Scientists have long known that traits in species, such as feather coloration, can arise for a variety of reasons. Two forces that shape such traits are natural selection and sexual selection. Natural selection is often caused by external factors, such as environmental pressures or predation. On the other hand, sexual selection is often the result of breeding preferences of females.
The outlandish tail feathers of the male birds-of-paradise (Paradisaeidae) are a good example of a trait scientists are intrigued by. But what is the interaction between the 2 kinds of selection? And are sexually selected physical characteristics, such as the size and color of a bird's plumage patches, an indication of preferred behavioral characteristics?
To find out, scientists from the Smithsonian Migratory Bird Center, Virginia Tech, and the University of Maine studied plumage and behavior of two types of marsh sparrows: the coastal plain swamp sparrow (Melospiza georgiana nigrescens) and the inland southern swamp sparrow (Melospiza georgiana georgiana) in Delaware and Maryland.
The swamp sparrow has distinct patches of forehead feathers and cap feathers which vary in size and color. The cap patches range in color from dark brown or black in females to a beautiful rust color in males, and the forehead patches vary from brown to black. The team studied the connection between each different patch of plumage feathers and certain types of behavior, mainly aggression and parental care. They hoped to discover whether the differences were a result of natural selection, sexual selection, or both.
Female swamp sparrow on left; male on right. © Gerhard Hofmann
Over several breeding seasons, the team observed song sparrows in marshes along the Smyrna River in Delaware and inland on the Allegheny Plateau in Maryland. Birds were captured and banded, and measurements of the cap and forehead patches were taken. Additionally, they photographed birds so they could analyze the color of the birds’ crown feathers and determine the percentage of black or rust colored feathers in the crown.
Study sites in western Maryland and Delaware.
Part of the difference in plumage color is a result of environment. The coastal swamp sparrows tend to have more black (or eumelanic) feathers and fewer rusty (or pheaomelanic) feathers than the inland, freshwater swamp sparrows. This could be because birds with darker feathers are harder to see against the dark gray and black salt-marsh mud. Another theory involves bacteria found in the tidal marsh which degrade keratin in feathers. Melanin resists abrasion and is antimicrobial, which makes birds healthier and longer-lived, so natural selection favors birds with more melanin (and darker feathers) in the coastal marsh areas.
But the team hypothesized that the feather differences in males could also be a result of sexual selection—indicators of which birds were good parents and which birds were more aggressive when defending territory. To help determine which males were better parents, the team observed nests to see when the nest was built and how often the males fed the chicks. To measure aggressive behavior, the scientists watched each male in its territory, both in an unprovoked state and also in a controlled experiment simulating territory intrusion with song playbacks of aggressive male calls.
After collecting both the physical and behavioral data from their observation experiments in the field, the scientists began an evaluation to determine if there was a relationship between the size and color of the crown feathers and male aggressive and parental behavior. Typically, male birds invest energy in aggressive behavior or parental care, but not both. So the team was looking for clear links between each type of behavior and the different plumage characteristics.
The scientists began to see that rusty cap feathers were an indication of good parental behavior, whereas black forehead patches correlated with aggressive behavior. In both coastal and inland males, the team found that individuals with larger, rusty caps fed their young more than their black-feathered counterparts. But males with more black feathers in their caps exhibited more aggressive behavior. The size of the black forehead patch also correlated closely with aggression: the bigger the patch, the more aggressive the bird.
The team noticed the coastal plain swamp sparrows in particular had fewer rusty feathers in their crowns, and the black forehead patches tended to be larger. And these birds were definitely more aggressive. This was suggested by team’s observation of more frequent singing—coastal males sang almost 3 times as often as the inland males—and frequent changes of perch location during the territory intrusion experiments.
Aggressive behavior is valued in the coastal birds because the best habitat is highly contested. Coastal males are often fighting for small territories of high-quality habitat, and males with the smallest territories were able to produce the most chicks. Not surprisingly, these successful coastal males had more black feathers in their crowns. This variation in the crown feathers in coastal males was directly related to claiming and holding territory as well as the production of young.
This was a clear indication that the differences in the male crown feathers between the 2 subspecies is related to behavior and can be attributed to sexual selection. The increased instance of black feather badges along the coast indicates sexual selection for aggressive behavior. Aggression is clearly a more important and desired trait among the coastal males. On the other hand, inland males with larger rusty cap patches produced more offspring. This difference in cap color and behavior indicates aggression to defend habitat is not as important inland.
So at the end of the day, it is the female swamp sparrows' choices for certain kinds of behavior which are driving the badge and cap coloration in the males. Female sparrows in both areas seemed to prefer rusty coloring in the cap feathers of males. And as the scientists discovered, males with rusty caps make better fathers—the inland males with rusty-colored caps typically nested earlier and fledged more young. And in both subspecies, males with larger rusty caps fed nestlings more.
In addition, the coastal females are not only selecting the potentially superior fathers with rusty caps, but also the aggressive, territorial males with larger, black forehead patches. They know these birds will defend smaller areas of high quality habitat, which will provide a better chance of survival for their nestlings. Even though some of the overall coloration of the swamp sparrow is a result of natural selection, it is the female's desire to find a mate who provides good parental care and quality habitat that drives these specific plumage characteristics of the male swamp sparrows.
This article summarizes the information in this publication:
Olsen, B.J., R. Greenberg, I.A. Liu, J.M. Felch, & J.R. Walters. 2010. Interactions between sexual and natural selection on the evolution of a plumage badge. Evolutionary Ecology 24(4):731-748.
The evolutionary stability of signals varies due to interactions between sexual and natural selection. A tidal-marsh sparrow, Melospiza georgiana nigrescens, possesses darker pigmentation than an inland-marsh sparrow, M. g. georgiana. Studies of featherdegrading bacteria and convergent evolution among salt-marsh vertebrates suggest this dark coloration is due to environmental selection. Sexually dichromatic swamp sparrow crowns, however, may be additionally under sexual selection. We investigated ties between two plumage patches (rusty cap and black forehead) and two behaviors (male-male aggression and parental care) in the coastal and inland subspecies to test the effect of sexual versus natural selection on badge evolution. Across both subspecies the extent of rusty feathers in the cap patch was correlated positively with parental care and negatively with aggression, and the extent of black feathers in the forehead patch was correlated positively with aggression. Males with larger forehead patches produced more offspring along the coast, while males with larger cap patches did so inland. The date of the first nesting attempt for both subspecies correlated with cap patch extent, suggesting a similar role for female choice. Natural selection likely accounts for darker coastal females. Coastal male head color, however, is darker due to increased selection for larger forehead patches via intrasexual competition, yet it remains largely rusty due to female choice for larger cap patches. Increased sexual dichromatism among coastal plain swamp sparrows thus provides a clear example of the interplay between sexual and natural selection in subspecies divergence.
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