Sometimes they spook easily. These took flight, and I shot them as they were moving rapidly away. You can tell that the shutter speed I was using was a fast one, because the birds' wings and tails are sharp. The fast speed stopped the action. Now we know how the shutter and aperture work together to make an exposure.
Speed is also important because of camera shake.
A common rule of thumb is to check the focal length of your lens. If you're using a tripod, your camera should be steady enough to enable you to use any shutter speed. Always be sure to adjust the aperture to balance the exposure, however. This means that you control the exposure equation. You set both the aperture and the shutter speed.