It is a question as old as time, for animals ranging from pint-sized crocodiles to apprehensive college freshman: when are you ready to leave the nest? Transitioning between life-stages involves both prospects and perils. For many animals,this doesn’t mean completely striking out on one’s own, but does involve managing a novel set of risks and rewards. The very process of transitioning from one life stage to the next often involves high mortality rates if a young animal leaves before it is large or developed enough. Timing is everything.
One of the most cinematic life-stage transitions observed in nature is seen amongst the murres (also known as guillemots), members of the auk family. Before they can even fly, murre chicks literally take a leap of faith–they jump 300-400 feet from their cliffside colonies to the cold ocean waters below.
Why would these inexperienced, bite-sized baby birds leave their cozy colonies for a dip in the predator-filled ocean? The answer involves a few different trade-offs, and also highlights the tough work involved in being a murre parent.
Trade-off #1: How do murre parents decide how much care to give to their offspring? Obviously, a chick would benefit from mooching off of mom and dad for as long as possible, but the parents have other priorities once their chicks are capable of fending for themselves.
Calculation: In 1974, Robert Trivers established the term “parent-offspring conflict,” which refers to the disparity in the ideal level of parental investment from the perspectives of offspring versus their parents. Obviously, youngsters want to take as many resources as possible, while parents want to save some time and energy for their next batch of offspring. As Trivers pointed out, the fundamental cause of this can be traced back to genetics. Parents are equally related to all of their offspring, and thus it makes evolutionary sense to invest equally amongst them. Offspring, however, are either 50% or less related to each of their siblings, but 100% related to themselves.* Obviously, they are better off taking resources for themselves rather than relinquishing them to partially unrelated siblings.
*(If you’re not clear on how full siblings can be less than 50% related, check out this useful site).
Outcome: While murre parents can’t provide for one chick or clutch forever, they do need to at least make sure that offspring survive their initial fledge, or else all the time and energy they’ve invested in those chicks will be lost. When the offspring is able to provide for itself, though, the parents need to stop coddling them and begin preparing for the next breeding season.
Trade-off #2: When is it in a chick’s best interest to sacrifice safety for opportunities to obtain more energy and grow?
Calculation: Activities that increase an individual’s fitness–foraging, mating, grooming, etc– often result in elevated vulnerability. As a wild animal, the more time you spend searching for a fish, or a flower, or a fly, the less time you spend scanning your surroundings for predators. On the other hand, an animal’s efforts to stay safe may be wasted if it doesn’t eat, search and search for mates, as those activities will ultimately determine its fitness. So, at some point, an animal has to decide what level of reward warrants increasing its risk level.
Outcome: If a murre chick decides to hang out in an ostensibly “safe” zone for too long, it sacrifices opportunities to obtain food, find mates, and do other murre things. Also, the cliff colony could be exposed to predators that don’t prowl in the waters below. When you are a young bird in a world full of hungry carnivores, you are never entirely safe. At some point, every chick has to leave the cliff. A chick usually does so when it reaches a size at which it needs more resources than it can easily obtain when it is hundreds of meters up a barren cliff.
Trade-off #3: For a murre parent to successfully raise a chick, should it spend more energy carrying food up cliffs, or keep the offspring more accessible, even if that involves bringing the chick down into the murky water?
Calculation: As long as a murre parent is providing for a chick, they both need to make sure they’re getting the most out of the situation. When an adult murre has to commute back and forth between the water and cliffs to bring food to its offspring, it is spending time and energy on the commute that it could otherwise use to catch more food for itself and its chicks.
Outcome: This is similar to Trade-off #2, except in this case a parent is making decisions about risk to its offspring, rather than the chick making decisions about risks to itself. No pressure.
So, with those issues in mind, we can investigate why a seemingly crazy custom — baby birds plunging from isolated cliffs into the ocean waters below before they can forage for themselves–is routine for murres. Kyle Elliot and colleagues took on this investigation, and recently published their results in The American Naturalist. Their findings are enlightening on several levels.
First, is a murre chick in a cliff colony truly safer than one floating in the open ocean? Surprisingly, no. Elliot and colleagues found that, after the initial plunging event, mortality rates for chicks of similar ages were essentially the same wherever they spent their time. So, although actually getting to the water can be a risky endeavor, predation risk there is no higher than it was on the cliffs.
We now know that chicks aren’t at significantly higher risk in the water than on the cliffs, at least once they’ve reached a certain stage of development. But it is still only worth the plunge (literally and figuratively) if they are obtaining more resources down there than they were before. Elliott and colleagues confirmed this: the average energy gained by a chick in the water was more than twice that of a chick that remained on the cliff. They determined that the primary reason for this was that it was easier for a parent to access the chicks to feed them, meaning they could spend more time hunting and feeding rather than wasting time going up and down the cliff all day.
And about those parents: this study also sheds light on the apparently uneven dynamics between murre mothers and fathers. Elliott and colleagues attached time-depth recorders to adult murres to track their foraging habits. On an average day, a male murre spent about 2.6 times as much time diving (i.e., foraging for food for offspring) as did the females. Males also foraged in lower quality habitats than the females could be bothered with. In fact, once a murre chick is in the water, the father is essentially a single parent.
Perhaps this is a leveling factor, as the female has to create and lay eggs around 11% of her body weight–the equivalent of a 140 lb woman having a 15.4 lb baby. With no epidural. Regardless of how the investment balances out, the parent-offspring-conflict involved in murre fledging is largely about the energetic costs to the father and how those factor into his best interests in terms of producing strong offspring. As Elliott and colleagues found, the father’s best interests seem to be served by having his chicks in the water once they’ve reached an appropriate size, so he can provide them with more food at less cost to himself.
The result of all this is the dramatic sight of baby birds leaping into the oblivion, with all involved hoping for the best.
Reference: Elliott, K. H., Linnebjerg, J. F., Burke, C., Gaston, A. J., Mosbech, A., Frederiksen, M., & Merkel, F. (2017). Variation in Growth Drives the Duration of Parental Care: A Test of Ydenberg’s Model. The American Naturalist, 189(5), DOI: 10.1086/691097.
Hyena with cubs: http://www.izw-berlin.de/life-history-and-social-behaviour-intra-specific-evolutionary-conflicts.html
Murre parent and chick: http://birdallyx.net/young-common-murres-in-care/
Murre family: https://commons.wikimedia.org