Having been away over Easter, I followed developments from a distance and was pleased to see a fourth egg laid on Saturday morning, after a further gap of another two days.
So it took a week (Saturday to Saturday) for the clutch of to be complete, as another egg at this stage looks extremely unlikely. With the same size clutch as every previous year at St George’s, there’s nothing to suggest a different female on that front, and the colour is also very similar to the deep ruddy shade of the eggs in each previous year. Views are mixed as to whether the screengrabs show the same female as last year or a different bird, and at present everything is proving inconclusive as to her identity.
As the male took over brooding duties earlier today, it was evident again how he struggles to cover the four eggs. In Tim Birkhead’s excellent new book on eggs, The Most Perfect Thing, shortly to be available in British bookstores, he discusses both the shape and number of eggs in a clutch. Many waders lay four quite pointed eggs that can fit – pointed-end inwards – into a neat group that can be easily brooded. Both the images above show that the Peregrines do not do this, partly because their eggs are much less pointed and more spherical than in the waders; and, as a result, it is not possible for the four eggs to fit together so neatly.
The other thing that more pointed eggs allows for is that a greater percentage of the egg’s surface area can be in contact with the adult’s body for the transfer of body heat to the eggs. So on this count too Peregrines are less ‘efficient’ in their brooding. And why do different species (or families) of birds lay eggs of different shapes? It seems this is one in which is still much to learn, although it may be related to pelvis shape (right-angled in raptors), which in turn bears a superficial correlation to the shape of the bird overall.
Raptors and their relatively rounded eggs are not, then, optimally efficient in their brooding, but with few natural predators and relatively secure nest sites perhaps this is less of an issue than for ground-nesting waders whose young are born at an advanced stage of development. Whatever the merits of the Peregrines’ strategy, these particular eggs will take between 30 and 32 days to hatch from completion of the brood, so we can expect them to hatch some time around 26-28 April all being well. In the meantime, there will be a lot of time during which the adults will be sitting tight, punctuated by handovers every few hours. Not the most exciting stage perhaps, but an absolutely vital one nonetheless.