Before covering the events of 19 July… most readers will know our Nest 1A and Nest 2 males, White YA and Yellow 37, hatched at Glaslyn in North Wales. Their half sister, Blue W7, has a problem with her right foot. We send our very, very best wishes to Glaslyn for her recovery.
UV has visited Kielder a few times now. On 19 July he spent most of the day here and there were one minute fixes for some of the time. He was at or near all four nests so for nest protection images are limited, but we gained great insight into his activity from such detailed data. Paul has written this blog, an important piece of analysis which is almost certainly a first for the UK. Over to Paul:-
Visits to active nests by non-breeding individuals are an integral part of osprey ecology. We use the word “intrusions” for these incidents, which implies some kind of hostile intent on the part of the intruder. In fact, this whole aspect of their behaviour is much more subtle and complicated than mere aggression.
It’s true that some ospreys may attempt to take over a nest site occupied by another pair. Such a case will often result in talon-to-talon combat – and yet the takeover attempt itself is far more often unsuccessful than not. Evolutionary theory tells us that the risk of physical injury or even death would hardly be worth it, when the attacker’s statistical chances of obtaining a breeding site are so low.
In fact, the vast majority of active nest intrusions have no aggressive agenda. To use a (slightly weak) military analogy, they are not offensive manoeuvres but more like intelligence-gathering missions. Young non-breeding ospreys can gather a great deal of crucial information by visiting a successful nest and the area around it. The fact that an active nest is present at all, tells the visitor that he or she is in a “good place” for ospreys. If the nest is large and well-formed, it means that there is plenty of construction material nearby. A brood of healthy well-grown chicks signify adequate food supplies.
Since the installation by Forestry Commission England of nest monitoring cameras and video recording equipment at Kielder, we have seen many such nest intrusions at all stages of the osprey breeding season. But – for obvious reasons – all of these observations are seen from the nesting birds’ “point of view”. We can’t really know what the intruder is doing during the period immediately before and after its fleeting appearance on screen.
But on the 19th of July 2016, Blue UV changed all that…
He had visited his home area before but, on this occasion, good weather (very rare during this summer of near-permanent overcast) meant that he could return for some serious nest-visiting. His detailed tracking data allowed us to visualise – perhaps for the first time in UK osprey studies – the way an intruding osprey behaves during these events.
To avoid giving away the locations of protected nesting sites, this blog will have to do without most of the usual fancy graphics and animations. (All times in the text are in UTC unless otherwise stated, the nestcam times are BST ie 1 hour ahead.)
Reconnaissance, Nest 2
UV left his roost site in eastern Cumbria just after 08:30 and by 10:20 was over the boundary of the park and heading towards Nest 2. On arrival, he positions himself by flying along a ridge line above and to the north of the nest, about 500m away. He is observing the nest, and what is going on there.
At 10:39, UV decides that a closer approach is possible and heads straight for the nest. He makes one pass over it at a height of about 70m, then veers away, chased by another bird – probably Uncle 37!
UV continues towards Nest 3, arriving there around 11:10.
Reconnaissance, Nest 3
UV adopts a similar tactic at Nest 3, favouring the higher ground behind the nest platform but this time keeping an average distance of around 350m. He remains in the area for only 10 minutes.
Interlude – local exploration
After this, UV flies a long semi-circular loop around the northern sector of the forest – a path which eventually brings him back towards Nest 2 at 12:10.
Intrusion, Nest 2
Again, UV stands off at a distance of some 500-600m for 2 minutes, and then makes a direct pass over the nest at 12:14.
This time he is much lower – only about 20-30m above the platform height. Immediately after this, he flies 750m back up to the ridge line, and perches in a tree for the next ten minutes. His next batch of points show a zig-zag path away from the nest area. We interpret all this as UV being chased away by one of the nesting adults.
Nest 2 again
At 13:13, UV makes another close approach at Nest 2.
He wheels away, and comes in again (13:22) for second pass. His speeds in these manoeuvres are fairly high, 25-30 knots. (45-55 kph). Again, he returns to the ridge line and perches, this time for 15 minutes. At 13:53, another low-level pass at the nest. Immediately after this, he leaves the area and perches some distance away to the south- east.
This image, with UV’s overall track shown FROM BELOW, shows the pattern of movements associated with a series of nest intrusions and subsequent chases.
Back to Nest 3, but not for long
After another leisurely tour of the forest with several perching stops, UV arrives again at Nest 3 (15:27). This nest does not seem to interest him quite as much as Nest 2: he spends a few minutes watching it from the same range as before (350m) before flying away northwards gaining altitude on the way.
By 15:40 he has made a detour towards high fell overlooking his parental nests at Nest 1 / Nest 1A. However, they are over 1000m distant and he does not approach any closer than this. Instead, he backtracks along his previous path. UV has not done with Nest 2 yet…
Nest 2 Intrusion #4
UV arrives back at the ridge overlooking Nest 2 by 16:29. Unlike his previous visits, he doesn’t fly along it but makes another direct pass by the nest (16:30), turning back sharply and gaining height to over 700m. There is video evidence that he was intercepted again during this pass. Here is a screengrab of EB taking off after UV.
Still climbing (1000m+) he alters course to the west and flies out of the Park in the direction of Newcastleton.
A striking aspect of the day’s activities is the amount of ground covered by UV within the boundaries of Kielder Forest, and the frequent and pronounced altitude changes during it.
This kind of flying must consume a great deal of energy, UV flew 130 km around Kielder – much more than normal daily foraging and flights to and from roost sites. The whole behaviour of nest-visiting and intrusions must have a considerable benefit for it to have evolved in the face of such an energy budget.