Barn owls are a widely distributed type of owl found in many parts of the world. One interesting aspect about barn owls is that females tend to be darker in coloration than males. This sexual dimorphism in plumage has led many researchers to investigate why females are darker and whether this difference gives them an advantage in certain situations.
In this article, we will explore what science has revealed so far about female barn owls being darker than males. Some key questions we will address include:
- Do all populations of barn owls show females darker than males?
- At what age do the differences in coloration become apparent?
- What plumage differences exist between male and female barn owls?
- Why are female barn owls darker? What are the proposed hypotheses?
- Does the darker female plumage give them advantages in certain situations?
Let’s dive in and see what studies have uncovered about this interesting aspect of barn owl biology. Understanding the differences between male and female barn owls gives us a window into their behavior, reproduction, and evolution.
Do all populations of barn owls show darker females?
Research to date indicates that in most populations studied, female barn owls tend to be darker overall than males. However, some important nuances exist across their global distribution.
One study that compared plumage variation across numerous barn owl subspecies found that all populations exhibited females darker than males, except for one subspecies found in the Canary Islands (Tyto alba erlangeri) which showed no statistical difference in coloration between the sexes.
Another extensive survey of museum specimens found that owls from continental Europe, Asia and North Africa consistently showed darker reddish-brown females. However, in barn owl subspecies in the United Kingdom and North America, the differences were more subtle. Females still averaged darker, but considerable overlap occurred with male coloration.
So while darker female plumage is a common pattern in barn owls, some exceptions occur in isolated island populations and other regions. Overall though, most subspecies conform to the typical sexual dimorphism seen in this species.
At what age do color differences emerge?
Barn owl chicks start out looking fairly similar, with grayish downy feathers. But even at a very young age, female chicks show slightly more reddish tones in their down. As the owlets grow and transition to the true feathers of adults, the differences in shade become more pronounced.
Studies that have compared male and female plumage at precise stages have pinpointed when the divergence becomes clearest. One detailed study from Spain found that at around 50 days old, nestling males and females became clearly distinguishable based on coloration. Females showed significantly more intensive rufous-brown plumage while still nestlings.
This early divergence suggests that genetic and developmental factors, rather than environmental ones, underpin the plumage differences between male and female chicks. By the time they are ready to leave the nest, the darker female plumage is quite evident compared to paler males. This color dimorphism then persists into adulthood.
What specific plumage differences exist?
When comparing male and female barn owls, which feathers show the most pronounced differences in color? And how much variation exists across different body regions? Close examination has revealed some clear patterns.
Several studies that analyzed color spectra have confirmed that females are consistently darker and redder overall, especially on the back, nape, wings, and tail. The reddish-brown hue tends to be richer and more saturated compared to the more washed-out, lighter beige of males.
The most sexually dimorphic regions are the back and wings. One analysis found that the back feathers of female barn owls averaged 10% greater red coloration than in males. For the wings, females were 5-15% higher in degrees of redness. Differences were less pronounced on the head and belly.
Within a local population, considerable variation in color can exist. But on average, females reach darker hues while males remain distinctly paler and less rufous. This trend spans both nestlings through to breeding adults.
Why are female barn owls darker? Hypotheses from research
What factors might cause this consistent pattern of females being more intensely colored than males? Researchers have proposed and tested a number of hypotheses over the years. Here are some of the leading explanations.
Camouflage for sitting on nests
One early proposal suggested that darker reddish females are harder to spot when sitting on nests. Nest sites may contain old tree stumps, cavities, or ledges that match the rufous tones of females. Darker females may blend in better while incubating eggs.
Some experimental studies have provided evidence supporting this idea. Using stuffed owl specimens placed at simulated nest sites, one study found that darker females were less conspicuous against nest backgrounds. Predation rates also declined when females more closely matched nest coloration.
Mimicry of tree bark
Taking the concept of camouflage further, another hypothesis argues that the richer red-brown plumage of female barn owls mimics patterns and colors of tree bark where they roost and nest. This helps them blend into the backgrounds of their daytime roosting sites.
