Mystery of the ornamented chicks
Looks do matter for nestlings to get a better deal.
In the story, the ugly duckling thrives and grows to be a swan. This, however, is not how it works, little ones need to look good if they want to be fed.
Bruce E. Lyon and Daizaburo Shizuka, from the Universities of California, at Santa Cruz, and Nebraska-Lincoln, in a paper in the Proceedings of the Academy of Sciences (PNAS), describe an instance that is the reverse of the ugly duckling, of brightly
coloured chicks that are the most likely to grow, and end up as drab and dull, but healthy adults. And along the way, the authors of the paper discover mechanisms behind the colours of these chicks’ feathers, according to the order in which they are born.
The American coot, also known as the mud hen, is an ordinary, grey and black, waterside bird, found in North America. A long-standing surprise is that many of the coot’s eggs hatch into surprisingly brightly coloured chicks. “American coot babies are among
the most ornamented offspring found in nature, sporting vividly orange-red natal plumage, a bright red beak, and other red parts around the face and pate,” says the paper in PNAS. What could be the purpose of this feature? Even Charles Darwin, the paper says,
wondered how the appearance of the feature could be understood – it led to no advantage of survival or foraging, nor in mating, as the features did not persist in the adult coot.
The authors of the paper in PNAS note that offspring ornamentation is typically found in birds or animals where young ones need parental care for some time. In the case of the American coot, they observe, parents show distinct preference, at feeding time,
for chicks that have the most ornamentation. Why the parent birds show this preference, however, has not been resolved.
In order to understand the context of this parental preference, and to determine whose interest the ornamentation and its effects served, the authors took a close look at how the nature and extent of coloration varied, both within families and between
different families. The reason for looking at how colours varied between different families was that the practice of brood parasitism is common among American coots.
Brood parasitism is when birds lay their eggs in the nests of other birds of the same species, as a means of promoting their own offspring at the cost of others. If parasitic chicks could manage to receive better attention from their foster parents by
virtue of their bright colours, this could be a driver for colouration to develop.
What is seen in the field, however, is that the chicks that hatch from the earliest eggs are the least brightly coloured – and it is these, early eggs, that the parasitic birds lay in the nests of other birds. As the chicks that emerge from the first eggs
that are laid have the least bright colours, and it is the biology of the mother that determines the coloration of the chicks that emerge from the eggs, this rules out the possibility that a mother makes use of colouration to benefit her own chicks when they
are born in foster nests.
The fact that it is the later born chicks that are more brightly coloured, however, suggests that colouration could act as a signal of the chicks’ age to the parents, to guide a parental strategy of rationing out the nourishment that they bring to the
nest. Different aspects of the colours of chicks, and how they fared – the associations between the different colour traits that were found, the relation of the colour and the hatching order and the difference in colouration between parasitic chicks and chick
born in their own nests, were hence studied, based on a survey of 1,431 chicks.
A feature that suggests an explanation of the coloration and the parental behavior is the fact that the American coot lays many more eggs than the number of chicks that she does or can raise and feed. The coot thus needs to limit the number of chicks that
survive and then to see that those that do receive a fair share of the limited food resources.
The American coot usually lays nine to ten eggs, which hatch, one after another, over the course of two to eleven days. During the first ten days, or so, after the eggs have hatched, the parents show no preference and feed whichever chick that reaches
them. The chicks that are born first, from the eggs that were the earliest laid, have a head start and they dominate, grabbing the best of the food the parents bring. The first few days after the last egg has hatched is thus a scramble and the period of the
greatest chick mortality, and as many as half the chicks perish from starvation.
In about ten days, however, the parents move in, to equalize the competition – which they do by preferential feeding of last born, brightest coloured chicks, and even hostility towards the older, and stronger chicks. The result is that smaller but brighter
coloured chicks start feeding better and they soon catch up with their plainer coloured siblings.
“They start out by creating an unequal playing field, which allows culling the brood, and then they intervene and level the field. The orange plumage seems to be a feature that helps them do that,” co-author Bruce Lyon is quoted as saying, in a notice
out by the University of California at Santa Cruz.
The paper describes a strategy of the American coot, to start by producing in plenty what they can without consuming great resources, and then to trim the brood, to leave just the number of chicks that can be supported, to grow into adulthood. This is
more efficient than to start by laying a limited number of eggs. This is a small economy, but fewer eggs would lead to less than the optimum number of chicks in a year when food is plentiful. In good years, if there were more eggs to start with, more chicks
would be there to survive the first, harsh days.
What the study shows in that unlike devices where chicks maximize their fitness, the feature of juvenile coloration acts as a signal that helps the parents recognize the youngest chicks and implement a process of optimizing the fitness of the species,
the paper says.
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