Polygamy in Ospreys

In 2016 at Cors Dyfi we have a polygamous situation where Monty has mated with two females, Glesni and Blue 24, on two separate nests and both females have laid eggs. Let's look at some definitions first.



Polygamy - having more than one sexual partner. Polygamy can be defined more narrowly as:

Polygyny - one male, more than one female (as in our case)

Polyandry - one female, more than one male


For the sake of this article, we'll refer to the state of one male osprey having two nests with two females as the more accurate term - polygyny. This is the most common form of polygamy in ospreys, but polyandry does occur also where two males will breed with one female - see this BBC video here of two male ospreys breeding with one female in a desert/ocean habitat.

On 17th April 2016, a Rutland born female, Blue 24, laid an egg on Nest 2 at Cors Dyfi reserve. Just under 20 hours later, Glesni laid an egg in the main nest - Nest 1. The nests are 300m apart.


 Monty (right) and Blue 24 on Nest 2 observing an intruder osprey during switching over incubation duties - 19th April, 2016

Polygamous reproductive strategies are very common within birds, probably because most of them can fly and travel between locations very quickly without the restrictions and obstructions of moving around over terrain. Around 90% of all bird species are polygamous including many raptor species. Many flightless birds are polygamous too - the rhea being a rather famous case.

We have a unique situation at the Dyfi. Since we opened the 360 Observatory in 2014 we have unparalleled views of the Dyfi nest and the whole Dyfi environment - we can see the ospreys literally miles away and observe them for practically all the time during the five months that they are with us every year. Five High Definition cameras and 24 hour continual recording add to our observational abilities.

New nest (Nest 2)


Here's what we know from scientific observations over the last few years:

1. Polygyny is more prevalent when there is an excess of female ospreys (true in Wales at the moment). Females are less philopatric than males, meaning they will disperse more widely from their ancestral colony than males when old enough to breed. This is exactly why we have (had) several females from Rutland in Wales: Nora (2008), Glesni (2010), Blue 24 (2010), Blue 5F(2012) , Blue 3J(2013); we have yet to record a male from the same Rutland colony.

Males tend to return to their natal colonies when they are ready to breed (see here as to why this didn't happen to the Glaslyn males (2005 - 2009)). Out of six offspring from all three nests in Wales in 2013, all six were females. We haven't had enough males from Welsh nests yet to re-address this distortion in sex ratios. Over time, we will see this balance out.

2. During polygynous trios in ospreys, in most cases the male seems to prioritise one nest and partner over the other. One nest becomes a 'primary nest' and the other a 'secondary nest'. This makes sense when there are productivity pressures such as bad weather or increased predatory influences. The primary nest will be favoured in terms of paternal investment and the secondary nest becomes a bit of an insurance nest.

Monty and Glesni - is this Monty's primary nest?

It is too early to say yet whether Monty will adopt this primary/secondary stance and if he does, which nest is which. There is evidence that some ospreys only fall back on this position when there is a limiting factor such as too many chicks to feed as they get older or periods of bad weather.


3. In his book; Ospreys: A Natural and Unnatural History (1989), Alan Poole documents three separate polygamous events in his study area in New England, USA.

"Males all defended the second sites, usually against other males, and copulated regularly with both mates (exactly like Monty, and Glesni/Blue 24). Secondary females were short-changed, however; males rarely fed them and spent most time at their primary nests. Not surprisingly, secondary females usually had to hunt for themselves and rarely hatched eggs"

Real-life Examples:


1. Rutland, England - 2014

Site K: one male, Blue 06(2009), mated and had eggs with two females, Blue 00(2009) and Yellow 30 (2005), at two different nests. At some point during the incubation period the male chose the nest with Blue 00 as his primary nest and successfully raised one chick. Yellow 30's eggs failed to hatch. Both females went on to breed in future years with their own respective males. With thanks to Tim Mackrill at Rutland Water.


2. Aberfoyle, Scotland - 2014

A male osprey, named Drunkie, mated with two females at two nests within 200m of each other. Two chicks fledged from the primary nest, none from the second.

Drunkie at his 'primary nest' in Aberfoyle, 2014


3. Minnesota USA

We are extremely grateful to Vanessa Greene for the following accounts of polygamy in a population of ospreys in Twin Cities, Minnesota. Vanessa has almost a quarter of a century of experience of researching ospreys - thank you Vanessa.

