I am going to a venue that I know will have lots of swallows feeding on the wing over a river.
I haven't had much success in the past so I wondered if the forum members had and advice on photographing these fast flying birds. A high shutter speed is obvious, but I have always struggled to get them in focus. I will be using a Nikon Z8 or Z9 so should I try Auto-area AF with bird detection on and let the camera decide which bird to focus on, can the Nikon focus that fast? Or is there a better way to setup the camera so I can try and select the bird I am most interested in?
I've used both Auto AF Area and a large custom C1/C2 area on my Z9 and Z8 with pretty good success. I'll opt for a fairly large horizontal custom area if there's things down by ground or water's edge that could trip up the AF tracking and full auto for a clean scene with just open water or other situations where the AF is more likely to stay on the swallow.
I agree with Doug above that the most important thing with erratic air hunters is repeatability so if you can find a place where a lot of them are feeding and better yet when as you observe them they frequently fly in a similar pattern like keep coming up a small water channel or keep flying into the wind over a particular place the odds of getting a shot go way up. My best swallow shooting days have been early morning or evening sessions when there's been a recent bug hatch and there's a ton of birds feeding. Often I haven't set out to photograph swallows but when I see this kind of stuff happening I'll jump on the opportunity.
I've captured swallow flight shots with lenses from 300mm to 800mm and it can all work but to get started and while shooting a high resolution camera it's a lot easier at the start to shoot with a wider lens and then crop a bit if needed. With practice and tuning into repeated flight paths it becomes easier to predict where they'll be and acquire them with longer lenses. Again, small side channels, canals and the like are awesome as they tend to constrain the birds to similar flight paths.
One trick is to refocus on something like shoreline grass at roughly the distance where you think you'll initially acquire focus and then wait for the next bird to come into the scene. IOW, I like to shoot when they fill more of the frame but try to pick up initial focus at a distance in a place where they seem to be repeatedly flying. So for the birds that I'm able to track in close my focus adjusts to a close distance, before trying to pick up the next bird I'll quickly refocus on a distant object close to where I picked up the previous bird to be ready again.
Another thing to look for is common perching places. There's often a tree, bush, fence or something else where birds might perch while taking a break from hunting. If that's not too far away it can be a good place to focus on a bird, wait for takeoff and track them in as they hunt closer to your position.
Even when all of that happens expect many birds to just erratically dart out of your frame. That happens a lot and usually I move on to the next potential subject unless I see that they loop around and give me another chance. That's just part of it and there's a lot of times I'll acquire initial AF tracking and still lose them as they swerve and dart around but hopefully there's more around so I get more chances.
