Moving subjects – like birds in flight or dogs chasing after a ball – are more difficult to keep in focus than static ones using the standard (auto) autofocus mode. While auto AF mode can alternate between single and continuous autofocus when it identifies a moving subject, selecting continuous AF mode (AF-C) will give more consistent results.
AF-C enables you to keep the focus bang on track, focusing on your subject continuously all the while the shutter-release button is half-depressed.