Common loons defend chicks according to both value and vulnerability

Submitted by Johan on 29 June 2015.

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During the spring of 2012, Gabriella Jukkala was just about to graduate from Northland College. After extensive discussions, Gabby and I became interested in how adult loons might protect their chicks during their fragile first weeks of life. While adult loons are robust 10-12 kg animals that are almost immune to predation, chicks hatch at less than 100g and face a variety of hazards, including snapping turtles, predaceous fishes, bald eagles and other loons.

In fact, conspecific intruders – which sometimes approach and kill young chicks in the course of their efforts to learn about and usurp territories – are responsible for more chick deaths than any other threat. How might loon parents cope with the danger that conspecific intruders represent to their young?

The wooden decoy used to investigate chick defense in common loons.

Fortunately, we knew from prior work that loon parents react behaviorally to decoys of adult loons, so we had a tool with which we could simulate conspecific intrusion. We sought to ask whether loon parents behaved so as to protect their chicks when they were most vulnerable and/or when they were most valuable – that is, when the brood consisted of two chicks (the maximum number) rather than just one.

Loons proved to show robust chick defense but to do so in ways that differed from many other species. Intruders that land in the lake with parents and small chicks can dive and appear suddenly right next to chicks, which they can peck to death within a few seconds. Parents, moreover, cannot track intruders underwater. When we confronted them with decoys, parents of small chicks did not approach and attack the decoys as most birds would; instead they remained close to their young to defend against sneak attacks, and males yodelled (see photo below) to discourage intruders from approaching chicks. (Fathers of two‑chick broods were especially vociferous, yodelling almost four times as often as fathers of singleton chicks.)

Male on Muskellunge Lake yodels at a flying intruder, as his mate and two chicks watch. (Photo by Linda Grenzer.)

Parents of chicks that had reached four weeks of age – which can dive and elude intruders effectively – tolerated intruders, yodelling rarely and often leaving chicks in order to socialize with intruders. In short, loon parents defend young chicks according to both value and vulnerability – and do so in a quirky way that reflects the limited visibility of loons within aquatic territories.

Male from Razorback Lake (3rd from left) penguin dances, causing an intruder to flee across the water. (Photo by Matt Erlandson.)

One finding that surprised us was the crucial importance of the male parent to chick survival. Only males yodel, so only they can vocally deter landing by flying intruders. Our experiment also revealed that male parents are more than twice as likely as females to carry out the aggressive “penguin dance” (see photo above), by which adults convey the likelihood of aggression to conspecifics close at hand. Finally, males spend more time in close association with chicks, while females often wander off within the territory and even to other lakes nearby. Considering the vital role that male parents play in protecting their chicks, it is a rare thing, indeed, for a chick to fledge after losing its father within the first week of its life. But that is just what happened on Squash Lake in 2012, the first year of Gabby Jukkala’s research.  

Kristin Brunk (left) and Gabby Jukkala (right) with “Miracle Chick”, a young loon that lost its father when it was a week old to lead poisoning, yet was reared to fledging by its mother alone. 

Written by Walter Piper (Author)

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http://onlinelibrary.wiley.com/doi/10.1111/jav.00648/full

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