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Morning All.  Your humble servant is guest hosting this week.  You may have noticed the rather trashy title I have chosen in a cheap ploy to draw in more readers.  Think of this diary as a late night movie that is really really late.  So late that it isn't night any more.

Hopefully this will be a bit better than 'The Giant Claw' (1957, distributed by Columbia Pictures) one of relatively few bird-oriented horror movies (other than 'The Birds' itself) and the only one, to my knowledge, that features a giant bird.  But with the same sensationalistic appeal.

OK, let's start over.  The birds that I'm going to discuss today are pretty weird and they do bear a general resemblance to our friend above.  However they don't eat planes or perch atop the Empire State Building.  They are Hoatzins

The Hoatzin, Opisthocomus hoazin, is a South American bird found in the Amazon region and neighboring areas to the east of the Andes.  Hoatzins are well known for a couple of features that set them apart from all other birds.

1) The juveniles have claws on their wings, something that appears to be found in no other living bird species.  This caused many scientists to speculate, when these birds were first described, that they represent an ancient lineage that has remained unchanged since the early days of bird evolution when wings with claws were common as dirt.  The nests are fairly low in vegetation emerging from the water.  When threatened by predators the chicks apparently dive into the water and then climb back out after the danger is past using the claws.

It is now known that Hoatzin claws are not a characteristic passed unchanged from the origin of birds.  Although the evolutionary relationships of the Hoatzin are a bit uncertain it does seem certain that they fall within the main group of modern birds, most likely related to the cuckoos.  It would be very interesting to study the developmental genetics of the claw formation as it probably involves the reactivation of a developmental pathway present in dinosaurs that was deactivated during the evolution of modern birds but not lost.

2) Hoatzins eat leaves.  I got a little of bit of flack on this topic in an older diary in which Hoatzins were mentioned.  Readers mentioned several other birds which are known to eat leaves.  What sets the Hoatzin apart is that it is the only bird known to eat a diet that overwhelmingly consists of leafy vegetation.  A study of Hoatzin diet cited in Wikipedia found that over 80% of their food intake was leaves with the rest being fruits and flowers. Leaves are an abundant but poor source of nutrition.  Grazers often have a fairly low energy lifestyle, reflecting the lack of a need to move around very much to get food and the relatively low amount of energy coming in per unit of food consumed.  Given that leafy food is bulky is has long been speculated that both the energetic and aerodynamic demands of flight preclude flying vertebrates from being grazers (the small size of insects means that flight is much less energetically expensive).  Hoatzins seem to be the one exception to this rule.

For an exotic bird in a remote part of the world, Hoatzins are relatively easy to see for the ecotourist.  The birds live and nest in colonies along the banks of lakes, swamps, or very slow flowing small rivers.  Given that they are large, colonial, sedentary, and noisy it is not hard to find them.  I have seen them every time I have gone to Ecuador.  The majority of my observations have been at a small ox bow lake at Tiputini Biodiversity Station.  However I have also seen them in the area around Kapawi Ecolodge in southeastern Ecuador.  The Hoatzin is the symbol of Kapawi and the birds are abundant there.

Hoatzin habitat at Tiputini.  This was taken during a period of low water level (note mud on the vegetation).  I'm not sure how the birds deal with the extreme fluctuations in water level while nesting

So that's a little general info on our friends the Hoatzins.  I thought I would poke around in the dusty electronic archives of academia and see what new and exciting stuff scientists have discovered about the sole member of the family Opisthocomidae. (Note: Opisthocomus means wearing long hair behind which is apparently a reference to the crest of feathers).

It turns out that the major focus of Hoatzin research has been the microbiology of their digestive tract.  There has also been some work done on the impacts of ecotourism on their colonies, and a fairly small amount of basic ecological work.  Surprisingly, I didn't find much on their social behavior.

Diet and Gut Microbiology

The major area of Hoatzin research is linked to their largely folivorous diet.  Like grazing mammals, Hoatzins have a number of digestive features to help them get the most out of their diet.  Instead of the complex stomach of a ruminant, Hoatzins have an enormous crop which allows them to both physically and chemically break down food.  They also have a complex community of micro-organisms in their gut which gives them assistance with the breakdown of plant compounds.

