Shift Change On The 'Ol Brooklyn Mourning Dove Nest

I've got a mourning dove nest on my terrace, and I've been shooting time lapses of them every day, hoping to catch something interesting.

 

While I initially thought I only saw one bird on the nest, it turns out that the mating couple share nesting duty.  And they do their shift change pretty quickly, often within a few seconds. Once, both birds hung around for about 30 minutes, and today, they once overlapped by a few minutes: (one picture each minute). 

\