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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.

Estimating First Egg Date

In the case of most small birds (e.g., small shorebirds, woodpeckers, many ducks, most passerines), you can calculate a first egg date by backdating using the assumption that one egg is laid per day (the interval between laid eggs is about 24 hours).

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, it would not be correct to assume that if you find a nest with only 2 eggs in total, 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.

Some species require more time between each egg laid. Ostriches, rheas, herons, storks, cranes, bustards, gulls, doves, owls, hummingbirds, swifts, kingfishers, geese, swans, as well as some accipiter hawks and cuckoos tend to lay eggs every other day. A few groups may lay eggs at even longer intervals (e.g., parrots, penguins, megapodes, pelagic seabirds), making it very difficult to estimate when the first egg was laid. If you aren’t able to observe or estimate the first egg date, feel free to skip this question and move on with data collection.

Estimating Hatch Date

You may be able to record the hatching date if you checked a nest and saw both hatched and unhatched eggs, with small young that have obviously just hatched, or are in the middle of hatching. Broken eggshells are normally seen at this time too, although parents often take them away shortly after the nestling frees itself. Note that sometimes one or more eggs in a clutch won’t hatch, so if you see both eggs and small nestlings in the nest, but don’t see birds actively hatching, you will want to confirm whether those remaining eggs hatched on your next check. If the eggs remain in the nest, then they are likely nonviable, and you may not know the hatch date with certainty (although you can indicate if your date is estimated or observed, more on that here). Similarly, if you find more developed nestlings (i.e., some feathers erupting, able to sit upright) in a nest alongside unhatched eggs, it is probably safe to assume that it is not hatch day.

Because nestlings grow at different rates depending on a variety of factors including habitat quality, parental experience, and weather conditions, we do not suggest trying to estimate nestling age and backdating to approximate a hatch date. If you miss the window for observing or reasonably estimating the hatch date, simply skip this question and move on with data collection.

Estimating Fledge Date

Fledge date is particularly challenging to estimate because we ask that you do not approach a nest closely near the potential fledging window (to avoid startling the young into prematurely leaving the nest). However, sometimes we are able to witness the fledging event directly or can estimate it reasonably well for some species (e.g., for a nest on your front porch, or a nest with a camera installed on it). If you miss the window for observing or reasonably estimating the fledge date, simply skip this question and do not worry about it. Indeed, some species leave the nest before they can fly (the technical definition of “fledging”) and may wander away with their parents (e.g., Killdeer, shorebirds, geese), never to be seen again. In this case, do not worry about reporting a fledge date as it will be nearly impossible to know!

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Cornell Lab of Ornithology

Cornell Lab of Ornithology