How the dates are worked out
The first egg date counts back one egg a day from the first count, as the NestWatch manual does. Incubation starts with the last egg (the day before it for swallows, wrens and chickadees), so the hatch window is the last egg plus the incubation days in the table. Once you see young, enter their age from a growth chart and the hatch date firms up.
The last day to open a box is the earliest possible hatch day plus the stop age, minus one, so it errs on the safe side. An age read from a growth chart can be a day or two out, so the last day to open is counted as if the young were 2 days older than the age you entered. Past it the box says "Do not open", and before it the list shows the last date to open.
| Species | Incubation | In the nest | Stop at |
|---|---|---|---|
| Eastern bluebird | 11 to 19 | 15 to 21 | 13 |
| Western bluebird | 12 to 17 | 18 to 25 | 13 |
| Mountain bluebird | 10 to 18 | 18 to 24 | 14 |
| Tree swallow | 11 to 19* | 15 to 25 | 15 |
| House wren | 13 to 15 | 12 to 18 | 12 |
| Black-capped chickadee | 12 to 13 | 12 to 16 | 11 |
| Carolina chickadee | 12 to 15 | 16 to 19 | 11 |
| Tufted titmouse | 12 to 14 | 15 to 17 | 12 |
Bluebirds: NABS fact sheets and Get to Know Bluebirds. Others: NestWatch species pages, Sialis.org and, for house wrens, the Tennessee Wildlife Resources Agency (do not open after 12 days; this log stops at 12 because wrens can leave that early). *Plus up to two weeks when cold weather delays the start (Sialis.org). Change any stop age under Trail settings.
Example: box 3 on the example trail
Four Eastern bluebird eggs on May 1 and five on May 8 put the first egg on April 28 and the last on May 2. Hatching was due May 13 to 21. On May 22 the young looked 6 days old, so they hatched about May 16. In case that age is a day or two out, the last day to open was May 27 (eggs were still unhatched on May 15) and fledging was due May 30 to June 6. Open the example trail to see its walk sheet and report.
Questions
When should I stop opening a bluebird nest box?
Once the young are 12 to 13 days old, counting hatch day as day 0. Opening the box after that can make them leave before they can fly. The North American Bluebird Society growth chart says day 13 and the Virginia Bluebird Society trail sheet says 12. This log uses 13, and you can change it.
When will bluebird eggs hatch?
Usually 12 to 14 days after the female starts sitting, which is when the last egg is laid. NABS gives a rule of thumb of about 17 to 18 days after the first egg. The young then stay in the box for about 16 to 21 days.
How often should I check my nest boxes?
At least once a week. NestWatch asks for every 3 to 4 days but accepts weekly checks. Go in late morning or afternoon, skip cold or rainy days and keep each visit under a minute.
What counts as a nesting attempt on the annual report?
The Virginia Bluebird Society counts a nesting attempt only once at least one egg is laid. The Bluebird Society of Pennsylvania asks for completed nests. The report shows both, so you can copy whichever your society asks for. A nest you only watched from outside is not counted until an egg is recorded.
Why does a box I have not opened for weeks say Do not open?
A nest could have started unseen, with young now past the stop age. Watch it from a distance for at least 10 minutes and record what you saw. If no adults came or went, tap the box and listen for begging; then lift the lid only a little, and close it at once if you see feathered young.
Can I send this to NestWatch?
NestWatch takes one nesting attempt at a time, not trail totals. The nest list download has each attempt's first egg, hatch and fledge dates, clutch size and number fledged, ready to enter there.
How are house sparrows counted?
House sparrows are not native and are not protected by the Migratory Bird Treaty Act, so their nests are kept out of the native totals. The Virginia form lists them under disruptions and the Pennsylvania form asks how many were eliminated. Native birds and their nests are protected.
Where is my trail data kept?
Only in this browser, saved with every change, and it works with no signal after the first visit. Save a backup file now and then, and open it on another phone or computer to carry on there.
Sources
- North American Bluebird Society fact sheets: Monitoring Bluebird Nestboxes, Eastern Bluebird Nestling Growth Chart, Getting Started, House Sparrow Control.
- NestWatch Nest Monitoring Manual and Code of Conduct (Cornell Lab of Ornithology).
- Bluebird Society of Pennsylvania annual nestbox report and Virginia Bluebird Society monitoring forms.
- Sialis.org nesting timetables and species pages.
- Tennessee Wildlife Resources Agency: house wren nest box.