Getting intimate with ospreys

With a full clutch of eggs now sitting in the nest, mating attempts have come to an end for this year. This has given us the chance to analyse, and compare to last year (the first year LM12 and LF15 paired up) the mating behaviour that staff and volunteers have recorded over the past few weeks.

Luckily for us, mating between ospreys nearly always takes place at the nest, as this is where the female spends the majority of her time, so we can be sure we’ve a reasonably complete dataset. Thankfully, we can also tell fairly easily if a mating attempt is successful – the male will land on the female’s back, she will raise her tail, and the male will bend his tail down and under the female’s — in short, everything will be in the right place.

Total Numbers

Figure 1 – Total mating attempts 2016
Figure 1 – Total mating attempts 2016

Our male osprey (LM12) arrived back at the eyrie on the 25th of March to join LF15, his mate from last year, and by the end of the day we had already recorded several mating attempts. The pair continued to mate for 25 days, until the 18th of April, when LF15 laid the third egg. During this time they attempted to mate 217 times — an average of 8.68 attempts a day — of which 67% were recorded as successful.

Figure 2 – Total mating attempts 2015
Figure 2 – Total mating attempts 2015

The numbers for last year are similar. Over the 23 days from LF15’s arrival, 228 mating attempts were recorded, a slightly higher daily average than this year at 9.91. However, 16% of these attempts were recorded as outcome unknown. Accordingly, there is a range of possible success percentages, from 46%, if all the unknowns were unsuccessful, to 62%, if they were all successful. Inevitably the true situation will have been somewhere in the middle but a difference between copulation success rate in new and established osprey pairs has been reported (37% for new pairs vs. 72% for established pairs) , which may give us reason to believe the success rate was at the lower end of this range.

Frequencies

Previous studies have concluded that generally the number of mating attempts between osprey pairs reaches a peak in the 4-8 days before egg laying starts. So, how does our data stack up?

Figure 3 – Mating attempt frequency 2016
Figure 3 – Mating attempt frequency 2016

Although the graph for this year shows a slight spike in mating attempts on the 6th of April this is only a small difference and mating attempts have been, contrary to the previously reported studies, fairly evenly spread over the period from the 28th of March to the 12th of April.

Figure 4 – Mating attempt frequency 2015
Figure 4 – Mating attempt frequency 2015

Last year’s graph on the other hand, is a bit more interesting. If we ignore the large number of attempts on the 1st of April, there is a definite peak on the 8th of April with a subsequent decrease in frequency thereafter. Last year’s data certainly seems to accord better with the previously mentioned studies.

But why so much?

Perhaps the obvious question arising from this data is why do ospreys mate quite so much when only a tiny fraction of the observed matings would be more than sufficient to fertilise an entire clutch of eggs? The short answer: to make sure she’s going to lay his eggs.

For a bit more detail we need to understand a bit about how ospreys behave when they pair up to nest. Specifically, two things: female ospreys spent the overwhelming majority of their time at the nest and the male is responsible for feeding the female. That time fishing means one thing for a male osprey – long periods of time spent away from the nest and, therefore, the female. It’s this separation that’s the reason for so many mating attempts.

A male osprey has one major drive, to pass his genes on – he wants the female to lay his eggs – but while he’s away from the nest other males are able to try and mate with the female (it’s called extra-pair copulation, in case you were wondering). Potentially, the female could lay another osprey’s eggs. Of course, there’s no guarantee that the female will cooperate in any of these attempts, or that they’ll happen. But it’s a numbers game for the male – if he can mate more with the female than any other osprey, the likelihood it’ll be his sperm that fertilises each egg is much higher. So what does he do? He tries to mate with the female whenever he can.  And that means a lot of mating attempts!

Duncan and Freya,

Species Protection Officers.

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Preface

With a full clutch of eggs now sitting in the nest, mating attempts have come to an end for this year. This has given us the chance to analyse, and …

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