Bald eagles are back, but what’s on their menu?
Several years ago some folks mused about the possibility of bringing extinct species back into existence, a process that’s been dubbed “de-extinction.” It’s an intriguing idea, but one important question that’s been asked as people have wrestled with the implications of de-extinction is just what it would mean to be an individual of a de-extincted species, suddenly thrust into an ecosystem for which it did not evolve. It would be cool to see a woolly mammoth, but those animals were not simply elephant analogues. They lived in a particular time and place, became sick when infected by particular pathogens, used particular features of the landscape to orient themselves, ate particular foods, competed with particular species over access to those particular foods, and were hunted by particular predators.
The truth is, we don’t have to bring back a long extinct sabercat or mastodon to begin to understand what it means to introduce an organism into an environment different from the one it expects. We do that sort of thing all the time, when we attempt to reintroduce species into areas that were once a part of their natural range but from which they have since become extirpated. The process is called “translocation,” and just such an experiment is going on right now—with bald eagles on California’s Channel Islands.
Comparing ecological information from the past with a species current habitat use would shed important light on the process by which a group of animals build homes in an altered landscape that was, at least at one time, home to their ancestors (or their cousins, in some cases). That’s why University of New Mexico biologist Seth D. Newsome, together with Santa Barbara Museum of Natural History researcher Paul W. Collins and Peter Sharpe from the Institute for Wildlife Studies, turned to the bald eagles.
“Even when historical, archaeological, and paleontological information is available to characterize a species’ former presence, abundance, and/or ecological role, essential ecological conditions such as habitat quality and food supply may have changed since a predator’s local extinction in the area slated for translocation,” they write. To complicate things even more, as humans have altered the ecosystem, we may have introduced new sources of food that could not have been exploited by earlier animals, and those prey species may contain contaminants (like rat poison) or be themselves of conservation concern, making the whole enterprise far more knotty than simply dumping a bunch of raptors onto some trees on a few islands in the eastern Pacific.
At one time, bald eagles (Haliaeetus leucocephalus) graced the skies over all eight of California’s Channel Islands archipelago, hunting and scavenging off the plant and animal life both within and around them. But by the 1960s, they disappeared. They declined in part due to hunting and egg collection and in part thanks to an overabundance of environmental contaminants. Those contaminants – namely DDT and PCBs – didn’t just affect them directly, but also indirectly, via their prey.
Then, starting in the early 1980s, efforts began to reintroduce bald eagles to five of the eight islands on which they historically bred: Santa Catalina, San Clemente, Santa Cruz, Santa Rosa, and Anacapa. By all accounts, it’s a fairly successful conservation effort.
Now, Newsome and his team want to know just how the raptors have managed to survive. Are they utilizing the same prey as the birds that came before, or have they adapted to an altered ecosystem? What impact do they have on other recovering wildlife populations, like seabirds or island foxes?
To find out what they’re eating, the researchers examined eagle nests in 2010 and 2011 for prey remains as well as feathers, from which they could extract carbon isotopes to determine on what they had been snacking. Feathers from nestlings were sampled as well, during annual banding efforts.
In all, the researchers collected more than 6,200 bits of eagle leftovers, representing 72 species of prey items from 38 taxonomic families, though there were differences in which species or families dominated from island to island. Of the bits that could be identified, half were fish (especially rockfish, toadfish, and surfperch), 42% were birds (especially gulls, cormorants, alcids, fulmars, shearwaters, and waterfowl), and the rest were mammals (ungulates, rodents, and skunks). Thankfully, island foxes comprised just 0.3-0.5% of prey leftovers recovered. The stable isotope analysis revealed similar trends, though it indicated that they had more small fish in their diet (critters that are less likely to show up in prey remains).
Eagles from the northern islands ate more seabirds than did those from Catalina, while those from Catalina relied more heavily on a wider variety of fish. That could be because recreational fishing, which occurs more often off the coast of Catalina, provides an easy and reliable source of nutrition for the crafty raptors. In addition, while breeding seabird colonies are found on all the Channel Islands, the most abundant colonies, comprising some 72,000 individuals altogether, are found on San Miguel, Santa Cruz, and Anacapa Islands – not on Catalina, where there are just 300 or so breeding seabirds.
The eagles were also quite flexible in what they chose to eat. Data from Santa Rosa Island, which at one time was privately owned, showed that eagles there scavenged on deer and elk carcasses, which were hunted for trophies. (The National Park Service now operates the island, and hunting no longer occurs there.) Meanwhile, a nest from Catalina Island’s interior was far more likely to contain terrestrial food scraps from freshwater fish and ground squirrels.
Prehistoric eagles seemed to dine far more often on seabirds than on fish, suggesting that something in the ecosystem has shifted in recent years that resulted in a higher proportion of fish on the menu. That could in part be opportunism thanks to recreational fishing, but it could also reflect the decline of seabirds. And the particular seabird species preferred by the bald eagles has shifted over time, from a larger historical reliance on ducks and a larger modern reliance on gulls. That eagles are gobbling up gulls more often tracks with increasing gull populations in the North Pacific, a trend thought to be associated with human development.
This information not only highlights the diversity of prey items that Channel Islands bald eagles can feast upon, but also that they’ll scarf down just about anything.
Most importantly, Newsome and his colleagues calculated that the bald eagle population has not yet rebounded to historical levels, nor to the levels that could be sustained by the amount of food available. In the early twentieth century, there were at least 25 pairs scattered throughout the islands, and at least fifty pairs earlier than that. Today just nineteen breeding pairs are resident on the islands, half of which nest on Catalina alone.
Ultimately, the researchers hope that comparing ecological data from the past and the present can help wildlife managers set targets for species reintroduction efforts by gaining a more nuanced understanding of the ways in which human behavior – both historical and contemporary – affects wildlife populations. – Jason G. Goldman | 31 July 2015
Source: Newsome, S. D., Collins, P. W., & Sharpe, P. (2015). Foraging ecology of a reintroduced population of breeding Bald Eagles on the Channel Islands, California, USA, inferred from prey remains and stable isotope analysis. The Condor: Ornithological Applications 117, 396-413. DOI: 0.1650/CONDOR-14-213.1.
Header image: Breeding male Bald Eagle is shown landing at the West End on Santa Catalina Island, California. Credit: P. Sharpe.
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