I have spent the last 3 summers monitoring cuckoos and their reed warbler hosts at Wicken Fen Nature Reserve, guided by the steady hand and vast wisdom of Nick Davies: the master cuckoo whisperer. What an immense privilege it has been to learn about this fascinating system and landscape first-hand! If you also want to immerse yourself in the intricacies of life as a cuckoo on Wicken fen, then read Nick's exquisite book 'Cuckoo: cheating by nature'. To see the fen and it's inhabitants through Nick's eyes is to unlock a secret world. And this little world has firmly etched a place in my heart.
When I first started out on this project, it was a massive sensory adjustment for me to be surrounded by water and lush greenery after 6 years of Kalahari desert and red dust. I can't help but identify with the reed warblers and cuckoos who excitedly sing with the arrival of spring, and then pack up in search of Africa before the autumn seeps in. Late September every year I experience my own 'Zugunruhe', and dream about filling a backpack for African migration. But in summertime, I have begun to feel quite at home amongst the reeds.
This season's cuckoo highlights have been....
Watching a reed warbler build a nest insulated with sheep's wool and then discovering that a cuckoo had been watching too! She certainly laid her egg in the cosiest of nests that was available in mid-May. I thought to myself, that's where I'd put my egg if I had one! But of course, cuckoos lay lots of eggs, so not all of the nests that she used were so superb.
Observing the fascinating deimatic behaviour of cuckoo chicks. Cuckoos remain dependent in the nest for much longer than the reed warbler's own young. Presumably the reed warbler nestlings leave as soon as they can in order to avoid predation, since the longer they spend in one spot and the bigger and noisier they get, the greater the risk of being discovered and eaten. The cuckoo takes longer to fledge and uses a different strategy - any predator approaching the nest by this stage will presumably be as surprised as I am to see a flash of flame red and a lunging, snapping mouth, rearing up out of the nest! It definitely doesn't evoke the impression of a delicious baby bird.
As this season draws to a close, although slightly weary and worn from fieldwork, I am already missing the sneaky cuckoos and the gentle reed warblers. The ephemeral nature of this habitat makes it feel as though it might have all been a dream - one day a peacock butterfly can be feeding on lush blossoms as swifts scream above, but soon enough the same scene might feature only the rustle of dry yellow reeds and a sullen sky.
Even though there will be no chance of cuckoos, the pink skies and starling murmurations will entice me to return to visit the fen over the winter. Or perhaps because absence truly makes the heart grow fonder, I will simply visit the winter fens to mourn the summer, with regret for any day that I took for granted amidst the hustle and bustle of busy data collection.
Travelling through Southern Africa in October, I was excited to photograph these characters...
The weaver birds (Ploceidae) are famous for their elaborate nest construction. They mainly inhabit sub-Saharan Africa, although a few species occur in Asia too. For many of the Ploceus weavers (e.g Village and Masked weavers), the complex nest is crafted by the handsome male, who weaves strips of grass and leaves together using a similar technique to human weavers. Somehow he can complete this exquisite construction within a day or two!
Once the nest is complete, the males try to attract females by hanging upside-down underneath it and fluttering their wings whilst producing a metallic 'churring' song. If a female approves of a male's nest, she lines it before she mates with the male and lays her eggs. Afterwards, the male's attention rapidly shifts towards building more nests and attracting as many females as he can.
Female weaver birds have some battles of their own to fight. For starters, other female weavers in the colony might try to sneak their eggs into her nest! But there is a far more sinister invader about which she should be concerned. A number of these colonial breeding weaver bird species (such as Village, Masked and Cape weavers) are hosts for the brood parasitic diederik cuckoo (Chrysococcyx caprius).
Although the diederik cuckoo males are gorgeous metallic green-backed birds with vivid red eyes, their females are somewhat duller and more difficult to spot. The females have to be sneaky, so that they can slip in and deposit their own egg in a weaver bird nest the moment the opportunity arises.
