The period between eggs being laid and hatching is the quietest time in the osprey breeding season. But it’s a chance to do a bit more research into these wonderful birds. In fact, as far as eggs go, they haven’t got unique features compared with other birds – unlike their specialisations for catching and digesting fish.
When a pair of ospreys meet and start forming a bond the female wants the male to prove he will be able to provide for her and the chicks. She begs for food on his nest and he tries to meet her needs. Followers of events at Dyfi recently witnessed incessant calling by Seren (a new and very noisy female) and the resident male Monty’s attempts to satisfy her appetite for fish. She was reluctant to allow him to mate, wanting that assurance that he would supply enough food.
The female calls the shots for successful mating; she needs to raise her tail and tilt forward so that the male can curl his tail under hers and achieve a ‘cloacal kiss’. This contact enables the sperm to pass from his cloaca into hers. The image of the Nest 2 pair shows a successful ‘kiss’, with his tail wrapped under hers. Although she hasn’t tilted forward as much as she might!
A male will land on a female’s back many times without this final contact being made. Studies have shown only 30-40% of attempts are successful. Early copulations stimulate the growth of eggs within the female’s ovary and strengthen the pair bond. The last 3 or 4 days before eggs are laid are the most critical for fertilisation.
In a successful copulation the sperm travel to the female’s oviduct. This is where the various stages of egg development occur. First, the sperm fertilises an egg which has been produced during ovulation and already has a yolk. Then the principal coating of albumin is applied before the outer and inner shell membranes are added. These stages take about 5 hours. Next the calcareous shell forms. The background pigments are laid down. The egg stays within the uterus for about 20 hours and tiny glands excrete the streaks and patterns that result in the darker markings on the shell. Finally the egg is laid! The eggs weigh between 60 and 80g and are about the size of duck eggs. The background colour ranges from off white to pale brown, the mottling is red or dark brown. The first egg is usually the largest. Fascinating stuff!