Every spring during reproduction, birds have a decision to make: should they use nutrients derived locally for egg production, or use reserves transported to the breeding ground? Well, it's actually not so much a choice as it is a matter of species life history.  But for aquatic toxicologists, knowing what strategy a species prescribes to is critical if one chooses to sample eggs for contaminant analysis.



What we are talking about is income breeding and capital breeding. Income breeders use nutrients and energy derived from their diet on the breeding grounds (exogenous nutrients) for synthesizing eggs. Capital breeders incorporate nutrients stored in their tissues and transported to the breeding ground (endogenous nutrients). The advantage to using exogenous resources is that a bird doesn't have to expend energy transporting excessive nutrients, but it only works when there's sufficient prey at the breeding grounds. But because biologists often use contaminants in eggs as environmental health indicators, its important to know what environment the eggs represent.

A recent study by Alex Bond and Antony Diamond looked at resource allocation in eggs of six seabird species breeding in the Gulf of Main. From smallest to largest, the species are: Leach's Storm-Petrel, Arctic Tern, Common Tern, Atlantic Puffin, Razorbill, and the Common Murre.  The author's hypothesized that the small seabirds (first three) would be income breeders whereas the latter three might be capital breeders.

To carry out the study, Bond and Diamond sampled eggs from the above species at a mixed-species colony. They sampled the first laid egg for all individuals. But to tell if the eggs are of exogenous or endogenous origin, the two had sample adults as well. To do this, the authors sampled feathers and blood  from several adults and analyzed carbon and nitrogen stable isotopes. The isotope signatures in the feathers (grown over the winter) have endogenous isotope signatures whereas the blood (synthesized locally) has an exogenous signature. Then, by comparing the isotope signatures in the eggs to the blood and feathers, the author's modeled what proportion of egg nutrients were synthesized with endogenous or exogenous nutrients.

The results suggest that two of three small species are in fact income breeders: Arctic and Common Terns. But not the smallest of the study species, the Leach's Storm-Petrel, which was classified an intermediate. Surprisingly, Common Murre's, the study's largest species is also an income breeder. Razorbills and Atlantic Puffins fell into the intermediate category with the Storm-Petrel's.

These results are important for any biologists working on contaminants in seabirds. Particularly those that use eggs as bioindicators for contaminants and diet - particularly for biologists working in the Gulf of Main, as endo vs. exo nutrient allocation can shift across breeding territories.

But unfortunately there is one flaw in the study design that the authors pay only minor attention to. The authors sampled feathers and blood from different nests then the eggs. Their argument is that the results are therefor representative of the population mean. But because individuals could have migrated to different places over the winter, I think this needs further testing. Once all samples come from the same nests I think we can be more confident in the results