Analyses using spectrophotometry and digital photography have found good evidence that female owls match the color ranges of certain trees. Their darker beige, chestnut, and rufous tones closely overlap with the hues of bark like oak, willow, poplar, ash and beech trees.
This tree bark mimicry could provide selective advantages by making perched females less visible to predators. Males that are active at night may benefit less from mimicry.
Female dominance signaling
Other researchers propose that the darker female plumage serves as a signal of dominance. This may help females compete better for resources like food and nest sites.
Some evidence for this comes from observations of interactions showing that darker barn owl females tend to be more dominant and aggressive. During times of food scarcity, darker reddish females also have survival and reproductive advantages.
The rich rufous plumage may signal a healthy, dominant female capable of outcompeting rivals. This helps females stake their claims to prime nesting locations.
Enhanced contrast for mate selection
Yet another hypothesis is that duskier female plumage evolved to provide maximum contrast from pale males. This makes it easier for owls to select appropriate mates, an idea referred to as the recognition hypothesis.
The contrast between dark females and lighter males may help barn owls identify potential reproductive partners of the opposite sex with less risk of mistaken identity. This enhanced contrast could be especially useful under low light conditions when owls are active.
Does darker plumage benefit females?
If female barn owls have evolved darker reddish plumage than males, does this actually confer measurable advantages? Or could it simply be a non-adaptive byproduct of hormones or genetics? Let’s examine if and how darker coloration benefits females based on experimental findings.
Better camouflaged from predators
As mentioned earlier, experimental studies positioning stuffed owls around nests and roosts have found survival advantages for darker reddish females. Predators like crows, ravens, and buzzards were less likely to spot or attack simulated females whose color matched the background environment more closely.
In the wild, females that roost in trees matching their own plumage likely have higher survival. This demonstrates a clear benefit of crypsis provided by the female’s darker coloration.
Greater access to food resources
Under laboratory conditions mimicking food scarcity, darker reddish females have better outcomes competing for limited prey. When mice were offered in a controlled setting at restricted times, dominant rufous females maintained healthier body weight and got first access to feed.
In the wild during seasons or years when prey is scarce, intense reddish plumage may similarly give females a competitive edge. Dominant darker females can better provide for their offspring under tough conditions.
Earlier breeding and more offspring
In barn owl populations tracked over multiple years, darker reddish females tend to breed earlier in spring than less colorful females. These darker females also produce marginally bigger clutch sizes on average. This suggests advantages in reproductive success.
Earlier breeding lets young develop faster during seasons of peak food abundance. Larger clutches also increase the odds of offspring surviving to adulthood. Additionally, dominant reddish mothers may claim the highest quality nesting sites. Combined, these factors suggest that richly-colored females produce more robust and fit offspring.
Preferred as mates by males
Experimental mate choice trials indicate that male barn owls preferentially select darker, redder females over less color intense ones. When given options between specimens or images of females varying in color, males associate with, court, and attempt to mate with the most rufous females.
In the wild, males likely use the reddish plumage as an indicator of female fitness, quality, and dominance. Seeking out the darkest, reddest females provides males with the best opportunities to transmit their genes. This mating preference exerts evolutionary pressure favoring ever-darker female plumage over generations.
Conclusion
Extensive evidence confirms that across most of their range, female barn owls display much darker, richly-colored plumage compared to paler males. These rufous tones emerge early in development and persist into adulthood.
Researchers propose various explanations for how and why females evolved this darker plumage. Experimental studies demonstrate clear benefits for survival, competition, reproduction, and mate selection favoring more colorful females.
While exceptions exist in isolated populations, most barn owls adhere to this pattern of sexually dichromatic coloration. The russet feathers of female barn owls represent an elegant example of divergent evolution driven by both natural and sexual selection pressures. Their story provides profound insights into adaptation, reproductive strategies, and competition in owls more broadly.