Weaver birds and diederik cuckoos have provided an interesting natural experiment because weaver birds were introduced from mainland Africa to the islands of Mauritius and Hispaniola, where they live life diederik-free. Studies conducted by David Lahti suggest that the absence of diederik cuckoos on the two islands has resulted in the weavers eggs becoming less variable in appearance across females and less consistent within a given female's clutch. So without the pressure of brood-parasitism, weaver birds appear to have reduced the pigmentation that they add to their eggshells. However, the weaver birds are no less likely to reject foreign eggs than mainland birds, when the harder task of distinguishing between eggs due to the greater within-clutch variation is accounted for.
So it seems as though weaver egg appearance on mainland Africa has evolved in response to parasitism by diederik cuckoos. This has arisen via an evolutionary arms-race between cuckoos producing mimetic eggs to fool the weavers, and weavers producing eggs that minimise the risk of accepting a cuckoo egg or rejecting their own eggs by accident.
I enjoyed reflecting on these fascinating interactions between diederik cuckoos and weaver birds as I watched this male diederik relentlessly calling out to attract a female... "dee-dee-dee-dee-derik". Meanwhile, in a tree not more than 50 meters away from their foe, the weaverbird males hung upside down flapping their wings and churring to impress their own ladies, blissfully unaware of the imminent dramas that would unfold in their carefully woven nursery baskets.
I have a passion for robins. And a passion for Africa. Out of these two separate interests a quest was born! A special zoological quest that took me back to my childhood home in Zimbabwe, to marvel at one of the most elusive and rare robins on the continent...
My first independent research project as an undergraduate was to study the song of European robins in the green spaces around the City of Bristol. This left me with inordinate fondness for these confiding, beady-eyed balls of bright red fluff! On finding Terry Oatley and Graeme Arnott's delightfully illustrated "Robins of Africa", I realised it was time to return to Africa once more, but this time to visit Zimbabwe for the first time as an adult.
I left Zim when I was 8 years old, but it had a lasting impression on me and is no doubt the reason why I study the behaviour of animals. As a kid I was fascinated by the chameleons and agamas that I would find and then carry around. I stared in wonder at the big grey mammals that could destroy your tent or car, or even you if you made the wrong move, but were typically gentle giants. The big predators struck instinctive awe and fear into me, especially after one time hearing a lion roar not meters away from our tent in the dead of night. But the birds, the birds were often flitting gems that I couldn't quite capture and comprehend in the detail they deserved.
The youngest of 4 kids, I travelled in the boot of our Renault hatchback with the luggage. A benefit of this position was ready access to the field guide books that I would flick through to while away the long bumpy hours over corrugated dirt roads. In these guides, I glimpsed the beauty and variety of flora and fauna that Zim had to offer. And now, over 25 years later, I have sought to capture some of this with grown-up eyes, and lots of optical gear. So I decided that it was time to go in search of Swynnerton's robin in the Bvumba forests in the Eastern highlands of Zimbabwe.
Swynnerton's robin is interesting for two main reasons. The first lies in its name: Swynnertonia swynnertoni. The latin and common names were given to commemorate the entomologist, and Fellow of the Linnean Society, Charles Swynnerton, who tragically died en route to collect a prestigious award for his contributions to tsetse fly research. Swynnerton was a keen naturalist and his interest in cuckoos led him to suggest that host birds might paint their eggs with 'signatures' to help them detect foreign cuckoo eggs in their nests. And in time these ideas were vindicated by the work of Claire Spottiswoode on cuckoo finches. Interestingly enough, Swynnerton's robin is also recorded as an occasional host for the red-chested cuckoo.
The second reason that Swynnerton's is a particularly interesting robin is because this is the only African robin species for which the female can be readily distinguished from the male in the field based on her olive-green instead of dark grey face. The male puffs out the sharply contrasting black and white bib on his neck when he conducts his bobbing displays to the female to entice her. However, we don't yet understand why this sexual dimorphism has emerged in just this one African robin species.
