Lately I’ve been watching young Robins Grackles and Blackbirds harass their parents for food, and their constant squawking reminded me of the various parenting strategies, phenomenons and conflicts that animals find themselves in as they raise their offspring.
Blinded by the plight
It was a hot, summer day. I was in a bird blind in Wood Buffalo National Park and sweat was dripping off my brow. My blind was in the boreal forest, near Fort Chipewyan AB, and I was watching Downy Woodpeckers bring insect prey to their brood of chicks. The parents made frequent trips, and about the only time those youngsters weren’t making their dreadful racket was when they were having their gullets stuffed with caterpillars, moths and beetles. Eventually I began to wonder: given the likelihood of nearby predators, isn’t it a bad idea to constantly broadcast your location?
In a previous article on migration, we discussed how the changing seasons create an incentive for birds to migrate north, to breed. Long hours of daylight means more photosynthesis and therefore a bounty of plants and insects which attract the migrants. Once they’re here, those migrants are keen to turn that bounty into baby birds. But there are different strategies on how to take advantage of all that food.
Some animals, such as Mallard ducks, raise large batches of youngsters every year. Others, like elephants and whales, raise only a small number of young. In ecology terms, the ‘produce a ton of babies and hope a few survive’ strategy is called R selection. Mallards, a species which live for about 10 years, typically lay two clutches of about eight eggs each year. If the hatchlings all survive, that adds up to 160 ducks in the next generation. The other strategy, which we’ll dub ‘produce just a few offspring, but put a ton of effort into each,’ is called K selection. Humans may be the ultimate K-selected species because we often only have one or two offspring in our entire lifetime.
A problem for R-selected species can occur when these animals want to raise lots of young, but spend too much time gathering food for the kiddies, then don’t have enough in the tank to make it back to the tropics for the winter. This phenomenon is called ‘parent-offspring conflict.’ In 1974, biologist Robert Trivers published a paper on parent-offspring conflict which outlined his basic theory: individual offspring should always want more effort from their parents than those parents are willing to give, because it is in the parent’s interest to save some effort for the parent’s own well-being. But it’s in each offspring’s interest to get as much as possible from the parent. It’s like when your kid wants a car for a graduation present instead of the bicycle you had in mind.
A closely-related topic is sibling rivalry—not only do young animals want more from their parents than their parents want to give, but offspring also want to monopolize their parents’ attention (and food rations) at the expense of the care their brothers and sisters get. It seems that in the animal world there is more family drama than in an episode of Succession.
Squawk so you don’t drop
It was parent-offspring conflict that motivated those noisy woodpecker chicks in Wood Buffalo National Park; the shrill, constant calling was an attempt to get more out of their parents than the adult birds wanted to give. That calling also meant that predators might catch on and wipe out the year’s reproduction efforts, so there was a big incentive for the parents to shut the kids up.
Parent-offspring conflict can manifest in several ways, and it’s not just for the birds. In mammals, the classic example of parent-offspring conflict is the disagreement between mother and offspring over when to cut off the milk supply, aka weaning. Ungulates like moose, for example, will provide milk for their calves for about six months. At that point, Mom needs to start fattening up for winter, so the dairy bar is closed. Junior may not be keen on this arrangement, so occasionally Mom has to be quite insistent. This spectacle also plays out in the elk populations around Jasper—watch for the small bands of disgruntled calves that wander around sullenly after Mom has kicked them out in preparation for the next batch of youngsters.
But parental investment has its limits, which is where sibling rivalry ramps back up. To get their beaks on the limited menu, those woodpecker chicks had to outshout their brothers and sisters. This rivalry can be intensified by the reproductive cycle of some birds. If the avian parents start incubating after all of the eggs have been laid, the eggs will hatch at the same time. This is called hatch synchrony, and when it occurs, all the chicks will start out at about the same size.
However, if incubation starts on the first egg, rather than waiting for all the eggs to be laid, the chicks will hatch a couple of days apart (hatch asynchrony), and the first chick will enjoy the size advantage of a two-day head start. This strategy often arises in response to a variable food supply. In other words, when times are good, everyone survives, but when food is in short supply, the parents know it’s better for the smaller chicks to die off quickly and to leave all the food for the first-hatched. Sometimes the competition is even more to the point: the chicks of many predatory birds will actually kill their siblings outright if there’s not enough food.
Whooping Cranes are one of the species that use hatch asynchrony to nearly guarantee the demise of one of the brood. Whooping Cranes usually lay two eggs, but the first-hatched chick is almost always the only one that survives. In years of exceptional food supply, many second-hatched chicks will make it to independence, but those years are rare.
In 1999, I had the privilege of observing the family drama play out on a Whooping Crane project in Wood Buffalo National Park. We had built blinds near Whooping Crane nests, in what was essentially an enormous Black Spruce swamp (the mosquitoes were heinous!). I watched that nest for a few days before the family packed up and headed for parts unknown. In that time, the elder chick got almost every scrap of food. Not only that, but if the young chick so much as raised its head, the elder chick would pummel it into submission. The only time the little chick got any food at all was when the elder hatchling was so stuffed from gorging itself that it passed out in a food coma. After three days, both chicks walked away from the nest, each following a separate parent a few meters apart. Since they were no longer tied to the nest, the little guy could more easily avoid his elder sibling.
A few years ago, in Jasper, I was again reminded of parent-offspring conflict, but this time it was in another species of woodpecker. The Three-toed Woodpecker is normally fairly scarce around Jasper, but they are known for taking advantage of outbreaks of certain types of insect (recall our irruption article from this past winter).
The Mountain Pine Beetle outbreak had created a massive insect smorgasbord for Three-toed Woodpeckers and there were quite a few nests scattered throughout the park. One summer day I was walking back to my car from the Maligne Canyon trail when I heard that familiar racket: young woodpeckers pleading with their parents to bring home the bacon – or in this case a beak full of mountain pine beetles!
Mark Bradley // firstname.lastname@example.org