Why the hornbill shuts its nest

Von der Decken Hornbill nest cavity - it's tiny!

 This weekend I enjoyed a camping trip out to Simanjiro with some friends, and as well as finding some very cute new-born wildebeest, we found a number of nests. One of these, just near our campsite, was a pair of Von der Decken's Hornbill. All the hornbills of the genus Tockus have two fascinating pieces of nesting behaviour that it's well worth thinking about: firstly, after choosing (and sometimes modifying) a suitable nest cavity, the female climb inside and then plugs the nest hole with mud, faeces and other grot, sealing herself in until there's only a tiny slit through which the male feeds her and, later, her chicks. Secondly, as she's sitting there she carries out a simultaneous moult, meaning she drops all her flight feathers at once and is unable to fly. Nearly all birds moult their feathers once per year (larger birds sometimes take longer), but most do a sequential moult, meaning they drop feathers one after the other and replace them as they go, thus retaining the ability to fly throughout. (There are exceptions, of course - notably ducks and geese who also drop all their flight feathers at once.) In the case of the female hornbill, however, she goes in the nest hole, shuts the door and a little while later drops all her flight feathers. Interestingly, if she doesn't breed, she moults sequentially, just like the male, so there's some suggestion that the simultaneous moult strategy is triggered as a hormonal response to the dark interior of the nest. Now, that's the story that you'll read in all the papers (e.g. here and here), but it can't be the whole story as I was trying to find some pictures to point you to of birds inside nests looking all naked, and I couldn't. In fact, here are a few pictures that show females in the nest - this one is an African Grey Hornbill with smallish chicks, but clearly well feathered mother with fairly worn plumage - she's not just completed a moult. And here's an Asian species also with young chicks who certainly doesn't look in moult to me. I'm sure it happens though, and may be the norm, but there are obviously exceptions that haven't yet made it into the literature.

Male Northern Red-billed Hornbill brings insect to nest

Still, when looked at in detail there are some remarkable things happening here. Firstly, note that it's the female who seals up the nest, not the male. This helps us understand what the evolutionary purpose is, ruling out the theory that it's a defence by the male seeking to ensure paternity. In fact, the adaptations required to manage this sort of breeding strategy would also lend themselves to reducing male paternity assurance, as the hornbill has evolved a mechanism to store sperm for a very long time. Typically, the female enters and seals the nest cavity a week or two before she starts laying eggs. She therefore has to be able to store sperm from the male to fertilise the eggs as she develops them - most birds can only do this for a few hours at most, but she managed for weeks - once she's shut in the nest she doesn't come out again. What's more, she can fertilise several eggs (hornbills often have up to five eggs, a remarkably large clutch for such large birds in Africa) from the same store of sperm. That ability would mean that in the week or two before she enters the nest she could easily find time to sneak of and find the top male in the area for a quick mating. But interestingly, and very unusually for birds, there's no evidence this happens when the DNA of the chicks is compared with the parents. There's no clear reason why the female doesn't take advantage of this opportunity, but from the fact she clearly doesn't in at least the studied species, if seems likely it's strongly in her interests too, not to seek extra-pair copulations. The best guess at the moment is that since the female and her chicks are completely dependant on the male for food a female should choose the father of her young who is best at finding food, rather than one that has particularly good genes. Though I have to say I can't really see how that would stop her seeking good genes too...

Male Northern Red-billed Hornbill feeding female

Still, why do hornbills seal themselves into the nest? The obvious answer is nest defence. Threats can come from two sources - from obvious predators, potentially including Harrier Hawks, and a variety of snake and mammal predators, but also from other hornbills and other species that like to live in cavities too. Certainly predation risk is extremely low for hornbills, especially when compared with the generally high levels experienced by other tropical birds. So defence against predators is a strong advantage of the sealed nest approach. but there's also evidence to suggest the biggest threat is having the nest cavity taken over by other hornbills, who would kill any young they find in the nest if given the chance. I suspect the combined effect is a complete winner!

Feeding takes place through the sealed cavity. Note the black eye.

There is a risk though - if the male should die whilst the female has moulted all her feathers and is unable to fly, that's it for her and the chicks. Yes, she can break out of the nest (usually female Tockus hornbills wait until they've completed their moult, then break out of the nest cavity sometimes resealing the chicks in, sometimes the chicks themselves do that), but if she can't fly that's no good for anything. Happily, it's extremely rare - monitoring of 600 nests of one species found only two cases where males vanished, perhaps due to predation. And if it were to happen early in the nesting attempt, or once the female's moult was complete she can break out early (it has certainly been observed before, contrary to what some suggest) and, although she will be unlikely to succeed in the breeding attempt, will at least live to see another day.

Ruaha Hornbill, Serengeti. Note the pale eye and dark skin.

So, all in all, a pretty remarkable breeding strategy. Hornbills are very interesting birds! And that's before we even mention their distinctive 'ballistic transport' method of feeding or their major ecological role as seed dispersers in tropical forests (and probably woodlands too). One last thing to mention, perhaps, is the fact that taxonomists are busy splitting a number of groups into newly recognised species. In East Africa the Red-billed Hornbill group is the most obvious of these, with two species now recognised in Tanzania, the Ruaha Hornbill, from Serengeti down to Ruaha and the south, but Northern Red-billed in the north - Tarangire to Mkomazi (check that link for the original description and maps). A further undescribed species might be lurking in Samburu area of Kenya, so if you're up there keep your eyes open (and take some photos too!). Here in the Tanzania, as you can see in the photos, Northern has a black eye in yellow skin (all the breeding birds are this species, taken in Tarangire), whilst Ruaha has a yellowish eye in black skin. And if you see Red-billed hornbills in northern Serengeti / Maasai Mara, look very closely and take a GPS record so we can discover where the distribution limit it. My furthest north observation of Ruaha Hornbill so far is in central Ikorongo...

[With many thanks to Ron Eggert and Daudi Peterson for use of their photos of nesting hornbills (and a fun camping weekend! Check their website for many amazing pictures of many Tanzanian birds.] 

Main References:
KALINA, J. (2008). Nest intruders, nest defence and foraging behaviour in the Black-and-white Casqued Hornbill Bycanistes subcylindricus Ibis, 131 (4), 567-571 DOI: 10.1111/j.1474-919X.1989.tb04791.x  

Stanback, M., Richardson, D., Boix-Hinzen, C., & Mendelsohn, J. (2002). Genetic monogamy in Monteiro's hornbill, Tockus monteiri Animal Behaviour, 63 (4), 787-793 DOI: 10.1006/anbe.2001.1975  MUVRGH6EAW78

MILLS, M., BOIX-HINZEN, C., & PLESSIS, M. (2004). Live or let live: life-history decisions of the breeding female Monteiro's Hornbill Tockus monteiri Ibis, 147 (1), 48-56 DOI: 10.1111/j.1474-919X.2004.00340.x