Conservative estimate between 500 and 1000 birds. It was an amazing experience!, they were everywhere I looked, flying around and perching on vegetation, fences etc. The chirping was quite loud due to the number of birds. Hundreds were flying surprising high in the sky for a small bird, flying around swiftly in the strong wind. At the time I couldn’t recall ever seeing one before and had no idea what they were other than they looked somewhat like a dusky woodswallow. A mid-sized prey bird was chasing them around in the sky. I only had my IPhone at the time which makes everything seem miles away, but I managed a few photos using my iPhone through some binoculars.