Today (Thursday) saw the arrival of a fourth egg, as we’d hoped. By coincidence, 27th March was a notable date in 2013 too: last year it was the first egg of the season that was laid then, also on a cold sleety day. So it’s taken a week, almost to the hour, for the clutch to be laid from start to what is expected to be finish, assuming this female follows the typical pattern of laying the same number of eggs as in previous years. Several people saw the fourth egg being laid a little before 08:30, after which it was left exposed as the female incubated the other three eggs for 15 minutes or so before pulling the new egg under her to form part of the clutch. Why it would be left out like this is a puzzle, but it’s another fascinating insight into their breeding behaviour (thanks to Andy D for the details).
So after the first egg was laid early Thursday morning, there have been gaps of around 52-54 hours, 70 hours and 49 hours between eggs. This conforms to the pattern identified by Ratcliffe in terms of gaps and preferred period of the day for laying, but the longest gap coming between second and third egg (rather than third and fourth) certainly does not.
Following the initial anxiety of the final egg being left out in the cold, for the rest of the day the male and female have been taking turns to incubate, sitting tight and only briefly leaving the eggs when carrying out a takeover. The literature states that the female does most of the incubation, but today the male has spent at least half the time on the eggs, apparently unwilling to hand over to the female on more than one occasion.
In the screengrab above the female arrived with much calling, to which the male responded by calling and sitting tight, with the female departing again to leave him sitting for another two hours. When they do make the changeover, the difference in size is quite obvious, as in the side-by-side comparison below.
The male (right) looks barely large enough to incubate the four eggs and takes a while to get settled, shuffling from side to side and pulling the eggs tight in under his body, as below.
Handovers of incubation duties took place today at 12:48 (female took over), 14:32 (male took over) and 18:29 (female back on). She’s still sitting now (21:10), with at least one eye open, and seems to do the night-time stints. I may have missed some changeovers, as I’ve used the calling to alert me to look at the webcam, but don’t think I have during the day, so my reckoning is that between 10:00 and 19:00 the male sat for almost 5 hours and the female a little more than 2. Given her greater size, might she be better equipped to incubate the eggs at night, when temperatures are lower? The other snippet of behaviour I’ve noticed is that the female is still busy improving the nest scrape, regularly picking at stones and dropping them close to her body to increase the lip on the nest cup, presumably to reduce the wind getting under her to the eggs.
I’ve not seen the male do this today, but could easily have missed it, so one to look out for perhaps. Is this part of their breeding behaviour we can help to appreciate? It has been suggested that this could be a remnant of nest repair activities undertaken by species that build a nest of sticks, as raptors other than falcons do.
If four does indeed represent the full clutch, then an incubation period of 29-32 days would give a likely hatching date of 24-27 April. In the meantime, we’re going to see a lot of scenes of sitting Peregrines! Anyone looking for a project could keep track of the time the male and female spend sitting, or note changeover times observed (and whether it’s the male or female that takes over), to build up a picture of how this pair shares out the incubation duties. Ratcliffe suggests that the male may do 30-50% of the incubation in the earlier stages, though in many pairs this has been seen to decrease later in incubation. So much to learn and so little time to watch!