"I have seen cases of polygyny (1 male, 2 females) five times now.

Four of those cases were one male going between two nests. (three of these times it was the same male attending these nests.) Two of those times the nests were a mile or more apart, not within view of each other. Two times a male had one female on a nesting pole, and another on a nest on a transmission tower , within view, about 170 yards away. This year (2016) the same male who engaged in polygyny two previous times, is doing it again. This situation usually results in one failed nest and one successful nest. But this male did pull it off successfully one year at the two nests which were not in view of each other - he fledged three chicks at each nest!

He had a fair amount of help from those females who had to help provide fish as the chicks grew. He did give up this secondary nest that was further away the following year...and allowed another male to move in, though he did visit the nest occasionally. These two nests were over a mile apart.
The same male subsequently claimed the two nests that were in sight of each other (there was originally a nesting pole set that was used for many years, but when the trees grew up and too much human activity near the nest became disruptive, the ospreys moved to a transmission tower nearby. Much higher and removed from the humans).

In 2015 the female who was on his secondary nest was rarely fed, and watched him bring food to the other female frequently. She spent a great deal of time food begging, and ultimately had to leave to feed herself and the eggs died and the nest failed. It can be difficult to watch a male not feeding or caring for the other female, but it's back to that survival thing...he has to decide how to best use his energy so that he survives.

In 2016 we have observed that when a visiting male arrives in the territory, this polygamous male quickly flies to the nest on the transmission tower to defend it, so that is clearly his primary nest.

I woke up in the middle of the night remembering yet another case of polygyny here! One male attending two nests that were almost in sight of each other. Chicks hatched on both nests....three on one and just one on the other. Sadly the nest with a single chick was probably predated, chick found dead below the nest. So another case where, in the end, only one nest was successful. Did the males absence or lack of defense contribute to this death? Hard to say for sure.
So that brings our total to seven instances of this behavior.

We also observed two females and one male sharing a single nest. Both females shared fish, shared incubation duties...but when a single chick hatched, one female seemed to lose interest and she departed. Perhaps she somehow knew it was not her chick? "

You can read more about the Twin Cities Metro ospreys on Vanessa's blog and Facebook page.


4. Glaslyn, Wales - 2016

Since he arrived back from migration on 12th April, the male, named Aran, has been observed mating several times with another female on a man-made platform 2,200m away (1.4 miles) from thre Glaslyn nest. This is another Rutland female - Blue 5F(2012).



So it appears that in ospreys, polygynous behaviour is triggered when a set of environmental conditions are met: two or more nest sites within relatively close proximity and perhaps line-of-sight of each other, an adequate food source and a lack of other competing males. This is exactly what we have at the Dyfi in 2016.

Here's our take on the evolutionary ecology going on in a polygynous osprey trio from both the male and female's perspective:

Males: A male osprey will hold a loosely defined territory during a breeding season. He will defend this territory from other males so that his nest and productivity are not compromised. If there happens to be another platform/nest close by, he will defend this nest as well, as it is within his territory. He will mate with females that choose to stay at these secondary nests, but as we have seen from the examples above, these secondary nests are rarely successful.

Females: When there is a lack of males holding a territory - as in most of the above examples, a female osprey has limited choices. She can wander around continually in the hope of finding a male, but this is time consuming and by mid to late May, the invisible cut-off period is reached where breeding cannot take place for that year. A polygynous arrangement may represent the only avenue to reproduce, despite it being a low probability strategy.

So, a female may choose to 'share' a male in a polygynous trio in the hope that (i) the male chooses her nest as the primary nest, (ii) the female at the primary nest dies, or (iii) both nests are productive which sometimes happens. Note: it's natural selection that 'makes' these choices of course, not the actual female at the start of every year making a conscious evaluation of her options.

Many of us have reservations about what is happening to the Dyfi ospreys in 2016, but this unique, unobtrusive to the birds, window into their ecology will enhance and enrich our understanding of their complex lives. This will ultimately equip us with the knowledge to better protect and conserve them.

One last thing.. we get many comments saying that Monty is being a 'two-timing so-and-so' or chastising an osprey for the decisions he or she makes. Some of them are made in jest of course, but we can't think of any better words than those of Vanessa's to explain our position on this:

Vanessa: "First let me address the big issue the public has with anthropomorphism. Here is what I tell people who think an osprey is being "bad" by engaging in polygamy. Ospreys are largely motivated by two things....survival and the urge to reproduce. A male is driven to pass along his genes and when an opportunity like this occurs, he takes advantage of it. Sometimes the situation is set up by humans who placed nesting poles too close together. A male will naturally defend them both as his territory. He will allow another female in the territory, but not another male. Normal survival and reproductive instincts."

