It seems that the AF has some difficulty to "catch" your target.
I suggest that you set first:
- AF-S
- Center-focus
In that mode, you need to have the bird in the centrer of your screen for AF confirmation. Yet it is not easy to focus on a flying bird moving away (or towards you) fast. I have a similar problem with my DA18-250mm at 250mm.
(THe AF-C mode does not need AF confirmation to shoot and your photos might be out of focus.)
However, with a faster moving object, AF is not simple and you might consider MF. At a recent school fair, I use my MF Voigtlander 58mm to shoot the children and I was quite successful.
Food for thoughts...