-container-four.variant-3 { animation-delay: 7s; }
.bird-container-four.variant-4 { animation-delay: 9s; }
.bird-container-four.variant-5 { animation-delay: 11s; }

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

@keyframes fly-right-one {
    0% {
        transform: scale(0.3) translateX(-10vw) translateY(8px);
    }

    10% {
        transform: translateY(12px) translateX(10vw) scale(0.4);
    }

    20% {
        transform: translateY(4px) translateX(30vw) scale(0.5);
    }

    30% {
        transform: translateY(16px) translateX(50vw) scale(0.6);
    }

    40% {
        transform: translateY(8px) translateX(70vw) scale(0.6);
    }

    50% {
        transform: translateY(20px) translateX(90vw) scale(0.6);
    }

    60% {
        transform: translateY(12px) translateX(110vw) scale(0.6);
    }

    100% {
        transform: translateY(12px) translateX(110vw) scale(0.6);
    }
}

@keyframes fly-right-two {
    0% {
        transform: scale(0.4) translateX(-10vw) translateY(20px);
    }
    20% {
        transform: translateY(15px) translateX(20vw) scale(0.5);
    }
    40% {
        transform: translateY(25px) translateX(40vw) scale(0.6);
    }
    60% {
        transform: translateY(18px) translateX(60vw) scale(0.6);
    }
    80% {
        transform: translateY(22px) translateX(80vw) scale(0.6);
    }
    100% {
        transform: translateY(20px) translateX(110vw) scale(0.6);
    }
}

@keyframes fly-right-three {
    0% {
        transform: scale(0.3) translateX(-10vw) translateY(32px);
    }
    25% {
        transform: translateY(28px) translateX(25vw) scale(0.4);
    }
    50% {
        transform: translateY(35px) translateX(50vw) scale(0.5);
    }
    75% {
        transform: translateY(30px) translateX(75vw) scale(0.6);
    }
    100% {
        transform: translateY(33px) translateX(110vw) scale(0.6);
    }
}

@keyframes fly-right-four {
    0% {
        transform: scale(0.4) translateX(-10vw) translateY(44px);
    }
    30% {
        transform: translateY(40px) translateX(30vw) scale(0.5);
    }
    60% {
        transform: translateY(48px) translateX(60vw) scale(0.6);
    }
    90% {
        transform: translateY(42px) translateX(90vw) scale(0.6);
    }
    100% {
        transform: translateY(46px) translateX(110vw) scale(0.6);
    }
}