Saturday, September 27, 2008


Recently Beverly asked me about bird vomit. In particular she wanted to know if the "material" ejected by the birds when it is used as a defensive mechanism was the same "material" used to feed the young.
I did a bit of digging and found out that prey derived stomach oils are used both as an energy source as well as a defensive/offensive tool, at least for petrels, the family of birds most often associated with defensive projectile vomiting. Oils are produced by both adults and chicks and it is indeed fed to chicks by the adults. The chicks also produce the oils as well.

Birds have 3 stomach-like organs as part of their digestive system - the crop (generally for quick food storage in seed eating birds), the glandular stomach, and the muscular stomach (for grinding food). The crop and gizzard are generally reduced in petrels and most of the digestion is done in the glandular stomach. All petrels except diving petrels (Pelecanoididae) are know to produce stomach oils and the ability to projectile vomit those oils and stomach contents is most developed in those species that nest in the open on the ground, such as the Southern Giant Petrel, which I have photos of here.

The oils are derived from the prey items of each petrel species and are thought to afford petrels a high energy, low volume energy source composed most often of wax esters and triglycerides. These oils may also retard digestion of food items and thus enable petrels to assimilate food over longer periods, which is advantageous for birds whose prey are distributed in widely spaced patches or whose young often endure long periods of fasting between meals. The oils may also provide water for fasting chicks through the internal combustion of the oils.

These oils are apparently quite effective in deterring predation and have been know to result in death of predators, particular bird predators whose feathers become matted and loose insulating value potentially resulting in death through exposure. Diving petrels are thought to have not developed these oils because they tend to forage close to breeding areas and nest in burrows.

Although it might seem like the loss of these oils while vomiting them at a predator may be maladaptive, the loss of life would most certainly be a greater impact to an individual that a loss of a meal. The evolution of this behavior may have also benefited petrels by giving chicks a way to independently defend themselves allowing both parents to forage when the chicks are younger therefor providing more food earlier in the chicks life, and nesting sites can be in more open areas, thus allowing expansion into habitats otherwise not available.

The petrel that I have worked most closely with is the Southern Giant Petrel, arguably the bird most adept in the art of projectile vomiting. They are large birds, nearly the size of a turkey and have a correspondingly large stomach. When disturbed they can project their oils about 2 meters. We called it "gack" and I can vouch for the fact that it is very oily, stinky, and persistent. Giant petrels are also well known scavengers, so the projectile often contains more than just prey derived oils! Their bills are well adapted to scavenging with the lower mandible shaped much like a curved wood chisel well suited to jabbing into a carcass; the upper mandible is sharply pointed and hooked to then grab and tear out chunks of flesh to wolf down.

Despite being known as the Antarctic vulture with photos often taken of individuals head deep in a recently deceased seal, these birds can be remarkably endearing. The birds I worked with had become accustomed to having researchers visit their nests rather often and although some individuals never did get used to us, others were quite accommodating. Often we were able to gently approach a nest and reach under an incubating bird to rotate the band on the bird's leg so we could read all the numbers on the band. One of my favorites was a pure white bird that often tried to move me under her with insistent downward nudges of her bill on my arm so she could incubate me as well as her egg.


Armstrong E. 1951. Discharge of oily fluid by young fulmars. Ibis 93:245-51.

Warham J. 1977. The incidence, functions and ecological significance of petrel stomach oils. Proceedings of the New Zealand Ecological Society 24:84-93


Beverly said...

Wow! Thanks so much for this, John. If you don’t mind…I’ll be linking back to your answer…and the fascinating link you left, too!

Dang! “Some 2796 gallons were sold after the 1976 season as a by product of the harvesting of 549352 squabs” ???!!! That’s a LOT of squab for a single season! You’d have thought we’d have learned manners (or at least restraint) by that time. [sigh obviously not even yet! ]

Am I to understand by reading this:

“Petrels readily feed on fat and oil released onto
the sea during whaling, sealing and fishing operations.
Indeed fat has long been used as a lure for petrels.
In the open oceans, oil slicks also occur naturally,
like those containing wax ester and triglyceride
described by Lee and Williams (1974) which probably
resulted from a massive mortality of Calanus species.
Such slicks may provide yet another source of petrol
stomach oil.”

…that the oil slicks which ‘occur naturally’ are those from the animals processed during fishing & hunting operations and NOT from leaks and crashes? Lordy…if there are massive die-offs from the birds eating our pollutions, in addition to being trapped in them, is more than sad.

But even sadder was the last piece I read:

“…it is unfortunate that the chicks of burrowing species
are unable to spit oil, for it is these that suffer the
greatest losses from predation by introduced cats.”

Gads…is NO place sacred; are there those damned feral cats EVERYWHERE???

Okay, back to my thanks; Thank you AGAIN, John…for bringing to us absolutely fascinating information and for so kindly answering my questions. I really appreciate your taking the time!

Beverly said...

Oh...and I meant to add something about that word: Gack!

I have three brothers...and we were pretty rough n' tumble and into nearly everything. Especially trying to gross each other out. 'Gack!' was something we used to say to one another (or school-mates) when something was disgusting or stupid.

At the time, we'd never even heard of fulmars! LOL

We were a vocal bunch...

John Carlson said...

Thanks Beverly,
Glad you liked the post. I think that you may have misread the section you quoted concerning naturally occurring oil slicks. The one specifically mentioned is a result of a massive mortality of a Calanus species, a small marine copepod. They are rich in the oils and they are often an important prey item for many sea creatures.

Beverly said...

LOL Yup, I don't know all those Latin names for our various flora and fauna...and thought the die-offs were birds. Sheeshhhh. But then again...that's why I asked the expert! :) I even had to look up 'copepod'...

I'm going to go ahead and post my 'update'...and link back to your post. If that's not okay, please let me know.

Thanks again...this has been most interesting! I also enjoyed reading the PDF file to which you linked.