So with all this in mind, it was a spectacular thrill to creep through the tangled undergrowth of the Bvumba and spy a pair of these special birds. We followed them intently as they foraged in the understory. The light environment was a photographic challenge, but then this handsome male briefly popped out into a clearing, lingering just long enough for me to take this single photograph.
A successful quest! But I am left even more curious and enchanted by the Bvumba forest and its inhabitants. Now listed as 'vulnerable' and with suitable habitat rapidly declining, this fascinating and unobtrusive forest-dweller could sadly fade away as revenue from international tourism and conservation efforts have declined in the area. This would be a real shame, not just for Swynnerton's robin, but for the many other fascinating species living in the region that deserve our appreciation and protection.
I hope that I will return to see more of these fascinating birds in the Bvumba one day!
The biennial meeting of the International Society for Behavioural Ecology in Exeter was exciting, inspiring and quite exhausting! Lots of brilliant talks, catching up with old colleagues and friends, and of course meeting some new folk, kept me very busy for the week! I chaired a fascinating session on brood parasites and presented a poster on my work on testosterone and song, which led to lots of interesting chats with people.
Can't wait for the next one in 2018!
Not long ago red kites were facing extinction in the UK. Thanks to hard work and dedicated conservation efforts, the red kite population is now doing quite well. So well in fact, that a visit to Gigrin farm (a red kite feeding site) is nothing short of a wildlife bonanza! The skies of this valley in mid-Wales are filled by hundreds of these majestic birds every afternoon.
Although it is an absolute privilege to witness the aerobatic displays performed by the red kites, for me the real treat is observing the interactions between the birds. And it's not just the red kites that turn up for the free lunch. Rooks, crows and buzzards were all keen for an easy meal!
You can almost feel the tension in the air as the birds circle above Gigrin valley. After the food is delivered, there is a hesitation before the feeding begins - no one wants to be the first to take the dive and swoop down to take a snack. It seemed to be the role of the buzzards to set the pace - as soon as a bold buzzard touches down, an air raid of red kites rains down to snatch tasty morsels and devour them on the wing.
Much of our current understanding of testosterone comes from studies of species that inhabit the northern temperate region of the globe, perhaps unsurprisingly, because northern temperate species are readily accessible to universities in the developed world. From these studies there is a reasonable amount of evidence that testosterone plays an important role in the production of song by male songbirds. But there are very few studies of birds in other regions and this is important because the role of testosterone is not necessarily conserved across species in these different environments. So we need to study tropical and subtropical species too.
And so I set out to ask:
does circulating testosterone regulate male song production in white-browed sparrow weavers in the kalahari?
Throughout the breeding season, I got up at 3am to monitor male song production (they sing their solo songs almost exclusively at dawn) and I caught the birds late at night to collect a blood sample (when individual males can be readily caught from their sleeping chambers). It was hard and tiring work that meant I had to sleep in short (3-4hr) shifts at night and in the middle of the day (when temperatures were over 30 degrees C!). I then spent weeks analysing the samples in a collaborator's laboratory in order to measure testosterone concentration, and more weeks analysing song files.
And when all the data was in, the answer came back: NO.
There was no evidence that circulating testosterone concentrations were directly related to dominant male song production. or that males who sang more had higher testosterone than males that sang less.
Digging into the literature it quickly became clear that correlative evidence for a direct relationship between song production and circulating testosterone was not all that common. On the other hand, experimental manipulation of circulating testosterone did affect song production in many cases. So did our result mean that there is a latitudinal difference in the role of circulating testosterone for male song production? Or was there more to it? If the relationship between circulating testosterone concentration and song production is not strong and consistent across studies, then perhaps there is something more complicated going on under the hood?
