An explanation of incubation timing, etc.
Our vendor fair was a total washout today so we left to come early. I will take this opportunity to follow up on yesterday's post. The typical clutch of kestrel eggs contains 4 or 5 eggs. Once the female begins laying she will lay, on average, one egg every other day until she is done. Once all the eggs are laid, it will be 30 days until the eggs hatch and then 30 more days for the young to fledge. This easy math makes the calculations for when banding should occur, etc. pretty easy to figure out. The young should ideally be banded somewhere between the ages of 15-18 days old. At that age their lower leg bones are adult size so there are no concerns about their legs growing larger and having the bands becoming overly tight. In addition, beyond the age of 14 days old the primary wing feathers and other areas are developed enough to be able to accurately sex the birds.
Now, let's hit the rewind button a bit. The reason I spent so much time going into the nestboxes during April and early May was two-fold. First, it was necessary to keep the starlings out. Secondly, I was waiting to see the first eggs in the boxes so that I could then calculate the best day to go into the box to band the young. For example, let's say that on April 16th I went into a box and found 2 eggs. At that point I would add 2 days for every additional egg that would be laid up to a maximum of 5 eggs. That takes us to April 22nd. I would then add 30 days to bring us the the hatching date. That would be May 22nd. I would then add 5 days to bring the young up to an assumed age of 5 days old. So after waiting 41 days from the time I found the first two eggs, I would go up into the box. At that point I would be able to fine tune the exact final banding date, wait for that date and band the young. The math is not as exact as I portray it to be in this post. Sometimes egg laying can be delayed, incubation times can vary, etc. but it's usually pretty close. If you band them too young, you may not be able to determine the gender of the bird. If you band them too old, you run the risk of having them leave the box too early and prematurely fledge. That puts them at very high risk.
I hope this rather long-winded explanation helps others to understand the level of vigilance that is required if one is to to study and truly understand the dynamics of American kestrel populations. It is important to note that this is day 89 of my kestrel season and there is still a long way to go. Fascinating stuff. In the next 10 days I will be doing pre-banding visits at many, many boxes. I will keep you posted on what I find.