Pirates of the Galapagos
It’s a bright Saturday morning on the humid Galapagos island of Genovesa, and magnificent frigatebirds—both males with their bright red gular pouch and females with their white underbellies—cut the skies like blades with their forked tails and angular wings. Already owning their lethal appearance in the sky, their power is further demonstrated when a red-footed booby roams the sky with sticks for nesting material. As if the booby sets off an alarm, two male frigatebirds begin their assault: both crowd in on the booby, and one attempts to grab its tail. Once the booby drops the nesting material, the other frigatebird swoops to catch the twigs in midair. The frigatebirds’ assault leave the booby without the nesting material to impress a female and possibly, without a mate.
Although an unfortunate situation for the red-footed booby, this incident would not be the first time another bird has been stripped of its food or material resources as if it was an innocent child on a playground, persistently bullied by the big kids. It looks like the booby as well as other bird species have to be on the look-out because kleptoparasitism, a behavior in which an organism steals food or resources from another, is common in Galapagos birds such as the frigatebird and albatross (1,2,4). Though this feeding habit may sound harsh and unnerving, kleptoparasitism in birds can provide useful insight about adaptation and the natural selection forces behind this behavior.
Clues such as physical size of the bird, selection of host victims, and habitat conditions all act as factors that determine whether kleptoparasitic behavior is favored in the environment and species (1). As a result, evaluating these factors allows us to learn how such unconventional behavior as kleptoparasitism persists in certain bird species to increase their survival fitness.
On the island of Espanola, I waited about half an hour—which felt like several hours under the beaming hot Galapagos sun—in order to see any sign of the now critically endangered waved albatross. Just when I thought I would dry up like one of the marine iguana carcasses I saw just several feet away, the wait was rewarded by the beautiful sight in the sky: the albatross flew fairly close and swept high across the cliff edge with its clearly massive wings. Measuring in at a whopping three feet long with a wingspan of seven to eight feet (5), it is no surprise that the albatrosses’ massive size makes a kleptoparasitic lifestyle much easier and even advantageous.
According to a study that analyzed the cognitive intelligence of kleptoparasitic birds, birds that are larger in size generally have a physical advantage over smaller birds to steal their food and resources. A larger size might mean an “increase [in] the probability of hosts yielding their food items, while reducing the probability [that] the host aggressively [defend] its prey”(3). Besides having a physically larger body, kleptoparasitic birds tend to have larger eyes that are better at surveying their surroundings, making them better at detecting prime opportunities to harass and steal. For these reasons, the study suggests that kleptoparasitism would be more “profitable” (3) in species of birds characterized for their large size, such as the waved albatross. Thus, kleptoparasitism is selected for and later evolves in these larger bird species. Although I did not get to see an albatross exhibit kleptoparasitic behavior, watching it soar in the Galapagos leads me to believe that a smaller bird like a booby or petrel would barely stand a chance against this bird with a wingspan longer than the height of most humans.
While in the Galapagos, the overwhelming presence of frigatebirds on and around the islands is hard to gloss over. However, their abundance in the Archipelago gives ample opportunity to view their various habits, behaviors, and lifestyles. On several occasions aboard the boat as well as hiking on land, I saw frigatebirds delicately pluck fish from the surface of the water. They would hover just above the surface with still, unflapping wings, cautious not to wet their feathers or else they would get waterlogged and possibly drown. Frigatebirds lack the preen gland that produces the oil to make their feathers waterproof, so getting their feathers even a little bit wet can make it much more difficult to fly (1,4). This inability to get wet and dive deep into the water is evidence that frigatebirds as well as other seabirds must rely on kleptoparasitism to obtain more food variety.
