Why Do Birds Need a Puddle? Feathers Do Not Work Dirty

A feather is a precise structure whose barbs hook together to form a continuous surface, and that surface does the work of flight and of insulation. Dirt, dust, dried secretions and parasites disrupt it, and a bird with disordered feathers flies less efficiently and loses heat faster, which in cold weather is a survival matter. Bathing wets the feathers and loosens that material, and the preening that follows is the actual maintenance, in which the bird draws each feather through its bill to rehook the barbs and spreads oil from a gland at the base of the tail across the surface. Water is therefore preparation for preening rather than an end in itself.

The requirements are specific and most ornamental designs fail them:

• •Water no deeper than about two to four centimetres at the deepest point
• •A rough surface, since birds will not stand on anything slippery
• •Sloping sides, so several sizes of bird can find a comfortable depth
• •Placement near cover, so a wet bird can reach shelter quickly
• •Not directly beneath dense cover, which conceals an approaching cat
• •Cleaning and refilling regularly, since droppings and algae accumulate fast

Water is one method among several and the alternatives are worth recognising. Dust bathing, in which a bird works dry soil through its feathers and then shakes it out, absorbs excess oil and dislodges parasites, and species including sparrows and gamebirds use it in preference to water. Sunning, with wings and tail spread and feathers raised, appears to help control parasites and may assist the production of vitamin D. Anting, in which a bird deliberately picks up ants and rubs them through its plumage or sits on a nest and allows them to swarm over it, is well documented and not fully explained, with the leading suggestion being that formic acid from the ants deters parasites.

The behaviour is stereotyped and worth watching closely, since the sequence is the same across a great many species. A bird approaches, checks repeatedly for danger, and enters cautiously, since being wet reduces the ability to escape and every bird appears to know it. It then crouches, ruffles its feathers to expose the skin, beats its wings to throw water over the back, and dips its head and shakes. The whole episode lasts under a minute in most cases and is interrupted constantly by looking up. Afterwards the bird moves to a perch, shakes vigorously, and preens for considerably longer than it bathed, which is the part that matters.

Providing water attracts a wider range of species than providing food and does so throughout the year, since every bird needs to drink and bathe while only some visit feeders. That is most valuable in two situations. In hot dry weather natural sources disappear, and a reliable supply concentrates birds from a wide area. In freezing weather every open water source may be frozen, and access to liquid water becomes genuinely limiting, which is why keeping one ice free matters more than any amount of food. Hygiene is the trade, since a shared water source transmits disease efficiently, and outbreaks of trichomonosis in finches have been linked to contaminated drinking sites, so regular cleaning is not optional.

Feathers form a continuous hooked surface that dirt and parasites disrupt, so bathing wets and loosens that material and the preening afterwards is the actual maintenance. Water two to four centimetres deep, a rough surface, sloping sides and nearby cover are what make a dish usable. Dust bathing, sunning and anting do comparable work. Water attracts more species than food and matters most in drought and in frost.