Maybe the EV was set negative?
This still hasn't been answered. It's the most likely reason, if all exposure settings were on auto (filter on or not). I don't know what remote you have, but on my P3P remote, EV is controlled with the dial at the back of the remote at your right finger-tips. Be sure to set it to 0 rather than a negative value. Display the histogram and/or blinkies to see how your video is affected by the value.
The scene should have looked dark on your screen while filming.
If you don't know what EV means, but you're doing these jobs for money, you should take some basic photographic/video courses
"Almost always on sunny days like this I use an ND 8 filter. Usually produces sharp, crisp video."
That's not what ND filters are for. In fact, it's mostly the opposite, slowing down the shutter speed to achieve more motion blur, which typically adds to the cinematic look of the finished footage, as Franky mentions above. Here you would want something like ~25 / 30fps and a ~1/60 shutter speed
A very sharp and crisp video would require a higher shutter speed and frame rate, something like 60fps and 1/125 (or higher) shutter speed, which an ND filter would work against / take you further away from. If you want faster video, the last thing you want to do is reduce the light with an ND filter.
If you know what you're doing, you can still use an automatic exposure mode, if the filter is bringing down your shutter speed to the desired setting. But then, these birds fly around and change orientation in respect to the lighting (the sun), so manual can give you finer control.
Chris