Stealing from birds that hunt in deeper waters allows access to resources from different locations that are otherwise inaccessible to frigatebird offenders due to their inability to dive into the water. In an investigation that studied seabird piracy in the Galapagos islands, seabird species that typically foraged in shallow depths were more likely to steal food from other bird species that foraged in deeper water; in turn, parasitic birds were able to obtain food sources from different localities without having to exert additional energy to go in deeper waters2. In fact, out of the fifty-nine piracy incidents observed in the investigation, 98 percent “were by species which foraged less deeply than their victims”(2). Taking this high percentage into account, kleptoparasitism works as a major advantage for birds that do not have access to the wide array of resources beyond the depths of their foraging capabilities; exhibiting kleptoparasitic behavior almost seems to level the playing field with bird species that feed in deeper waters. Unfortunately, this “level playing field” means that many deep water foragers can put hard work into several food catches but have nothing to show at the end of the day.
A magnificent frigatebird soars over the open ocean near our yacht, The Fragata. Unobstructed expanses such as this open ocean provide prime opportunities for frigatebirds to scope out their hosts. Picture taken by Brianna Mims.
Besides the benefits that can come from selecting an appropriate host victim, certain habitat conditions can also affect the prevalence of kleptoparasitic behavior. Since the Archipelago is characterized for its often barren landscape and sparse vegetation, there is more access to open land as well as open sea around the coast of the islands. Out of the harassing and stealing incidents I had the opportunity to see, one had occurred over the open ocean near the coast while the other occurred over the flat, open land. In both incidents, the bird victim had nowhere to hide and was forced to relinquish its food or material. These areas, which happen to be where the frigatebird, albatross, and other pirate seabirds live, tend to facilitate kleptoparasitism. Areas that have more open space—whether above water or above grassland—provide better opportunities to easily target and attack vulnerable hosts without protection3. Consequently, certain habitats have influence on kleptoparasitism’s favorability in a bird species; based on the available opportunities to steal, different environmental conditions such as open space provide an ideal harassing ground while more cluttered or tree-dense environments make hosts less vulnerable and attacking difficult.
Compared to herbivores, carnivores, omnivores, and scavengers, kleptoparasitic organisms seem like the pirates of the animal kingdom. David Shealer, who researches roseate terns at Loras College in Dubuque, Iowa admits, “There's this stigma attached to individuals who steal things to make a living: that they can't catch fish or forage on their own” (6). However, observing the behavior and patterns in the Galapagos birds suggest that these birds are adapted to exhibit a beneficial behavior meant to increase their survival. It is not that these birds have completely lost the ability to forage on their own; rather, it is that natural selection has favored this behavior as more advantageous than solitary feeding methods. When it comes to behaviors such as kleptoparasitism in the animal kingdom, it can be extremely rewarding to understand what exactly drives this behavior and how populations adapted and evolved these behaviors over time. Often times, we underestimate that the forces of natural selection are constantly at work in the world we live in. Thus, learning more about these forces gives us all better knowledge and skills to recognize the variety of biological concepts that exist right before our eyes and appreciate both its structure and chaos. Right now, the spotlight is on “Pirates of the Galapagos”, but if you watch closely enough, there are more concepts in the animal kingdom to cover just about every movie Disney ever made. In fact, if I were you, I’d get in line to see “The Blue-Footed Booby’s New Groove” next.
- Brockmann, H. Jane, and C. J. Barnard. (1979). "Kleptoparasitism in birds." Animal Behaviour 27: 487-514.
- Duffy, David C. (1980). "Patterns of piracy by Peruvian seabirds: a depth hypothesis." Ibis 122.4: 521-525.
- Morand-Ferron, Julie, Daniel Sol, and Louis Lefebvre. (2007) "Food stealing in birds: brain or brawn?." Animal Behaviour 74.6: 1725-1734.
- Osorno, J. L., R. Torres, and C. Macias Garcia. (1992). "Kleptoparasitic behavior of the Magnificent Frigatebird: sex bias and success." Condor: 692-698.
- "Waved Albatross." ARKive. Wildscreen, n.d. Web. 25 Apr. 2016.
- Sohn, Emily. "Thieves of a Feather." Society for Science. Society for Science & the Public, 19 Dec. 2007. Web. 02 May 2016.