Blakeman says: not quite yet
Pale Male and Lola on nest in early February, 2008
Photo by Bruce Yolton Http://www.urbanhawks.blogs.com
John Blakeman read yesterday's blog and sent in some fascinating comments:
I seriously doubt that incubation will have started when either of the birds spend the night in the nest. Some observers seem to think the night-in-the-nest behavior indicates that an egg has been laid. I'm not so sure.
I think the formel (the female -- I like that term) begins to spend the night in the nest when she perceives that she's truly gravid, that the first egg is forming and beginning to descend the fallopian tube. It can take several days for complete egg formation. In fact, the formel's behavior may begin to change from elevated hormone levels long before an egg even begins to form.
We know from watching the birds in captive breeding programs that the formel's behavior changes markedly a few days or more before the first egg is laid. She gets a bit sluggish and seems to be in a bit of discomfort. She just tends to stand with a more vacant look on her face. Instead of diligently searching for prey and flying around with the tiercel, she just perches. Physiologically, her system is changing significantly, with notable stresses. Significant quantities of calcium compounds are being extracted from her bones, along with proteins for the embryo and the egg white. Lipids are being drawn from her body and being concentrated in the developing yolk.
Consequently, she has every good reason to rest in the bowl of the nest during the night. She can stay a bit warmer there, insulated by the now-thick nest lining. She can also shape her habits into egg-sitting, not something that she would ever do at any other time of year.
Your dates are very, very interesting. I notice that the first night-sitting dates are near the end of the first week in March, with most of the hatching dates in mid-April. If the first dates are real sitting, incubation-start dates, they put the incubation period in the realm of five weeks. In reality, it's closer to a bit over four weeks. I think this first week of sitting behavior actually happens before a real egg is laid.
I'm sure other observers have seen the following noteworthy behavior at 927. Until the last egg is laid, until incubation begins in earnest, the formel tends to sit just a bit higher in the nest. At times, she will hunker down deeply in the nest, but at most early times she will sit a bit higher at the start. But when the final egg has been laid, she will lay down very low in the nest at all times.
Incubation almost surely would not begin just now, unless Lola is to lay only a single egg this year. That's not likely. She's healthy. There's plenty of food. She will most likely lay two or three eggs this year, as before.
This time, she will not have to contend with the eggs wedged between the pigeon spikes. She will be able to instinctively roll them and keep them warm.
The reproductive season has begun -- with so much better hope than the last three.