We'll keep you updated on the changing dynamics of the 2016 polygamous situation at the Dyfi below:



13th May: Blue 24 is still with us and has been bringing nesting material back to her nest this week. Thankfully, we have seen no evidence of any more eggs despite the odd visit from Monty.


4th May: The inevitable has happened - a crow has been seen pecking/eating the eggs this afternoon. Blue 24 had been investing less and less time incubating her eggs.















30th APRIL: Blue 24 is still incubating her eggs, but is leaving them unattended for several hours at times. There was a severe hail storm today and Blue 24 elected not to incubate her eggs despite a layer of ice forming on them.


They are almost certainly not viable by now.




27th APRIL: Blue 24 has three eggs - see blog here


25th APRIL: Blue 24 has been incubating all day again today. She went fishing for herself at 2pm (unsuccessful) and she's just gone again now. Monty has been over to her once and mated briefly.

Over at the Friends of Dyfi Osprey Project FB page, Live Streamers there have been recording Monty's incubation times around the clock. Here are the results so far.

Notice that on the first two days of incubation, Monty incubated Glesni's egg very little. On DAY 3, when Glesni laid her second egg, this is when he seems to have decided that Glesni's nest was his primary nest and broken his paternal bond with Blue 24.

The last five days have seen Monty incubating between approx 15% - 20% of the time, which is exactly what we would eggspect to see.

Citizen Science putting some great data together and validating what we are seeing.


24th APRIL: Blue 24 is still incubating her egg(s) but we have noticed her spending more time off the eggs today. We haven't seen her eating since last Wednesday, four days ago, but we can't see her 100% of the time when she is not on the nest, so she may have caught a fish for herself, we just don't know. Monty was last seen mating with Blue 24 on Friday afternoon, so he seems to have broken off any bond with Blue 24 for the time being.

UPDATE 20:30 - Blue 24 feeding on her nest, she has caught the fish herself.


23rd APRIL: If ever there was a stark example of how cruel nature can be sometimes, we witnessed, and filmed it, this afternoon.

As soon as Monty caught a trout and landed on a perch 700m away, Blue 24 started food soliciting. She could clearly see him.

After feeding for almost an hour Monty carried his half eaten fish to the 'new nest pole', a feeding perch just metres away from Blue 24's nest. He stayed there for 40 seconds while she continually screamed for food. Despite Blue 24's frantic food begging calls, Monty took off with the fish and headed straight for Glesni's nest.

Monty seems to have made his decision as to which is his primary nest - maybe it's best he makes this decision now and not when both nests have chicks and a whole clutch starves to death.


Taken with a video camera and a long lens from the 360 Observatory



22nd APRIL: It is becoming evident from observations and recordings that Monty is favouring one nest over an other. He incubates and brings food to Nest 1, but is only seen occasionally at Nest 2, usually mating with Blue 24. It looks like Monty is mirroring the examples mentioned above.

Here is the quote from Alan Poole's book again; "Males all defended the second sites, usually against other males, and copulated regularly with both mates (exactly like Monty, and Glesni/Blue 24). Secondary females were short-changed, however; males rarely fed them and spent most time at their primary nests. Not surprisingly, secondary females usually had to hunt for themselves and rarely hatched eggs"

21st APRIL: Glesni lays her second egg. We are not sure whether Blue 24 has a second egg as it is impossible to tell by behaviours alone. There is no camera on Nest 2.


19th APRIL: Monty has been sharing parental duties pretty evenly up to now. He mates and incubates at both nests as well as proving food for both females.

 Monty shares his food with two females - travelling between both nests with his fish


18th APRIL: Glesni lays her first egg on her nest, Nest 1, just 16 hours after Blue 24 lays.


17th APRIL: Blue 24 lays an egg. At 12:10 Blue 24 got up from a sitting position at the other platform on Cors Dyfi and looked straight back onto what must be an egg. She stared at it for a while before she sat down again and has been incubating all afternoon.

Blue 24 (left) and Monty on Nest 2


3rd APRIL - 17th APRIL: Monty and Blue 24 are seen mating on Nest 2 several times a day.