How osprey eggs are formed - an eggtraordinary story!

When the ospreys return to their breeding sites in the spring, the only thing that is on their minds is to produce and successfully rear young.

This involves reproduction, sex, mating or copulation: various terms which mean the same thing. Apparently, in the world of birds it's normally referred to as copulation.

The next section details how ospreys mate, so please skip this if it’s not for you.

Unlike humans, most birds, and ospreys in particular, have just a single opening for the intestinal, urinary and reproductive tracts. This cloaca is the same in both male and female and is the exit point for solid waste, urine, sperm in males and eggs in females.

Incidentally, this does mean that it’s not straightforward determining the gender of ospreys when they are ringed at about five weeks. It’s not as obvious as identifying the sex of humans or animals. Instead, other factors come into play such as weight, wing length, thickness of legs etc.

In order to copulate successfully the male needs to position his cloaca alongside the female’s so that sperm can pass between the two. There is no elaborate ritual or display before copulation. It’s a quick manoeuvere, fast and furious, maybe lasting just fifteen seconds. It’s not a simple move and depends upon the female being receptive to the male’s advances. Inexperienced pairs can really struggle to get it right. Young females can make it difficult for the male if he hasn’t proved himself by getting her enough fish or convinced her of his nest building abilities.

Ospreys may attempt copulation ten to twenty times a day but possibly only 30 – 40% of matings are successful. The female is at her most fertile just before egg laying and mating continues up to, and often after, the eggs have been laid.

White YW and Blue 35 seem to have no difficulty in mating but the video of White YW attempting to mate the young female intruder, who was on the nest for a few days before Blue 35 arrived, shows how awkward an unreceptive female can be.

In the second clip of White YW and Blue 35 mating, the female has tilted forward with dropped wings and tail in the air so that the male can carefully balance on her back. A successful copulation happens when both cloacae meet for a few seconds and sperm passes to the female – often delicately referred to as a “cloacal kiss”.

The egg takes about twenty four hours to form. The female releases an unfertilised egg cell, ovam, from her ovary into the egg tube, oviduct. The ovam is attached to a yolk sac which will provide the nourishment that the embryo will need to develop into a chick in around five weeks time. If a successful mating has happened this is where fertilisation occurs and where the egg starts its journey.

As the egg progresses through the oviduct, glands produce the egg white, albumen, which will provide more nourishment as well as a buffer to protect the developing embryo. More protection is provided with membranes added before a porous, chalky shell forms. The pores in the shell allow oxygen to enter the egg and carbon dioxide to be expelled. The final stage, just before laying, is when pigment is added to the shell. This provides the distinctive markings that we recognise in osprey eggs. The first egg will often have stronger markings than the last egg, as you can see from the photo below.

After about twenty four hours, the egg will be ready to go out into the world. It is sometimes possible to see the bird’s contractions as she pushes the egg along the oviduct and out of the cloaca. She will often then stand up and check the egg and herself and fuss about in the nest to make sure the egg is secure before nestling down on it to keep it warm and safe.

That’s enough about reproduction but do keep an eye on the webcam for signs of hatching which could happen from about mid May. In the meantime, the pair still keep busy, building the nest walls to protect the hatching chicks and defending the nest from intruders. Blue 35 will do most of the incubating with White YW keeping guard and bringing her fish, but he does do his fair share of sitting on the nest when she wants a break.