03/16/2026
Rain is the biggest challenge for birds incubating eggs
Research supports the premise that rain is a significant, and sometimes the primary, environmental challenge for nesting birds, impacting them from the early stages of incubation through to the rearing of nestlings. Prolonged or heavy rainfall can lead to lower hatching success, nest abandonment, and high mortality among both eggs and chicks.
Why Rain is a Major Challenge
Rain creates several compounding problems for incubating birds that threaten the viability of the eggs and the condition of the parents.
1. Temperature and Thermoregulation
Chilling of Eggs: To survive and develop, the embryo inside an egg must be kept at an optimal temperature, typically between 34°C and 40°C. Rain cools the air and can soak the nest, rapidly drawing heat away from the eggs through conduction. If a parent is forced to leave the nest to forage, the eggs may cool below the lethal threshold, halting development or killing the embryo.
Energetic Cost to Parents: Maintaining a warm microclimate in a cold, damp nest requires significantly more metabolic energy from the parent bird. The parent must use its own body heat to re-warm cooled eggs and maintain their temperature, a process that can deplete its fat reserves. If the parent's body condition deteriorates too much, it may be forced to abandon the nest to save itself.
Vulnerability of Hatchlings: When eggs do hatch, the nestlings of many species are "altricial," meaning they are born nearly naked, cannot regulate their own body temperature, and are extremely vulnerable to chilling in wet weather.
2. Physiological Risks to the Egg
Blocked Gas Exchange: Bird eggshells are porous to allow the developing embryo to breathe (exchanging oxygen and carbon dioxide). If the shell is covered by a film of water for a prolonged period, it can effectively "drown" the embryo by blocking this critical gas exchange.
Bacterial and Fungal Infection: Warm, damp environments are ideal for the growth of pathogens. Excess moisture can promote bacterial growth or fungal infection that can pe*****te the shell and kill the embryo.
3. Reduced Resource Availability
Foraging Difficulties: Rain reduces the activity of many insect prey, such as flying insects, which are crucial food sources for many bird species. This makes it much harder and more time-consuming for the parent to find food, forcing it to spend more time away from the nest or go hungry itself, both of which jeopardize the brood.
Indirect Impacts and Evidence for Abandonment
Studies indicate that rain can have a greater impact on nest success than temperature alone. In some cases, the combination of high energetic demands on the parent and low food availability during wet weather is the main driver of nest failure.
A study on Great T**s, for example, found that the majority of nest abandonments occurred during the incubation period or early nestling phase, and that the amount of rainfall was a more important predictor of abandonment than temperature. This suggests that birds may sometimes make a trade-off, deserting a doomed brood to preserve their own condition for future breeding attempts when weather conditions improve.
How Birds Mitigate the Challenge
While rain is a formidable challenge, birds are not defenseless and have evolved various behavioral and physical adaptations to cope with it:
Nest Placement and Structure: Birds often select nest sites in sheltered locations, such as under tree canopies, within dense shrubs, under the eaves of buildings, or inside cavities (holes in trees or nest boxes). Some species build domed or enclosed nests, and many incorporate water-repellent materials like preen oil-coated feathers into the nest lining.
Parental Behavior as a "Living Umbrella": An incubating parent will use its own body to protect the eggs, crouching low and spreading its wings and tail feathers to form a waterproof shield over the nest. Feathers, particularly when maintained with waterproof preen oil, are remarkably efficient at deflecting rain.
Cuticle: Most bird eggs have a thin, waxy coating called a cuticle that acts as a partial hydrophobic (water-repelling) barrier, offering some protection against short periods of dampness. This cuticle is often thicker in species that nest in rainy environments.
