Failed nesting of grey-rumped treeswifts

I become quite accustomed to hearing the calls of grey-rumped treeswifts as I started to try to photograph these birds in flight.

As luck would have it, one bird led me to its nest, which I had originally thought was a resting perch. After a week's observation, I realized that the pair were nesting.

Male

Female

After close to a month of daily checks on the pair, I found the nest abandoned. I can only speculate that crows stole the egg as there were several crows in the area the previous day.

Abandoned nest