Just off the coast of Peru, the Humboldt Current produces one of the most productive marine ecosystems on the planet. Humans and animals alike have based their livelihood on the abundance of marine life that results from the deep, nutrient-rich waters of this coastal upwelling. Seabirds, which gather in massive groups off the coast to prey on schools of fish, have been completely sustained, until recently: Anchovy decline from overfishing and El Nino’s warmer waters have led to a major drop in seabird populations. One resilient bird, however, has held steady due to its solitary hunting style.
Peruvian boobies, like sea lions, penguins and seabirds in the area, rely on anchovies as a primary food source. But unlike the threatened Guanay cormorant, which shares breeding grounds and food stocks with other resident seabirds, the booby appears unaffected by the anchovy decline. Researchers from France and Peru have found that the key to the boobys’ resilience may be in its hunting style.
Henri Weimerskirch from the Centre National de la Recherche Scientifique in France and colleagues tracked Guanay cormorants and Peruvian boobies off the coast of Peru with GPS. They discovered that the cormorants use social cues to locate anchovies, whereas the boobies rely on personal experiences and memory to find the fish.
The cormorants hunt in large groups, diving into the water and driving anchovies closer to the surface; the remaining birds then pick off the fish close to the water’s surface. The birds continue in shifts until all the group members are well fed. Then, in a single file line, the birds return to a larger group of cormorants floating offshore. As the smaller hunting group returns, the waiting cormorants, forming a group called the compass raft, adjust their position to face the direction of the returning birds. That way, birds taking off from the raft in search of food are directed to the most up-to-date location of anchovy stocks.
Peruvian boobies, on the other hand, tend to use a memory-based search strategy when hunting, recalling areas where anchovy groups tend to gather and picking off those that rise to the surface. As the researchers reported in a study published yesterday in PLoS ONE, the boobies sometimes use the social cues of the cormorants as well by following the direction of the compass raft. In some cases, they even joined the large groups of cormorants, participated in the divings and returned in the same single file line to the raft.
And it appears that their methods of solitary and adaptive hunting have paid off. In the last 50 years, cormorant populations have fallen from around 21 million birds to about 2 million, whereas Peruvian boobies have remained relatively stable at 2 million birds. Perhaps it is the fall in cormorant numbers that has kept the population stable, or maybe it is their adaptive hunting style; either way, it seems the Peruvian bobbies are successfully riding out the boom and bust of the anchovy fishing industry. As the authors describe:
The observation that the populations of cormorants and boobies show different trends although the two species breed together and feed on the same prey led us to hypothesise that differences in foraging ecology between species may be implied in the differential trends of the species populations. In particular, the extent of personal versus social information used to find food patches may differ between species.
Weimerskirch, H., Bertrand, S., Silva, J., Marques, J., & Goya, E. (2010). Use of Social Information in Seabirds: Compass Rafts Indicate the Heading of Food Patches PLoS ONE, 5 (3) DOI: 10.1371/journal.pone.0009928