Alien Invasion

It was a little cooler today, so I thought it would be fun to take Natalie on a walk. There is a bike trail not too far from our house and it is a great place to walk. Natalie likes the woods and she loves bridges. The trail has a number of small bridges, but recently we found a really big one on the trail. In fact it is close to a half mile long! We got out of the car and started to walk on the trail. Finally we came up to the bridge, "Woah" she says. "That's a big bridge". She got really excited and did her little gallop/run onto the bridge. We got about three quarters the way down and I thought it would be a good spot for a picture. I told her to get in the middle a the bridge and I squatted down to take the picture. Just as I snapped the picture, I saw here look up at something. I turned around to see what she saw. Was it a giant swarm of mayflies, or a huge alien ship, an elephant swinging from tree to tree, or maybe a hundred beachballs that were falling from a plane that had the cargo door accidentally opened? Nope, it wasn't any of those. What do you think it was?