Petrels poisoning their own chicks

The seagoing petrels, an order which includes 108 species such as albatrosses, shearwaters, fulmars and storm petrels, can fly long distances in search of food for their own consumption and to take back for their chicks. To counter the energy expired in these long journeys, the birds have evolved an unusual way of storing the food that increases its energy value.

They swallow the food to an area called the proventriculus, situated between the crop and the gizzard, where it is partially digested into oily and aqueous phases. The various components of the prey are partitioned between the two phases, with oils, fatty acids, hydrocarbons, acylglycerols, cholesterols and alcohols accumulating preferentially in the oily phase to create a high-energy food source. In fact, the energy density has been estimated to be 5-35-fold higher than that of the prey from which it was produced.

The oil is retained in the proventriculus longer than the aqueous fraction and is available for regurgitation when the birds return to their nests. The nutritional benefits of these stomach oils have been well defined but they can also act as a Trojan horse.

As well as accumulating energy sources, the oils are a sink for chlorinated contaminants. Organo-chlorine pesticides and the ubiquitous PCBs become concentrated on the ocean surface, due to their hydrophobic nature, and are ingested by the birds, along with pollutants that are present in their prey, typically fish or crustaceans.

This source of contaminants has been little studied, so a team of scientists based in Canada has undertaken an initial assessment of their concentrations in northern fulmars. Karen Foster and co-researchers from the University of Ottawa, Canada, Sedna Ecological, Inc., Trent University and the Canadian Wildlife Service, studied birds from colonies on St. George's Island, Alaska, and Devon Island, Nunavut, Canada.

Collecting the oils was not as troublesome as you might think. The birds were captured from their nests by hand or with dip nets or noose poles and, via an inbuilt defence mechanism, they voluntarily regurgitated and expelled the stomach oils. These were collected directly in sterile polyethylene bags and transferred to glass vials.

Following extraction, the lipids present were separated out for analysis by HPLC and the chlorinated pollutants were identified and quantified by GC-ECD on a 5% phenyl methylpolysiloxane column. The target analytes included 32 PCB congeners as well as polychlorobenzenes, hexachlorocyclohexanes, heptachlor, aldrin, heptachlor epoxide, chlordanes, endosulfans, dieldrin, endrin, mirex, methoxychlor, DDT and DDEs.

The total PCB and total DDT concentrations were 13-236 and 5-158 ng/g wet weight, respectively, and were similar from both nesting sites, although the mix of compounds was different.

The energy density of the stomach oils was 8-9 times higher than those of crustaceans or fish, but the exposure to the contaminants was up to 5-fold higher. Intricate mass and energy balance calculations for total DDT indicated that the potential contaminant load was higher than that experienced by direct consumption of the prey due to concentration in the oil after consumption.

For instance, stomach oils produced from consuming crustaceans or fish contained 531 and 1434 ng, respectively, compared with 249 and 443 ng for whole crustaceans and whole fish.

These findings change the viewpoint of the contaminant cycle in the petrel food chain. Chicks fed a greater proportion of stomach oil will be more exposed than those eating less oil or whole prey. Those birds that fly long distances to acquire food might provide higher levels of the pollutants than those feeding close to home due to a longer digestion time during the flight home.

So, a new source of exposure of petrels to chlorinated pollutants has been confirmed and should be included in future models of dietary exposure. The oils can be sampled without killing the birds and will reflect the contaminants of recently caught prey. As such, at-risk petrel populations can be examined without killing any birds and the same individual can be tested again and again to assess the trends in exposure.

In addition, the data can be used to estimate the relative levels of the pollutants in the oceans, since they will be reflected in the prey and, subsequently, the stomach oils. Sampling birds from different colonies would allow the comparison of regional difference in oceanic contamination across the Arctic.

The research team recommended "petrel stomach oils as a non-invasive and non-destructive alternative to traditional techniques of monitoring contaminants in the marine environment."

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

 Images: US Fish and Wildlife Service