And indeed, recent work indicates that this is very much the case. There are at least 3 important reasons why a direct relationship between circulating testosterone concentration and song production might not be necessary or could be obscured:
1. variation in receptor expression - the androgen receptors that circulating testosterone bind to can vary in number or density, which could account for the effectiveness of circulating testosterone concentration without variation in circulating testosterone levels
2. local conversion - aromatase coverts circulating testosterone into estradiol, which may then bind to estradiol receptors at target sites, therefore the concentration of estradiol may be more directly related to behaviour than that of circulating testosterone
3. local synthesis - testosterone and other steroids can also be synthesised locally at target tissue sites, which means that peripheral measurements don't necessarily tell us an awful lot about what is going on in the brain or at other specific sites.
So what does this all mean? Well it means that there is still a lot more work to be done! We need to better incorporate an understanding of the complexity of neuroendocrine mechanisms into evolutionary hypotheses about the role that testosterone may play in life-history trade-offs and animal communication systems. Often, working hypotheses are based on the assumption that there is a simple concentration-dependent effect of testosterone, which need not be the case.
More generally, we need to take the approach that hormones are more than mechanistic links in the translation of genotype to phenotype. Due to their pleiotropic effects on gene expression, hormones can structure genetic variances and covariances that determine how a population responds to selection. But hormonal mechanisms are more than simply the circulating hormone acting alone, so we need to characterise the other components too.
Further studies of songbirds outside the northern temperate zone are now essential in order that we can begin to understand whether endocrine systems are conserved or divergent across species, and why this is the case.
You can read more about this work and the associated literature here in the open access article.
Orchids belonging to the genus Ophrys, such as this early spider orchid (Ophrys sphegodes), have an elaborate pollination mechanism. They emit pheromones that mimic those produced by female solitary bees to attract a male!
Male bees are attracted to the scent and in trying to copulate with the flower, the pollinia becomes attached to the bee and is carried away to another flower to complete pollination. While these early spider orchids might not look exactly like a female bee, the combination of the sexy scent, and a few key tactile and visual cues, is enough to confuse the amorous male bee!
I was thrilled to spot these early spider orchids in flower just near Dover. Other Ophrys species include the bee orchids and the fly orchids.
Male birdsong is known to perform two key functions: attracting a female and repelling rival males. In kalahari white-browed sparrow weavers (Plocepasser mahali), the dominant males monopolise access to breeding opportunities, both within- and extra-pair mating all goes to the dominant males: subordinate males don't get a look-in. Dominance tenures are also very long, therefore in order to breed, subordinate males must acquire a dominant position.
Dominant male white-browed sparrow weavers sing pretty much every day at dawn during the breeding season. Their territories are very densely-packed so a group's roosting tree might be just 100m from their neighbour. Waiting in the darkness at dawn, it is like sparrow weaver surround-sound as all the males in the area one-by-one begin their dawn song performance. You really get a feel for what's going on in the neighbourhood. So it occurred to me, perhaps subordinate males monitor the dominant males in the neighbourhood. This might give subordinates a good idea about their future prospects of promotion - if a dominant male's song performance is poor, then perhaps he is on his way out or is temporarily weakened and now is a good time to attempt a take-over.
So the first step was to ask: is dominant male dawn song performance a good indicator of current condition? I gave dominant males a small immune-challenge to see whether this would affect their dawn song performance. Would they stop altogether or would they sing less than the previous day? And across males, would the strength of their immune response be related to their dawn song performance?
The answer was yes... dominant males sang less the day after an immune challenge, but interestingly, they all still sang. The more a male sang on the day before the immune-challenge, the stronger was his immune-response to that challenge, suggesting that, at least in this species, dawn solo song might tell other birds something about a given dominant male's current immune status. The open access paper is available here if you want to know more.
The next step would be to find out whether subordinate males utilise this information during dispersal, in order to maximise their chances of winning the jackpot and becoming the dominant male in a group.
If you are considering a trip to Madagascar ... just go!
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