The eastern tiger swallowtail is one of North America’s most familiar butterflies, large enough and common enough to catch most people’s attention in backyards, parks, and forest edges across the eastern half of the continent. But what many people do not realize is that the females come in two completely different color forms, while the males are always the same bright yellow with black stripes.
Knowing how to tell males and females apart, and understanding why females evolved two different looks, makes watching these butterflies considerably more interesting. This guide covers the visual differences, the behavioral differences, and the evolutionary story behind the dark female form.
Key Takeaways
- Male eastern tiger swallowtails are always yellow with black tiger stripes and a narrow black border on the wings.
- Female tiger swallowtails come in two forms, a yellow form similar to males and a dark form that is largely black with iridescent blue on the hindwings.
- The dark female form mimics the pipevine swallowtail, a toxic species that predators have learned to avoid.
- The proportion of dark females in a population increases in areas where pipevine swallowtails are common, suggesting the mimicry works better where predators have more experience with the model species.
Male Appearance
Male eastern tiger swallowtails are consistently yellow with four black vertical stripes on each forewing that give the species its tiger name. The outer edge of the wings has a black border containing a row of yellow spots, and the hindwings have additional blue scaling along the inner portion of the border, along with orange spots near the tail. The tails themselves are long, narrow projections from the hindwing that are characteristic of the swallowtail family.
The underside of the wings is a paler yellow with a similar border pattern and orange and blue accents. One reliable field mark for males is a solid yellow forewing with no dark scaling in the body of the wing, which helps distinguish males from yellow females at a distance. Males tend to be slightly smaller on average than females, though there is enough overlap that size alone is not a reliable sexing method.
Male tiger swallowtails are highly active and often seen patrolling territories or nectaring on tall flowers like Joe Pye weed, thistle, and butterfly bush. They also puddle frequently, congregating on wet gravel, mud, or even road surfaces to drink and absorb sodium and other minerals. Puddling groups of male tiger swallowtails are a common midsummer sight along rural roads and stream banks.
Female Yellow Form
The yellow form female looks similar to the male at a casual glance, but there are several reliable differences visible with a moment’s attention. Yellow females have significantly more blue scaling on the hindwings than males, often extending the blue area into a broad band across much of the hindwing. This gives the hindwing a notably bluer overall look compared to the male’s mostly yellow hindwing.
Yellow females also tend to have more orange scaling in the submarginal area of the hindwing, creating a warmer color tone that distinguishes them from males. On the forewing, the black border may show some darker scaling that bleeds slightly into the yellow wing surface, a feature that is much less common in males. These differences are subtle but consistent in photographs where you can see the full wing surface clearly.
Behaviorally, yellow females are more likely to be seen visiting host plants to assess egg-laying sites than males. They use host plants including wild cherry, tulip poplar, sweetbay magnolia, and several other tree species. A detailed look at the caterpillar stages that follow egg-laying is covered in this article on the eastern tiger swallowtail caterpillar.
Female Dark Form (Mimicry)
The dark form female is startling if you expect a tiger swallowtail and suddenly see what appears to be a completely different species. Where the yellow form has yellow wings with black stripes, the dark form has wings that are predominantly black or very dark brown. The forewing shows faint shadow stripes that mirror the tiger pattern when held in certain light, but they are barely visible against the dark ground color.
The hindwings are the most striking part of the dark female. They show extensive iridescent blue-green scaling across the inner portion, and the border contains orange spots similar to those of the pipevine swallowtail. The tails remain, which helps confirm the identity as a tiger swallowtail rather than a pipevine swallowtail, but the overall appearance is close enough to cause confusion even among experienced butterfly watchers.
The dark form is a Batesian mimic. The pipevine swallowtail, which it resembles, accumulates toxic aristolochic acids from its caterpillar host plant, the pipevine. Birds that eat pipevine swallowtails get sick and learn to avoid the distinctive dark blue-black wing pattern. The dark female tiger swallowtail exploits this learned avoidance by looking enough like the toxic model that predators avoid her too, without the cost of being actually toxic herself.
The mimicry is only seen in females, not males. This is thought to relate to the relative vulnerability of the two sexes. Females spend more time perched and egg-laying, making them more vulnerable to sit-and-wait predators like birds than males, which spend more time in active flight. The protective mimicry therefore provides more benefit to females. A fuller picture of the different swallowtail species and their characteristics is available in this guide to swallowtail butterfly species.
Behavioral Differences
Males and females behave differently in ways that reflect their different biological priorities. Males spend a substantial portion of their adult lives patrolling territories along forest edges, hilltops, and other landmarks where females are likely to pass. They chase other males aggressively and investigate any large flying object that enters their territory, including birds and occasionally passing humans.
Males also engage in mate-seeking flights along tree canopy lines and over flower patches. When they locate a receptive female, courtship is brief but involves aerial pursuit and wing fanning that may transmit chemical signals. Unreceptive females signal their status by flying vertically, opening their wings wide, and raising their abdomen in a posture that discourages male approach.
Females’ primary activity after mating is finding appropriate host trees for egg-laying. They move through the landscape methodically, landing on and tasting leaf surfaces. A female may visit and reject dozens of leaves before finding one she judges suitable for laying. Individual eggs are deposited one at a time on the upper surface of leaves, and a female may distribute her eggs across many host trees over the course of days.
Females also nectar actively to fuel egg production, which requires more energy than the male’s primary activity of patrolling. Both sexes visit many of the same nectar sources including milkweed, ironweed, and Joe Pye weed, but females can often be seen spending more time at each flower cluster than males, which tend to visit briefly and move on quickly.
Frequently Asked Questions
How common is the dark female form?
The proportion of dark females varies geographically. In the southern parts of the range, particularly in areas where pipevine swallowtails are common, dark females can make up 80 percent or more of the female population. In the northern parts of the range where pipevine swallowtails are rare or absent, yellow females predominate because the mimicry has no protective value where predators have no experience with the model.
Can you tell males from females while they are flying?
Dark female forms are easy to distinguish from males in flight since the overall wing color is completely different. Yellow females are harder to tell from males while flying, but the broader blue area on the hindwing is often visible even in motion if the light is right. Behavior is another clue. A tiger swallowtail spending time on foliage and walking across leaves is almost certainly a female assessing egg-laying sites.
Do dark female tiger swallowtails ever appear in the north?
Yes, dark females are found across the species’ entire range, but they are much less common in northern populations. Individual dark females do appear in states like New York, Michigan, and Ontario, but they represent a small minority of females in those areas compared to the dominant yellow form.
Is the dark form female actually toxic?
No. The dark female is a Batesian mimic, meaning she gains protection from resembling the toxic pipevine swallowtail without being toxic herself. Batesian mimicry only works as long as the proportion of mimics stays lower than the proportion of genuine toxic models in the local population. If mimics become too common relative to the model, predators encounter the wing pattern more often and more often survive the experience, weakening the learned avoidance.
What is the lifespan of an eastern tiger swallowtail?
Adult eastern tiger swallowtails live for about 2 to 3 weeks under typical conditions. Most of that time is spent nectaring, patrolling, and, for females, egg-laying. The species produces 2 to 3 generations per year across most of its range, with the final generation of the year overwintering as a chrysalis in diapause before emerging the following spring.
