Urban birds sing more loudly and at a higher pitch. The city slickers sport duller plumage feathers than country cousins. This time of year is harder on them food-wise, and too much "fast food" handouts can cause problems.
These are some of the insights from new studies of urban birds compared with their rural counterparts. Urban areas are expanding, so it is an important habitat to be taken into account. And intensification of farming has left some Irish birds more reliant on urban spaces. There are perks that come with residing in towns and suburbs, such as garden feeding, but downsides too, including fewer insects during summer.
“During winter, urban birds are heavier, whereas during the summer they are lighter than rural birds,” says Dr Caroline Isaksson, evolutionary ecologist at Lund University, Sweden, who studies urban living tits. “Feeding them is definitely good for their survival over winter.” She is less certain that feeding is always positive.
Isaksson has investigated nutrient quality and found some bird feeds are poorer nutritionally than wild sources. They had high levels of omega 6, compared with omega 3, and this combination is bad for health in humans. Isaksson's advice is to continue feeding birds, but she likens birds overeating sunflower seeds and peanuts to humans overeating pizza and hamburgers. "It is better to give them a variety of foods, rather than just sunflowers. Fruits, for example," she says.
Birdwatch Ireland's Brian Caffrey says the advice is to feed beyond the winter, not with bread but with appropriate foods such as niger seeds for gold finches, fat balls for tits and apples for blackbirds. Even birds such as robins, blue tits and thrushes are under increasing pressure in the countryside, and urban areas have become important for them, says Caffrey. But there are downsides too: pollution, declining green space and lots of cats.
City stresses show in feathers. The yellow-breast plumage of great tits is dimmer among urban birds. Isaksson theorised that the supply of the necessary carotenoid pigments were being drained off by an upregulated immune reaction in response to stressors such as air pollutants.
Her studies drew a more complicated picture, but pollution may be to blame. “We traced it back to urban trees and found their leaves had lower levels of these pigments, as did the caterpillars, as did the birds,” Isaksson say. “The breast plumage signals that the urban birds are in poorer condition. There must be consequences.”
Insect supply is another challenge. Birds generally switch to eating plenty of insects for protein in spring-summer. But insects can become scarcer in urban landscapes.
“Little patches of green spaces tend to be developed, people pave their gardens and school playing fields get sold off for houses,” says conservation biologist Dr Karl Evans at the University of Sheffield, who organised a recent conference on urban birds.
Indications are that urban birds are breeding less successfully than rural birds. And monitoring data points to starlings, song thrushes and blackbirds declining in the greater London area, says Dr Evans.
There isn’t adequate data in Irish urban areas in part because bird research in Ireland is poorly funded. But house sparrow populations are falling in both Ireland and Britain.
“One reason for house sparrow declines is insufficient insect food during the breeding season. We could help by improving the amount of good-quality native vegetation in cities,” says Dr Evans.
He suggests some mown-grass areas can be changed to urban meadows, with longer vegetation and more flowers. This would increase the quantity and variety of insects.
Another difference between city and countryside songsters is predators. Townies might encounter fewer natural predators, but some house cats snare impressive numbers of adult birds and fledglings.
Ecologist Dr Becky Thomas in Reading earned her PhD by studying what the cat dragged in. Any of 250 cat owners could phone her and she would drive over to pick up what the cat had caught.
She was especially busy in springtime. Cats caught an estimated 18 prey items each year, although that average misses the tale. Some cats never killed anything. Some cats killed one or two. But there were “super predators” killing lots that pushed up the figures. “There was one house I had to drive to every single day,” Thomas recalls.
Field mice were most common as prey items, but songbirds, especially robins and blackbirds, proved popular too. Cats can indirectly harm breeding success by simply finding nests and bird boxes.
There are positive steps cat owners can take. Birdwatch Ireland advises bell collars and being conscious of when cats are out. Thomas agrees.
“Birds are most vulnerable during the early hours of the morning. They need to feed when they first wake up and will be more willing to put themselves at risk. Keeping cats in in the early morning can reduce predation rates,” Dr Thomas says. For urban birds, every little helps.
CATASTROPHE: THE HARM CAUSED BY PREDATORS
The mere discovery of a nest by a cat can prove harmful: this finding comes from a study that positioned a mock cat close to a blackbird nest for just 10 minutes.
Predictably, the parent blackbirds fussed and nosily mobbed the predator. But another result was hungry chicks. Adult birds reduced their feed to the nest by one-third afterwards – feeding hadn’t returned to normal even after 90 minutes.
“Predation of the blackbird nests increased in the 24 hours after exposure to the model cat, which was surprising” says ecologist Dr Karl Evans at the University of Sheffield.
“Our data suggests that the loud noisy display by the parent birds to drive the cat away acts to draw other predators to the nest site.
"There were then signs of avian predators afterwards," says Dr Evans, most likely caused by crows such as magpies. Owners can help by limiting the amount of time cats spend outdoors, while being cognisant of cat welfare. One alternative to cat walkabouts is to walk cats on a leash. There are "how to" videos on YouTube.