Once again, we’ve enjoyed a fascinating season with our Nightjars on Dersingham Bog NNR. After 25 churring males and 23 nests in 2018, we hoped this season would be just as productive and so it proved, with similar figures to last year.
Our evening surveys produced a minimum total of 23 separate churring males holding territory and we had a minimum of 16 pairs. We found 22 nests, which breaks down as 14 first broods and 8 second broods, and a mix of 15 twins and 7 single chicks were spread across all our nests.
This year we resumed ringing our Nightjar chicks and managed to ring 27, with a minimum of 3 chicks we missed to add to the total. These were from two 1st brood nests, as we later had fledged juveniles on one territory when we found the 2nd brood nest, and on another nest a well-grown juvenile was roosting with the female as she incubated her second clutch.
Based on the locations of some of the 2nd brood nests, we probably missed at least 4 more 1st brood nests and similarly, a minimum of 4 more 2nd brood nests. Unusually, some territories which have been consistently occupied for years were empty this season so it’ll be interesting if that pattern continues next year.
Our first nest was found in late May and our last discovered in mid-August so we had the usual spread of breeding attempts throughout the season. We didn’t experience the long hot spell that characterised last summer but the bad weather in early June which wiped out our 2nd brood Stonechats didn’t appear to affect our Nightjars too much. Although we’ll never know if some of the 1st brood nests we didn’t find perhaps failed as a result of the weather.
We suffered 8 known failures – 5 at the chick stage and 3 on eggs, including clutches where the females disappeared. It’s an unfortunately high ratio to lose more than a third of the nests found. Predation must play a significant part, although for some nests abandoned at the egg stage, we found the eggs still untouched in those nests even weeks later.
As is typical with Nightjars, the nest areas chosen were quite varied. While the nest site itself is the usual bare scrape, what surrounds it can vary hugely. Some nests were in dense cover, typically heather, where you had to almost stand on top of the nest before you could see it. Others were sparse bracken, sometimes on the edge but often in the middle of thick growth. One nest was on wetter ground, on a raised area with a rhododendron stump but in the middle of very boggy ground.
Other nests were out in the open, some with virtually no cover at all, and at least one of those was a failure. Open nest areas made it easier to check the progress of the growing chicks from a distance without disturbance but it presumably also made it easier for predators. Some nests were in a clearing provided by birch sapling stumps or by clusters of bramble bushes in the middle of heather. One common factor with a lot of our Nightjar nests is their liking for sites against stumps or cut wood, which lends a helping hand to their incredible camouflage. There were also a surprising mix of nests on the level plateaux but also on slopes, where only a small area of level ground on a slope can be sufficient for a nest site.
Distances between nest sites was also quite interesting to map. As is usual, 1st and 2nd brood nests can be quite close to one another – literally within 10m in one case. And different pairs can nest in fairly close proximity – either at opposite ends of the same plateau or in one case, 2 nests were either side of the main path, probably about 50m apart.
Another interesting aspect of our Nightjar pairs is the consistency of nest site locations, which perhaps hints at it being the same female from year to year? One nest was not only in the same bracken clump but on exactly the same spot within that bracken. Other nests we can almost predict where they’ll be within a few yards, which certainly lessens the effort needed if we can walk straight to an area and find them with minimal searching.
We also had something we’ve never experienced before. One of our 1st brood clutches consisted of an egg of normal size and colour but the second egg was smaller and completely white. There was lots of speculation if this would even be viable (or even what would emerge!) but a normal sized Nightjar chick with equally normal plumage eventually hatched, grew at the normal rate and fledged successfully, along with its sibling. Interestingly, what we presumed was the same female then went on to produce 2 normal eggs for her 2nd clutch.
Also unusual was that some of our 1st brood chicks were being brooded by the male a lot earlier than usual. We usually find 1st brood chicks are around 10-14 days old before the male takes over brooding duties while the female starts incubating her 2nd clutch. This year when we were ringing 1st brood chicks, we found males brooding when the chicks were just over a week old.
This year we also started swabbing the chicks for a DNA sample and collecting faecal samples. This is an exciting project and forms part of a wider project across the country to look at Nightjars’ diet, as well as looking at the DNA linkages between different populations of Nightjar.
A variety of nest-finding techniques were used this season and we want to do further trials next year so we have as many options available as possible. As usual, monthly evening surveys were conducted reserve-wide, along with individual volunteers making extra evening visits so we could target our nest-finding more effectively. Historical knowledge of past nest sites was also very helpful and enabled us to carry out targeted nest-finding with considerable success, which minimises both volunteer effort and disturbance. Later in the season we started to use a thermal camera and it has potential, although it also has its limitations in the wide variety of terrain we have on Dersingham Bog, and more practice will be needed to refine how best to use it.
Roping is a traditional nest-finding technique for Nightjars and it’s very successful and we found many of our nests using this technique. It became an in-joke at one point that wherever Les was on the rope line, the Nightjar nest would inevitably be found in front of him! He even managed to find one nest just walking back to the car park one day, minus rope…
But roping does take a lot of effort and time, and needs a lot of bodies to cover the ground more effectively, and roping slopes is really hard work but the camaraderie of group nest-finding can also be a lot of fun and very satisfying when it works. Some abiding memories include a double rope line of more than 10 people covering a lot of ground one day and watching Tony walk into a hollow completely out of sight (no mean feat given his height) and also Les disappearing into head high bracken. We figured as long as the rope didn’t go slack and the bracken kept moving, we hadn’t lost him…
Blind luck also plays a part in Nightjar nest-finding, as it always does. We found several nests just by walking from one part of the reserve to another, or when working on our Stonechats. One memorable occasion, having checked a Nightjar nest with Sophie, hoping to ring the chicks but finding it had failed, we walked off the heath and promptly stumbled on the female brooding her 2nd clutch of eggs within 50m, and that nest went on to be successful.
Huge thanks, as always, to our survey volunteer team of Roger, Irene, Alastair, Tony, Keith and Les, and equal thanks to our work party volunteers and NE staff, Tom and Nick, who pitched in with the roping. Everyone put in a huge amount of effort this season and we’re very grateful for all their time and enthusiasm. And once again, we’d like to extend our thanks to Sophie Barker of the Norfolk Ornithological Association who gave up so much of her time to come along and ring Nightjar chicks while Roger was away.
It’s always immensely satisfying to follow the fortunes of Nightjars from their arrival on the reserve, spending time experiencing their behaviour and unique calls on the evening surveys, through to finding and monitoring their nests, to helping with the ringing. Watching the chicks grow from literally a ball of fluff that can fit in the palm of your hand to almost the size of the adults in barely 3 weeks, and seeing the development of their amazing plumage, will never be anything less than a privilege.
It’s heartening that Dersingham Bog NNR remains such a stronghold for breeding Nightjars in Norfolk and what we learn from each season helps to inform management work across the site. It’s been an immensely enjoyable season and, as always, we continue to learn more each year as we work on such an incredible bird.
By Alastair Steele & Irene Boston & thanks to Les Bunyan for the photos