The Reddish Egret (Egretta rufescens) is a wading bird in the heron family Ardeidae. It is a medium-sized heron with reddish-brown plumage during the breeding season and all white plumage during the nonbreeding season. There are two distinct color morphs of the Reddish Egret which differ in the amount of reddish-brown plumage during breeding season. The white morph has little to no reddish-brown plumage, while the dark morph has significant reddish-brown plumage on the head, neck, back and wings. The morph is not linked to gender, age or location. Both morphs occur across the Reddish Egret’s range along the Gulf Coast of the United States and Mexico.
Description of the Two Morphs
White Morph
The white morph of the Reddish Egret has primarily white plumage year-round with little to no reddish-brown coloring. During the breeding season, the bill becomes two-toned with a black tip and blue-gray base. The legs become pinkish-red. The lores (the region between the eye and bill) may show a barely perceptible reddish or pinkish color. The irises of the eyes are yellow. Outside of breeding season, the white morph has uniformly slate gray legs and all yellow eyes.
Dark Morph
The dark morph has significant reddish-brown plumage during breeding season. The head, neck, back, wings, and chest take on a rich rufous (reddish-brown) color. The crown and crest may appear shaggy from elongated breeding plumes. The flight feathers remain grayish. The front of the neck has vertical white streaks. The legs become bright pinkish-red. The eyes are pale blue. Outside of breeding season, the dark morph loses most of the reddish-brown coloring and resembles the white morph but may retain some pale brown streaking on the head, neck and upper back. The eyes turn pale yellow.
Distribution of Morphs
Both the white and dark morphs of the Reddish Egret occur across its coastal habitat range along the Gulf of Mexico, including Texas, Louisiana, Mississippi, Alabama, and Florida in the United States, and along both coasts of Mexico.
There is no clear geographic separation of the two morphs. Individual breeding colonies may include both white and dark morph individuals. However, the proportion of each morph does vary in different regions:
– Texas: Approximately 90% dark morph, 10% white morph
– Louisiana: Approximately 65% dark morph, 35% white morph
– Florida: Approximately 90% white morph, 10% dark morph
– Mexico: Primarily white morph along the Yucatán Peninsula; mix of morphs along the western Gulf Coast.
Overall the dark morph is more common, estimated at approximately 70-75% of the total Reddish Egret population. However, the white morph becomes more prevalent towards the eastern extent of the Reddish Egret’s range.
Origins and Genetics of Morphs
The distinct white and dark breeding morphs of the Reddish Egret were first described by ornithologists in the late 1800s. However, the genetic basis and evolutionary origins of this unusual polymorphism remained unclear for many decades.
Recent genetic research has shed light on the morphs:
– The morphs are not separate sub-species – there is no genetic differentiation between the white and dark morph populations.
– The morphs are produced by a single locus autosomal gene with two alleles. The dark morph is the partially dominant phenotype.
– The dark allele originated from a mutation that occurred over 2 million years ago, predating the origin of the Reddish Egret species.
– The mutation was likely immediately selected for due to conferring advantages in hunting fish prey. The reddish-brown plumage provides camouflage against sunrise/sunset light over shallow water.
– The white morph is maintained in the population due to frequency-dependent selection. The rarer white morph may have an advantage catching prey startled by the more common dark morph.
Behavioral Differences Between Morphs
In addition to the striking physical differences, researchers have documented some subtle differences in behavior between the two morphs:
– Foraging Habitat: The dark morph tends to forage in mangroves and vegetation-fringed wetlands. The white morph tends to forage in more open habitats like tidal pools.
– Foraging Technique: The dark morph tends to be more stationary and stalks prey methodically. The white morph is more active and uses rapid foot stirring to startle prey.
– Nesting Density: Dark morph individuals tend to nest at higher densities than white morphs.
– Mate Choice: Pairs are not assortative – white and dark morphs interbreed freely. However, at higher nesting densities dark morphs have a slight tendency to pair with each other.
The behavioral differences are slight and relate to different foraging strategies. There does not appear to be major competitive exclusion between the morphs.
Conservation Status
The Reddish Egret has experienced population declines over the past century due to habitat loss and disturbance along its limited coastal range. It is considered a threatened or endangered species in many states throughout its US range. The combined population is estimated at only 6,000 to 10,000 breeding pairs.
Both white and dark morphs have declined proportionally. Neither morph has higher conservation priority. Protection of coastal wetland habitats is essential for stabilizing Reddish Egret populations and allowing polymorphism ratios to reach equilibrium based on natural selection.
Due to their spectacular appearance and rarity, Reddish Egrets have become prized by birdwatchers. Care must be taken by ecotourism operators and photographers to avoid disturbance to breeding colonies.
Conclusion
The Reddish Egret exhibits a rare plumage color polymorphism, with both white and reddish-brown morphs occurring in variable ratios across its coastal range. After more than a century of speculation by ornithologists, recent genetic research has shown that this polymorphism originated from a mutation millions of years ago and has been maintained by natural selection likely related to foraging strategy. Though subtle behavioral differences occur, the morphs freely interbreed and occupy overlapping habitats. Conserving remaining coastal wetlands will benefit Reddish Egrets of both morphs. The continuance of this striking polymorphism in a threatened species exemplifies nature’s endless creativity. Careful stewardship is needed to ensure this phenomenon endures.
