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Photo © Al Tuttle

What’s the difference between “observed” and “estimated” dates in data entry?

Usually, first egg, hatch, and fledge dates are estimated—relatively few monitors catch them occurring on the actual day. These dates should only be marked “observed” if you saw them happening or there is little chance for them to be wrong. For example, if you checked the nest one day and it was empty, and then you checked again the next day and found an egg in it, “observed” would be appropriate for your first-egg date entry. Since we know most songbirds lay one egg per day, and you know it was not there the day before, it’s unlikely that egg was laid any other day. For this reason, if you visit the nest during the egg-laying period and observe the number of eggs increasing, you can usually estimate the first-egg date by counting backwards one egg per day.

For hatching date, “observed” would be appropriate if you checked the box 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, though parents often take them away shortly after the nestling frees itself. Note that sometimes one or more eggs won’t hatch, so if you see both eggs and 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 should use “estimated” for the hatch day.

For the fledge date, an “observed” date would be appropriate if you watched the young leave the nest (either in person or via a nest camera). Otherwise, it’s best to use “estimated” dates. Be sure to follow the NestWatch Code of Conduct, which states that it’s best to hold off checking the nest near the expected fledging date, as this could cause the birds to fledge prematurely.

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Cornell Lab of Ornithology

Cornell Lab of Ornithology