A Song Sparrow fighting with its reflection in a window
Birds become extremely territorial in the spring, and sometimes see their reflection as a challenger who needs to be driven away. This futile effort will end within a few weeks as hormones fade.
■ As humans transform the landscape we also transform the soundscape. The industrialized world is awash in low-frequency noise. Since sound is such an important means of communication for birds, any extra noise has a significant impact on them. Surveys find reduced populations of many species near roads, industrial sites, and other noisy environments, and much of this decline is attributed to noise. Some species (such as the Mourning Dove, with its low-pitched song) simply avoid nesting in noisy places. Birds that do live in noisy places are shifting the pitch of their songs higher, which helps to differentiate their sounds from low-pitched noise. It’s not clear whether songs are actually changing as a direct adaptation to communicate better through the noise, or whether birds simply try to sing louder in noisy places, and produce higher-pitched sounds when they “shout.”
A Song Sparrow singing
■ Wild birds face two competing risks every day—starvation and predation. They must eat without being eaten, and they have to find enough food every day to last through the long night. Finding food requires searching, often in the open, and eating adds weight that slows a bird down. They are constantly assessing the risks and the benefits of any potential food source. Experiments have shown that when birds know that predators are around, they delay feeding until later in the day. That way they stay light and agile through the afternoon, and put on weight just before they go to sleep.
A Song Sparrow foraging at sunset
■ Across its wide range, from Atlantic to Pacific and Mexico to Alaska, the Song Sparrow shows a lot of variation in size, shape, and color. Most of this variation follows some general patterns that are shared with other wide-ranging species. Birds living in more humid climates (like the Pacific Northwest) tend to be darker than those in drier climates. One obvious benefit is that those colors match the surroundings better; an additional benefit is that the dark pigment melanin helps protect the feathers from bacteria, which are more active in humid climates. Another broad trend is that birds in hotter climates have relatively large bills and feet. This helps them regulate their body temperature by allowing more heat to escape from those uninsulated parts. In cold climates smaller feet and bills help reduce heat loss.
Song Sparrows from hot and dry Arizona (left), and cool and humid British Columbia (right)