Here's a photo of the 5 eggs in the Carolina Wren's second brood. This was taken on July 4th, so it's about 12 days after the first egg was laid, but only 8 days after the last egg was laid.
The second brood started hatching on July 9th. I checked the nest at 5:30 pm and 2 eggs had hatched. I checked the nest again a bit later, at 7:00 pm and 1 more egg had hatched. July 9th is 13 days after the last egg was laid. Their incubation period is 12 to 14 days, so they were right on schedule, counting from the day the last egg was laid.
One possibility I thought of to explain how the eggs can hatch within a day of each other even though they were laid a day apart is that the mother wren doesn't start the full incubation until they are all present. I think the embryo development is controlled by temperature and I noticed that the mother wren wasn't always in the nest until the last egg was laid. This would explain how the early eggs and the later eggs hatch within a day: they don't really start developing until she keeps them fully warm. This photo was taken on July 9th. There are 3 hatchlings although one is hard to see behind the rear egg.
One more egg had hatched when I checked the next day, on July 10th, at 7:30 am. Now there was only 1 egg remaining to hatch.
By the next day, July 11th at 4:00pm, the last egg had not hatched and I started to suspect that something was wrong. Either the egg had not been fertilized or something happened in its development.
On July 17th, the egg is still in the nest which surprised me because I thought the parents would remove it when it didn't hatch. The 4 baby wrens are developing nicely and I estimate they'll fledge around July 22nd which is 12 to 14 days after they hatched.
Another thing I observed whenever I peeked in the nest is that the baby wrens look exhausted. The first brood always looked active when I approached and frequently opened their mouths trying to get fed. The second brood just lies there. I noticed it's quite warm in the garage. The high temperatures are now approaching 100 degrees and there is no breeze in the garage.
What seemed like a good choice of nest location on the mother wren's part in the spring now appears to be not be so good.
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