Willow Warbler

Quick Take

• This bird weighs less than a AA battery, and yet it pulls off a feat of endurance that leaves scientists scrambling for explanations. See its tiny size →
• Unlike virtually every other bird on the planet, the willow warbler does something twice a year that scientists still can't fully explain. Explore the molting cycle →
• Its population numbers in the hundreds of millions, yet conservationists in one country have quietly put it on a warning list. Here is the tension behind those two facts. Check the conservation status →
• Three of its eggs fit on a single penny, and what happens in the weeks after they hatch is stranger than you'd expect. Follow the chick development →

The willow warbler is a small, migratory songbird recognized for its melodious song. You can see it in parks, gardens, and woodland areas all over Europe and Scandinavia. This bird undertakes a remarkable migration, traveling thousands of miles to Africa each year.

Classification and Scientific Name

The willow warbler’s scientific name is Phylloscopus trochilus. The willow warbler is one of around 80 species in the Phylloscopus genus, which combines the ancient Greek words for leaf, phullon, and skopos, meaning seeker. Therefore, the willow warbler’s scientific name means “Leaf Seeker”. Trochilus is the ancient Greek word for wren.

Subspecies

The differences in the three wood warbler subspecies involve slight variations in plumage, with centrally located birds being browner and eastern birds sporting grayish feathers. Breeding and wintering grounds are also unique to each subspecies.

• Phylloscopus trochilus trochilus: Breeding grounds are located from the Pyrenees and Alps northward, excluding northern Scandinavia. This willow warbler winters in West Africa.
• Phylloscopus trochilus acredula: Breeds in northern Scandinavia to western Siberia and winters in central Africa.
• Phylloscopus trochilus yakutensis: Breeds in eastern Siberia and winters in eastern and southern Africa.

Appearance

A willow warbler is a small, round bird with olive green top feathers and pale yellow feathers on the chest. It has pale legs and a stripe of yellow over each eye. It is around 4 to 4.9 inches long (11 to 12.5 cm) and weighs 0.25 to 0.53 ounces. The willow warbler’s beak is long and slender, which is handy for pulling insects out of trees and leaves.

For its size, the willow warbler has long primary feathers and long wings. It needs them for the long migration, which it makes twice a year. These birds travel huge distances to reach warm weather, and they travel long distances to return home.

Behavior

A warbler is any small bird that sings in a trilling or warbling manner. Willow warblers usually sit on tree branches singing. Like many birds, they are diurnal and prefer to hunt for food during the day.

One unusual fact about them is that they molt their feathers twice a year, unlike most birds that molt once a year. They molt during their breeding season and at their winter migration destination. Scientists are not sure why they do this. Molting helps bird feathers stay strong and healthy; new wings could be an advantage for this bird’s long migration. A 2009 study found that migrating songbirds fly much faster than scientists believe. The study estimated that a songbird could cover 300 miles in one day of flight.

Habitat

Willow warblers prefer open, wooded habitats of all kinds, including mixed forests. Their preferred trees are birches, alders, and the willows that give them their name. They use shrubs, brambles, and other low-lying plants to hide their nests. They also like human habitats, including farms and gardens, and environments with young trees. They make use of anything in their environment, including patches of low bramble, bracken, mosses, water features, and fallen leaves.

The willow warbler is a very common visitor to gardens and forests in Europe. The Wildlife Trust estimates there are more than 2.4 million nesting pairs that take their annual migration south from the U.K.. It is one of the most common birds in Scandinavia. There are about 24 million pairs in Sweden and Finland.

Diet

Insects and spiders are the main diet of a willow warbler. Its diet includes all bugs, but it will also eat berries and fruits in the autumn. Scientists believe migratory birds eat fruits and berries to help them store fat for their long journeys.

Reproduction and Life Cycle

The willow warbler is a versatile builder who makes nests on trees, thickets, and the scrub grass of the Arctic tundra. It will also breed and nest in shrubby areas away from trees. Willow warbler nests have been found in orchards, hedges, and dirt mounds. The warbler uses dry grass, leaves, moss, and strips of bark to create a distinctive, dome-shaped nest. The shape creates a warm place for the bird’s eggs.

The breeding season lasts from April to July. After mating with a male warbler, the female lays four to eight eggs. Their creamy, speckled eggs are tiny. The Wildlife Trust points out that three willow warbler eggs together are about the size of one penny.

The mother warbler hatches the eggs herself, but once the babies are born, both parents feed them. The babies are born blind and without feathers. For the first 10 to 11 days, the parents bring food to the nestlings. After that, the fledglings leave the nest. They may accompany their parents on foraging trips while they learn about the world. They also practice flying as their wings continue to grow. In about six weeks, they lose the last of their baby fluffiness and look more like their parents.

Population

Songbirds around the world are facing population losses because of climate change, human predation, and habitat loss. Like all songbirds, willow warblers have seen their population numbers decline in the past few decades. In fact, there are only about 400 million to 600 million willow warblers in the world.

Willow warblers are listed as “least concern” for conservation status under the International Union for Conservation of Nature (IUCN). However, they are listed as “Amber” status on the Birds of Conservation Concern list in the U.K.