The Guild of Avian Piscivores

June 14, 2021

Say What? Am I seriously going to lay this arcane ecological term on you?

Yes, I am. The subject of a recent discussion on YouTube Chat concerned the interactions between loons and other birds that eat fish. An animal that primarily eats fish is a piscivore. Birds are in the taxonomic class Aves. Avian is the adjective form of Aves. In ecology, a guild is a group of species that exploit the same resources, and usually in a similar manner. So we can translate the title to “The Group of Birds that Eat Fish.”

Loons share their lake with numerous other members of their guild. In New Hampshire this can include bald eagle, osprey, great blue and green herons, double-crested cormorant, common merganser and belted kingfisher. They all compete with loons to some extent but if we further divide them into three methods of fishing we’ll see that the methods are related to the intensity of competition.

The raptors (eagle and osprey) and the kingfisher all hunt for fish from the air. None of the three are good swimmers; they grab a fish at or near the surface and immediately fly off with it. There is no opportunity for any interactions with loons. Because the eagle is an opportunistic hunter and forager, eagle-loon interactions are only as predator-prey. However, anything an eagle catches in the water must be consumed at a terrestrial site. An eagle can only fly when carrying no more than a third of its own weight. Adult loons weigh about as much as an eagle (they may be smaller but they are much denser) so the only available prey items are chicks and eggs. Even so, an eagle needs stealth and cunning to get past a defending adult loon. Most eagle-loon interactions occur in open water when an eagle is after a loon chick. In the water, the loon has the advantage.

The herons are waders. They don’t like to get their feathers wet so they’re limited to shallow water that is no deeper than their legs are long. Loons prefer deeper water where they have more room to maneuver. However, loons and herons do occasionally meet but if a loon approaches a heron it will simply fly away. LPC has a few records of herons perched on the top of a nesting raft occupied by a nesting loon. Neither bird showed the least concern.

The third group includes the loons, cormorants and mergansers. These are the divers. All three spend almost all their time in the water and frequently encounter each other. The loon is the best adapted to an aquatic life, as well as weighing two to four times as much as the cormorant and merganser and having the most lethal, dagger-like bill. Avoidance is the responsibility of the cormorants and mergansers; if a loon shows up, get out of its way! This is normally not a big issue because, just like the herons, they can simply fly away. The only time there is a constant threat is when a merganser has a brood of chicks in tow. The chicks can’t fly and the adult is unlikely to abandon them. The loons find this to be irresistible. Picture a convoy of one ship and a dozen pleasure craft being targeted by a submarine.

The bottom line: The loon has evolved into an efficient specialist much more suited to an aquatic habitat than to a terrestrial habitat or flight. Within their guild, they rule as long as they stay in the water.