find out more

Photo © Al Tuttle

How do I estimate first egg, hatch, and fledge dates?

We define these dates as:

  • First Egg Date – Estimated date when the first egg was laid for each nest attempt.
  • Hatch Date – Estimated date that the first egg hatched for each nest attempt.
  • Fledge Date – Estimated date that the first nestling left the nest, for each nest attempt.

Since it is difficult to record a complete nesting chronology, we ask participants to provide estimates on first egg, hatch, and fledge dates for their attempt summary output. In the case of virtually all songbirds, you can calculate a first egg date by backdating using the assumption that one egg is laid per day.

Suppose you encountered 2 eggs in the nest on May 10 and you visit the same nest again on May 13 and discover 4 eggs. Counting backward one egg/day, we know the first egg was laid on May 9. The second egg was laid on May 10, the third on May 11, and the fourth and last egg on May 12. Once the clutch is complete, the female will start incubating after the ultimate (last) or penultimate (second to last) egg is laid. This backdating method does not work if the number of eggs is not observed to increase. For example, hummingbirds and doves typically lay 2 eggs, and it is easy to miss the egg-laying period. It would not be correct to assume that if you find a nest with 2 eggs, you should count backwards one day; you would need to see the egg count increase in order to estimate first egg date. The more eggs a bird lays, the easier it becomes to estimate the first egg date.

Return to FAQ list

Cornell Lab of Ornithology

Cornell Lab of Ornithology