Blue N0 and Telyn bonded with successful mating occurring for around a day and a half before Idris returned this year. So, the question that many people were/are asking was: "Who is the father of that first egg?"

While the timing might seem close to the naked eye, avian biology, seminal research by Dr. Tim Birkhead of Sheffield University, and the specific data from our Dyfi nest give us a very clear answer. Here is why Idris is almost certainly the father.

The Magic Number: 11.9 Days

Looking at our historical data for Telyn and Idris, a fascinating pattern emerges. Since they paired up in 2020, they have refined their "nesting rhythm" to an incredible degree. The chart below shows that the average time from their first mating to the first egg being laid is just 12.6 days.

In this current 2026 season, that turnaround was even tighter. From the moment Idris reclaimed the nest on the evening of 2^nd April to the arrival of the first egg on 14^th April, exactly 11.9 days elapsed. This "11.9 metric" isn't just a coincidence; it is a biological countdown. 

Female ospreys use this assessment period to determine whether their male for the season is up to the task of feeding her, himself, and several offspring. No female wants to waste a whole year with a 'dud'.

Idris brings in a whale of a mullet, further strengthening his bond to Telyn

Understanding Last Male Sperm Precedence (LMSP)

To understand why N0’s single day of "success" likely counts for nothing, we have to look at a phenomenon called Last Male Sperm Precedence (LMSP).

In the bird world, the female reproductive tract is a highly competitive environment. Ospreys have evolved a "last in, first out" system. When a female mates with multiple males, the sperm from the most recent partner has a massive mechanical advantage.

There are three main reasons why LMSP ensures Idris wins this race:

1. The "Flushing" Effect: Resident males like Idris don't just mate once; they mate frequently—sometimes up to 20 times a day—during the lead-up to egg-laying. This high-frequency mating acts as a "biological flush," effectively diluting and displacing any sperm left behind by a previous intruder.
2. Sperm Storage Tubules: Females store sperm in specialised tiny "tubules." Research suggests that the most recent sperm to enter these tubules is the first to be released when the egg yolk (the ovum) begins its journey.
3. The Fertilisation Window: An osprey egg is only fertilised about 24–48 hours before it is physically laid. By the time that critical window arrived on April 12^th or 13^th, Idris had been the exclusive mate for over ten days. The "N0 raffle tickets" would have been vastly outnumbered by the fresh, high-volume supply from Idris.

The Courtship Trigger

Beyond the microscopic battle of the sperm, there is the hormonal factor. Egg production in ospreys is heavily influenced by courtship feeding. The male brings fish to the female to prove he can provide for the family.

For nearly 12 days, Idris showered Telyn with fish and reinforced their pair bond. It is this specific 11.9-day period of intense care and consistent mating that physically prepared Telyn to lay. While N0 may have been a brief distraction, it was Idris who sealed the deal biologically. 

LMSP says Idris

Summary: A 99.9% Certainty

While only a DNA test could provide a 100% guarantee, the combination of Last Male Sperm Precedence, careful 24/7 behavioural recording and the 11.9-day timeframe makes Idris the overwhelming certainty to be the biological father.

As we look forward to the second and third eggs arriving (Friday 17^th and Monday 20^th April), we can be almost certain that the next generation of Dyfi chicks carries Idris’ and Telyn’s genes only.

Conclusion: The resident male, Idris, who returned and mated with Telyn every day for the 12 days before laying is overwhelmingly likely to be the father.

Additional – if you’re a Behavioural Ecologist nerd like me, - Dr. Tim R. Birkhead wrote a seminal paper in 1998 on Sperm competition in birds and LMSP –here’s the Abstract below, you can find it easily online: