Nice birds and umbrella!
I think if the number of birds is hard or inconvenient to count, eBird (from what I remember) prefers to have an 'X' as the number of individuals, with '1' reserved for single birds only. I think there's no way to input an 'X' directly from the search field above, but my experiments (and it took me a while to get to know about it all) have shown that if you type '-1', it turns into an 'X' by itself. Checked it just now, and it seems to be working (for me at least). Cheers