Prothonotary warblers (Protonotaria citrea) are stunning yellow songbirds found in wooded swamps and bottomland forests in the eastern United States. Their bright plumage seems almost too elegant for the boggy environments they prefer. These warblers have long been a source of interest for birders and ornithologists alike, in part because of their unique nesting habits. Prothonotary warblers are one of the only eastern warblers that nest in cavities, rather than building open-cup nests. But why do prothonotary warblers use cavities, when most other warblers do not? And what types of cavities do they choose for nesting sites? In this article, we’ll take an in-depth look at prothonotary warbler nesting ecology and behavior to understand why they are considered cavity nesters.
Prothonotary Warbler Nesting Habits
Prothonotary warblers nest in the abandoned cavities of woodpeckers and other wildlife. They do not excavate these cavities themselves, but rather rely on existing holes in dead trees (known as “snags”) located above or near water. Prothonotary warblers have a strong preference for nesting in flooded wooded wetlands and bottomland forests along streams, swamps, and lakes throughout their range.
Some key facts about prothonotary warbler nesting habits:
- Nest in pre-existing cavities in dead trees or “snags”
- Strongly prefer wetland habitats like wooded swamps and flooded forests
- Tree cavities are usually located over or near water
- Do not excavate their own nesting cavities
- Common nest sites are holes created by woodpeckers and natural decay
- Will also nest in artificial nest boxes placed in suitable habitat
Their dependence on appropriate cavity sites helps explain their close association with wooded wetland habitats. The waterlogged conditions in these forested wetlands create an abundance of snags and promote natural cavity formation through fungal decay. This provides plenty of potential nesting sites.
Advantages of Cavity Nesting
So why have prothonotary warblers evolved the behavior of nesting in cavities, when the vast majority of other warbler species build open-cup nests on branches? Cavity nesting actually provides several important benefits:
- Protection from predators – Cavities provide a physical barrier and seclusion from predators like snakes, squirrels, and raccoons. Nesting in holes makes eggs and young less vulnerable.
- Shelter from harsh weather – The enclosed cavity protects against heavy rainfall, high winds, and temperature extremes that could otherwise damage nests.
- Longer nesting season – Cavities provide warmer and more stable microclimates than exposed open-cup nests, allowing prothonotary warblers to breed earlier in spring.
- Reuse of nest sites – Cavities are a reusable resource that can save energy compared to building a new nest each year.
The protection and stability offered by tree cavities seems to outweigh any downsides, likely explaining why prothonotary warblers utilize this strategy rather than traditional open-cup nest building.
Cavity Attributes
Not just any cavity will do when it comes to prothonotary warbler nesting requirements. The warblers show strong preferences for cavities with certain attributes:
- Located over water or in flooded forests close to streams, ponds, etc.
- Excavated by woodpeckers, especially pileated woodpeckers
- At least 9 feet above ground or water
- Entrance diameter of 3 inches or greater
- Internal depth of at least 16 inches
- Lined with cypress leaves, grasses, pine needles, and other soft materials
Interestingly, one study in Louisiana found that cavities chosen by prothonotary warblers were on average higher above water, higher off the ground, larger in entrance diameter, and deeper than unused cavities.1 This suggests the warblers are able to assess cavity quality and selectively choose optimal nest sites.
Favored Tree Species
Prothonotary warblers also favor particular tree species when it comes to nesting cavities:
- Cypress (Taxodium spp.)
- Willow (Salix spp.)
- Maple (Acer spp.)
- Ash (Fraxinus spp.)
- Sycamore (Platanus occidentalis)
- Black gum (Nyssa sylvatica)
- Cottonwood (Populus deltoides)
These tree species are common in the forested wetland habitats used by prothonotary warblers, and are relatively prone to woodpecker excavation and natural cavity formation.
Adaptability
While prothonotary warblers clearly favor certain cavity characteristics, they also show adaptability in their nest site selection. In one study, the composition of tree species used for nesting did not always match the availability of snags across different wetlands.2 The warblers made use of whatever suitable cavities existed in a given area, even if that meant nesting in less preferred tree species. This flexibility likely helps expand their nesting options.
Threats and Conservation
The availability of suitable cavities limits prothonotary warbler abundance and productivity. Loss of flooded backwater forests and swamps reduces nesting habitat. Flooding and precipitation extremes with climate change may also decrease quality cavities. And competition with invasive house sparrows for cavities threatens nest success. Prothonotary warblers would benefit greatly from forest management and artificial nest boxes that augment nest sites. Their specialized nesting habits make cavity availability a key conservation focus.
Conclusion
In summary, prothonotary warblers are gram positive rocess that allows birds to take flight. Feathers provide the lift and thrust required for flight. They are lightweight, aerodynamic structures made of keratin, the same material found in hair and fingernails.
There are several specialized types of feathers that serve different functions:
- Contour feathers – Give the bird its shape and streamlined appearance. They cover most of the body.
- Down feathers – Very soft and fluffy feathers that provide insulation and warmth.
- Filoplumes – Hair-like feathers with sensory receptors that detect air movement.
- Flight feathers – Long, stiff feathers on the wings (remiges) and tail (rectrices) that provide thrust.
- Semiplumes – Small feathers with a central stalk and soft downy base. Help insulate birds.
- Bristles – Short, hair-like feathers around the eyes and beak.
The two main types of flight feathers are the remiges (wing feathers) and rectrices (tail feathers):
Remiges
Located along the posterior edge of the wing, remiges provide the majority of thrust needed for flight. They attach closely together in a tight, overlapping row to form a continuous lifting surface. There are three groups of remiges:
- Primaries – Longest and stiffest flight feathers located at the tip of the wing. Directly generate thrust and lift. Number varies from 9-11.
- Secondaries – Shorter flight feathers arising from the forearm. Help provide lift and control flexibility.
- Tertials – Innermost wing feathers that overlap and protect secondary feathers.
Rectrices
Found on the tail, rectrices help stabilize, steer, and brake birds during flight. They function as airfoils and rudders. The number of tail feathers varies widely by species, from 6 in some hummingbirds to 24 in peacocks. Rectrices are named based on their position:
- Central rectrices – Pair of feathers along the centerline of the tail.
- Lateral rectrices – Paired feathers extending outward from the central tail feathers.
Specialized structure and overlap of the remiges and rectrices allow birds to fly in different directions and perform aerial maneuvers with agility. The next time you see a bird take flight, look closely to see its feathers perform their elegant aerodynamic dance.
Conclusion
In summary, feathers are complex structures uniquely adapted for powered flight in birds. Their lightweight yet aerodynamic design allows the generation of lift and thrust. The precise arrangement and specialized types of flight feathers like remiges and rectrices give birds exquisite control over direction, speed, and maneuverability in the air. Feathers are a quintessential and beautiful example of an evolutionary innovation that literally helped life take off.