The diet of Hoatzins varies from location to location across their range and their gut flora varies as well.  They appear to have a core gut flora found in all populations and then each location has some bacterial types specific to that area.  Eating plants has two digestive challenges.  One is breaking down and extracting nutrients from plant tissue.  The second is dealing with the plant secondary compounds.  Secondary compounds are toxins produced by plants as defenses against herbivores.  Insects, which are typically specialists, feeding on a single plant species or a range of closely related species, frequently evolve resistant to the toxins of their hosts.  Mammals often deal with plant toxins by feeding on a wide range of different plants to avoid ingesting too much of any one toxin.  There are reports of Howler Monkeys falling out of trees because of accidentally overdosing by eating too much of one type of leaf.  Preliminary studies seem to indicate that Hoatzins select young leaves which are more easily physically digested but that they don't avoid eating plants known to have fairly toxic leaves.  Presumably their gut flora is helping them to detoxify their food but results seem to be pretty preliminary at this point.

The crop of the hoatzin is functionally equivalent to the rumen of a cow allowing for an initial breakdown and energy extraction from plant matter with a follow up in the hind gut.  A studied compared to foregut and hind gut flora of cows and Hoatzins and found that they had a common pattern indicating that the convergence between the two systems happened at the microbe level as well as the structure of the gut itself.

Ecology and Conservation

Relatively little basic ecological field work seems to have been published on Hoatzins which is a bit surprising given the ease of field observation.  Some basic life history work has indicated that breeding is seasonal and is timed to occur during the rainy season.  Speculation is that high water levels during this time period provide the best protection for chicks.  Clutch size is small, averaging two eggs per nest.  This is typical of tropical song birds and this result indicates that it may be true for tropical forest birds in general.

A couple of studies have looked at the impact of ecotourism on Hoatzins.  They found that the presence of tourists had a negative impact on fledglings but not nestlings or adults.  Noise seemed to be the more important factor and the abstract of one of the papers ends with this great piece of dry scientific understatement.

Although not tested, silence is probably the best strategy when looking for many wildlife species.  

That is it for our friends the Hoatzins.  Below are some of the studies I skimmed to glean info for this diary.  Most of the more general information comes from Wikipedia.

Differences in crop bacterial community structure between hoatzins from different geographical locations
Author(s): Godoy-Vitorino Filipa; Leal Sara J.; Diaz Wilmer A.; et al.
Source: RESEARCH IN MICROBIOLOGY  Volume: 163   Issue: 3   Pages: 211-220

Comparative analyses of foregut and hindgut bacterial communities in hoatzins and cows
Author(s): Godoy-Vitorino Filipa; Goldfarb Katherine C.; Karaoz Ulas; et al.
Source: ISME JOURNAL  Volume: 6   Issue: 3   Pages: 531-541

Sound the stressor: how Hoatzins (Opisthocomus hoazin) react to ecotourist conversation
Author(s): Karp Daniel S.; Root Terry L.
Source: BIODIVERSITY AND CONSERVATION  Volume: 18   Issue: 14   Pages: 3733-3742

Rumen-like methanogens identified from the crop of the folivorous South American bird, the hoatzin (Opisthocomus hoazin)
Author(s): Wright Andre-Denis G.; Northwood Korinne S.; Obispo Nestor E.
Source: ISME JOURNAL  Volume: 3   Issue: 10   Pages: 1120-1126

Nesting behavior and breeding success of Hoatzins
Author(s): Muellner Antje; Linsenmair K. Eduard
Source: JOURNAL OF FIELD ORNITHOLOGY  Volume: 78   Issue: 4   Pages: 352-361

Consumption of toxic plants by the hoatzin
Author(s): Dominguez-Bello M. G.; Aguiar R. E.; Garcia-Amado M. A.; et al.
Source: JOURNAL OF ANIMAL AND FEED SCIENCES  Volume: 16   Supplement: 2   Pages: 302-306

Exposure to ecotourism reduces survival and affects stress response in hoatzin chicks (Opisthocomus hoazin)
Author(s): Mullner A; Linsenmair KE; Wikelski M
Source: BIOLOGICAL CONSERVATION  Volume: 118   Issue: 4   Pages: 549-558

Originally posted to matching mole on Sun Jun 10, 2012 at 05:58 AM PDT.

Also republished by Birds and Birdwatching, J Town, and SciTech.

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