Birds and Buildings: Living on the Edge.

Long, broad ledges on city buildings, especially those providing shelter from the wind and the rain, and provide a good view of the surrounding landscape, are popular with many city birds. Ledges that are often used by birds are window ledges, shallow open horizontal (or near-horizontal) stormwater drains, and broad horizontal facades that are sheltered by overhangs. Birds use these locations for some or all of the following activities, depending on the species: roosting; resting; feeding; courting and mating; nesting; avoiding predators; and as vantage points from where they can observe the surrounding landscape. Ledges that are high above the ground  are preferred by most species that use them because they are less likely to be subject to anthropogenic and associated disturbances, provide more expansive views of the urban landscape, and are often sheltered better than lower ones. Favoured ledges are also usually located within viewing distance of the closest reliable food source.

Most bird species that use building ledges are communal species which scavenge on food scraps left by humans, or which are deliberately fed by humans. Communal species that commonly use ledges in many Australian cities (especially in the eastern half of the country) include Sulphur-crested Cockatoos, Little Corellas, Rainbow Lorikeets, Feral Pigeons, Welcome Swallows, European Sparrows, Common Mynas and Common Starlings. While most building ledges are usually not broad enough to accommodate Australian White Ibises and Silver Gulls, these species will use flat, low roof areas and canopies for loafing and as vantage points that allow them to observe and seize nearby food scraps.

Ledges are also important as brief resting areas for migrating and dispersing birds, particularly small passerines. While some cities are along established migratory pathways of many bird species, particularly in Europe, North America and the east coast of Asia, many individual birds are blown into cities by strong winds while migrating or dispersing, or are attracted by artificial light at night (ALAN). Individuals in this latter group are usually exhausted and disorientated and ledges provide them with the opportunity to rest before continuing their journey. However, ledges are alien environments to most, if not all, passerines, and those that use them are at risk of being preyed upon by aerial predators, rodents and cats. Therefore, they only generally rest on a ledge for a few seconds, sometimes for a few minutes, before taking flight again, provided they have the energy. Some of the more unusual passerines that I have observed on window ledges of multistorey buildings in the Sydney CBD over the years include the Silvereye, Yellow-faced Honeyeater and Brown Honeyeater, the latter two species not usually recorded within inner areas of Greater Sydney. The last European Blackbird that I observed in the Sydney CBD was in 1994, when it landed for no more than five seconds on a sixth-storey window ledge of an office building where I was working at the time.

Arthropodivorous (arthropod-eating) birds often glean spiders and moths from the edges of windows, either by hovering in flight, performing a quick snatch and grab, or landing on a ledge and reaching over to the prey item. This is more common on windows of houses or ground-level apartments, especially where there are gardens. However, I have occasionally seen medium-sized passerines such as Red Wattlebirds and Noisy Miners use lower-storey window ledges in the Sydney CBD for this purpose.

Raptors worldwide recorded nesting on ledges of occupied city buildings include Lesser Kestrel, American Kestrel, European Kestrel, Madagascar Kestrel, Seychelles Kestrel, Australian Kestrel, Lanner Falcon, Lagger Falcon, Orange-breasted Falcon and Peregrine Falcon. Individuals of many of these species nest on ledges of cliff-faces or in caves in natural habitat areas, and building ledges provide similar nesting habitat. Nests are usually located on a horizontal ledge on one of the upper storeys of tall buildings, and which is in a sheltered location that is also secure from predators. Broad ledges also provide locations for city raptors to feed on prey, away from anthropogenic and other disturbances, as well as ideal vantage points from which to search for prey in the surrounding landscape. Although I am not aware of Brown Falcons and Black-shouldered Kites nesting on building ledges in inner city areas, they are two additional Australian raptor species that I have observed using ledges or roofs of buildings located near the urban fringe while they were consuming their prey.

Raptors in urban environments generally breed earlier and have larger brood sizes than conspecifics in rural environments. However, some raptor species also fledge fewer young, caused by a lack of prey and, in some cases, increased anthropogenic disturbances. Raptors that prey on small mammals in urban environments are most likely to be least successful at fledging young (e.g. Australian Kestrel), whereas those that normally prey on medium-sized urban-dwelling birds (e.g. Feral Pigeons, Rainbow Lorikeets) are more successful (e.g. Peregrine Falcon). Eurasian Kestrels rely heavily on voles as a dietary item in natural habitats, whereas city populations prey heavily on insects, leading to malnutrition and hormonal stress in nestlings. Anthropogenic disturbances in urban areas (e.g. traffic and construction activities) can lead to elevated stress hormone levels in breeding American Kestrels and an increased rate of nest abandonment. Therefore, urban environments, particularly inner-city areas, can be an ecological trap for some raptor species.