Tuesday, February 17, 2015

This snake loves omlettes... al fresco!

An amazing phenomenon in the bird world is the fact that nearly every species of bird lays two eggs in the tropics. There is a general trend to have larger clutches as one moves towards the poles. This is partially due to different evolutionary lineages replacing each other as one moves away from the tropics but, even in the few species with huge ranges, clutches increase.  

Even with just two eggs/breeding season, most species in the tropics could potentially double their population in a single year should every egg turn into an adult. But tropical species can live for several years so populations could increase several-fold. However, avian population in the tropics are relatively stable. This means that a pair of birds, on average, only replace themselves. So if a bird lays 16-30 eggs in a lifetime, only 2 survive. 

That would be an average per individual in a population. The reality is much more noisy. There are probably birds that are never successful and don't replace themselves and other birds are extremely productive and have enough offspring to offset the losses from other birds. 

But why is failure so common? Could it help explain why birds are lost from forest fragments? 

Ph.D. Candidate Deb Visco and her advisor Dr. Tom Sherry from Tulane University (now deeply immerse in Carnival!) set out to measure nest predation in the Chestnut-backed Antbird in a large forest tract and a forest fragment in Costa Rica. 

Costa Rican understory at La Selva

To get predation rates and identify predators they set up digital video recorders. And record they did - over 22,000 hours of video were taken. What did it reveal? They captured forty-six predation events: one ocelot, one four-eyed possum (no idea what that is), a semiplumbeous hawk, two fire (not army!) ant swarms, and forty-one snake predation events. Amazingly, 80.4% of predation events were by one species of snake, Pseustes poecilonotus (see below). Please follow the link to the article to see pictures of the snake in action. I've seen my Ph.D. advisor give a talk to packed room on nest videography in the tropics and it makes for a great presentation. 

Photo credit Maxime Aliaga see http://maxime-aliaga.com for more photos
Were nest predation rates higher in the fragments? Nope. So why are forest fragments losing species. Nobody knows for sure but I strongly suspect it may have to do with a combination of dispersal abilities (can they get there?) with microclimate/microvegetation preferences (is there the right habitat if they do get there?). 

Hats off to the investigators for one of the most thorough investigations of nest predation in the tropics! Well done guys! 


  1. This is paywalled for me, so I must ask: how does a fire ant swarm eat an egg?

  2. It may have been the chicks but they chew away at the shell as far as I know. When I worked in GA on bluebirds I came across a nest that was recently invaded by fire ants and it was horrific - five living chicks smothered in hundreds of ants. It does seem like their little jaws would just slip on the egg - like us trying to bite into a beach ball sized jaw breaker.

  3. Great question! I did not expect it to happen the way it did, and my crew and I were quite confused at first! We found the nest abandoned and the eggs partially buried under a small pile of dirt in the middle of the nest cup. The fire ants had tunneled up through the bottom of the nest and buried their food! Eventually, the whole bottom of the nest fell out. The fire ants probably couldn't get to the goods inside the egg until it rotted or cracked, but they were able to stash it for later! I have video of when the initial swarms came through. The parent incubating was clearly disturbed and tried to pick off the intruders. Eventually, she abandoned.