What Does Diurnal Mean?
(adjective) Diurnal describes birds and other animals that are principally active during the day with all major life activities, including courtship, nesting, feeding, preening, and other behaviors. Most species of birds, including songbirds, hummingbirds, and waterfowl are considered diurnal. Diurnal birds generally roost and sleep at night and become active again when the sun rises.
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When Diurnal Birds Are Active
Diurnal birds are most active during the day, but not necessarily only during the brightest sunlight hours. Many birds that are active during the day are most active in the morning and evening hours, when insects and prey are most active and foraging is easiest. Even seed-eating birds may be more active in mornings and evenings, as they refuel after a long night and stock up on food before the daylight fades.
This does not mean that diurnal birds are not active later in the morning, in mid-afternoon, or in the early evening. There are many activities these birds do each day in addition to feeding. They will preen, sing, bathe, defend their territories, cache food, adjust nesting areas, tend to hatchlings, play, and more. Any of these activities may take place at any time of day. In general, cloud cover or direct sunlight does not impact how active birds may be, even though overall sunlight levels can be dramatically diminished on a heavily overcast day. Only in cases of severe storms or other disruptions, such as strong eclipses, will birds' activity levels change drastically during daytime hours.
When a bird species is exclusively active only during the twilight periods of very early morning or very late evening, they are not considered diurnal, even though these activity periods are technically during daylight. Instead, twilight-active birds are considered crepuscular. Birds that are the opposite of diurnal and are active primarily during the night, and sleep and roost during the day, are considered nocturnal.
Diurnal Bird Species
Most of the world's bird species are considered diurnal. Birds that are almost exclusively active during the day include songbirds and other passerines, hummingbirds, woodpeckers, ducks, geese, swans, plovers, sandpipers, herons, egrets, and most raptors. Many flightless birds are also diurnal, including many penguins, ostriches, emus, and cassowaries, though other flightless species are nocturnal and rely on darkness for protection, such as kiwis. There are exceptions in nearly every bird family, however, and some unusual birds can still be active at night even if most of their avian relatives are more commonly associated with daytime activities.
Diurnal Birds at Night
When darkness falls, most diurnal birds retreat to safe, secure roosts and sleep. During heavy migration periods, however, the pattern can reverse. Many diurnal bird species migrate at night, when cooler weather and less turbulent air makes flying easier for them but predators are more scarce. On exceptionally bright nights with full moons, diurnal birds may also be more active than normal, though their activity levels do not usually rival normal daytime energy and activity. Individual conditions, such as a large nest of hungry hatchlings, a dangerous habitat, or unique behavioral preferences, can also make some normally diurnal birds more active at night than usual.
Daytime birds can also be disturbed at night and may take emergency flight from hunting predators or unexpected disturbances, such as large fireworks displays or strong storms. When this happens, diurnal birds are unusually vulnerable because their senses are not as sharp at night, and they may fall victim to predators, collisions, or other hazards. A severe disruption can be the cause behind a localized die off, such as a flock of normally diurnal birds being startled into dangerous flight because of fireworks, and the flock succumbs to many collision injuries.
Helping Diurnal Birds
Birders can best protect diurnal bird species by helping protect their natural daytime behavioral preferences. Providing safe habitat, nutritious food, clean water, and secure nesting sites is essential for diurnal birds to thrive. Minimizing nighttime light sources such as backyard lighting, pool lighting, or unnecessary street lights will help ensure the birds are not subject to confusing lighting conditions that can cause problems. Other nighttime hazards, such as outdoor cats, letting pets roam after dark, or excessive firework use, should also be minimized to protect roosting birds so they can be properly refreshed for the next day's activities.