Urban Birds and Light Pollution

THE PROBLEM

Artificial light pollution has become a significant ecological issue as developments abut or even intrude into important wildlife habitat areas and migration corridors. Cities and towns around Australia have expanded upwards and outwards and have become more high-density residential over the last 10-15 years. This has been exacerbated further by developments occuring on or adjacent to land or coastal zones of moderate- to high-conservation value because most other sites amenable to development have been developed. Yet, light pollution impacts on wildlife are, at worst, ignored by most environmental consultants in Australia, at best, considered only in a cursory way.

The purposes of this article are to explain some of the light pollution impacts on bird species and populations, the fauna group that has been studied the most in relation to this topic, and to draw attention to some of the ways of minimising or avoiding those impacts.

Three groups of bird taxa, migratory shorebirds, nocturnal species (e.g. owl species) and coastal seabirds, are generally the focus of attention when relevant to a development or activity proposal. This is because they are listed as threatened, migratory or marine species under environmental legislation, and they live in habitats within or adjacent to the urban environment. But we should also be concerned about other bird species (and other animals) because, even though they are not yet threatened, many are known to be declining significantly in distribution and abundance. My discussion focuses deliberately on the ecological impacts to birds, whereas information about the vision physiology and sensitivity to light wavelengths by different bird groups should be consulted elsewhere.?

SOURCES OF LIGHT POLLUTION

Light pollution?is the presence of anthropogenic (human-generated) and artificial light in the night environment. It is exacerbated by excessive, misdirected or obtrusive use of light, but even light used carefully can have significant ecological impacts. Light pollution is often referred to ALAN (Artificial Light at Night) and comes in five forms:

Light Trespass (spill light). This occurs when a light fixture casts artificial light beyond property lines, illuminating other homes, businesses and wildlife habitat areas unintentionally. This is often caused by high or poorly-positioned street lights or outdoor lights around the home.

Glare. Unshielded light strikes the eye directly from the light source, e.g. floodlights at a sports stadium and headlights of approaching vehicles. Depending on the intensity of the light, glare can result in reduced visual contrast, colour perception and visual performance.

Glare occurs in three forms:

(a)??Discomfort (psychological) glare. This occurs when lighting causes annoyance or irritation, but does not decrease visual performance, and any discomfort is short-term.

(b)??Disability (veiling) glare. This occurs when stray light scatters in the eye, producing a veil over the retina. This reduces visual contrast, as well as colour and spatial perception.

(c)??Blinding (absolute) glare. This occurs when a light source impairs the field of vision, preventing the eye from seeing anything but the light source. Visual performance may remain affected for some time after the light glare incident.

Urban sky glow. This is light pollution that brightens the sky at night, often occurring as domes of light over cities. It is artificial light that is either emitted directly upward into the sky or is reflected from the ground and is scattered by dust and gas particles in the atmosphere, producing a luminous background. Sky glow increases the brightness of the dark areas of the sky, which reduces the contrast of stars or other celestial objects against the dark sky background.

Light Clutter.?This is excessive grouping of bright lights that cause confusion and distract from oncoming or surrounding objects. Typical examples of light clutter include light pollution from large refineries and street lights that are too bright and spaced too closely together. Clutter contributes to other forms of light pollution, including light trespass, glare, and sky glow.

Over-illumination. The excessive use of artificial light shone from the interiors of tall office buildings or on the exterior of landmarks or historical buildings. Over-illumination often contributes to light trespass and urban sky glow.

BIRD IMPACTS

Time and Energy Budgets

Several studies have shown that forest-breeding male songbirds near artificially-illuminated areas start their dawn chorus significantly earlier in the day than those in forested areas that are not affected by artificial light.?Birds that sing earlier may be getting less sleep and may be at a higher risk of predation.?Moreover, females gauge the reproductive fitness of males from the timing, duration and quality of their singing; therefore, females may be attracted to mate with lower-quality males who have been impacted by light pollution. Species that start their dawn song earlier in the morning under natural conditions are affected much more by the exposure to artificial light than birds starting dawn song later in the morning. This can be explained in terms of interspecific variation in eye size (early-morning songsters have relatively large eyes, which gives them better visual capability at low light intensities).

Diurnal bird species also begin foraging earlier under artificial lighting conditions. For instance, Australian Ravens and Little Ravens along the Hume Highway in NSW and Victoria fly low over illuminated sections at least one hour before dawn in search of fresh roadkill on which to scavenge. Australian Magpies, Magpie-larks and Masked Lapwings forage on large flying insects that are attracted to street lights throughout the night, and scavenge large dead insects on or to the side of the highway in the pre-dawn period. Whistling Kites are also soar low over the highway shortly after dawn, probably to find the best-quality meat from fresh roadkills before corvids and other early-morning scavengers consume all of it. While the timing of early morning activities of Galahs, Sulphur-crested Cockatoos and Little Corellas along the highway seem less affected by artificial lighting, Red-rumped Parrots are definite early-risers. Red-rumped Parrots leave their night-time tree roosts well before dawn and feed on copious quantities of grass seeds on the sides of well-lit areas of the highway.

More locally, Silver Gulls circling the Sydney Harbour Bridge at night and hawking moths and other large flying insects that are attracted to the bridge’s lights is an iconic feature of Sydney Harbour, particularly in spring and summer.

Pre-dawn activity in response to artificial light has the potential to negatively-impact on birds through depleted energy levels and exhaustion (longer days resulting in greater energy expenditure) and the attraction of predators. But it may be beneficial to some individuals if they use artificial light to advertise and patrol territorial boundaries, attract the best mates through courtship song, and if they are early arrivals at a rich source of food.

Artificial light at night shining on intertidal sandflats or mudflats can potentially assist shorebirds to forage visually for invertebrate prey, a more successful strategy to tactile foraging. But when roosting, shorebirds generally choose areas away from brightly-lit locations. Lengthy flights between roosting and foraging habitats can increase the risk of predation by raptors and reduce a shorebird's ability to build up fat reserves and complete their feather moult in preparation for long-distance seasonal migration. Therefore, they tend to forage and roost at the same or nearby locations. There is also some evidence that artificial light shining on roosting and foraging habitat can change the composition of shorebird communities because some species are more sensitive than others to light pollution.

Timing of Breeding

Under natural conditions, longer daylight hours stimulate gonadal growth and body fattening in birds, in preparation for breeding. Many diurnal bird species also vocalise at night during the breeding season to advertise for mates and defend territories, the intensity of this activity increasing under bright moonlight conditions. Birds subjected to longer daily light regimes in laboratory experiments undergo gonadal growth. It is not clear if artificial light in urban environments has the same effect, but it is possible because artificial light intensity at night can exceed the light intensity and duration thresholds that stimulate gonadal growth. However, this impact may be confounded by other environmental influences such as ambient temperatures and food availability, both of which are critical in the timing of egg-laying.?

Reproductive Success

Very little is known about the impacts of artificial light on the reproductive success of birds. A major influence is likely to be the availability of food at the time of nesting and dependency of young. For instance, Great Tits exposed to artificial light deliver insect food more frequently to chicks in the nest. On the one hand, this may be beneficial to the chicks because they are fed more food, but also potentially detrimental because the more frequently the adults travel to and from the nest, the greater is the risk of attracting the attention of predators. There may also be a significant energy cost to the adults associated with increased flight times to and from the nest.

Artificial light can also influence where birds nest. For instance, the early Black-tailed Godwit arrivals to breeding grounds in the Northern Hemisphere choose nest sites at a greater distance from artificial lights than the late-arriving birds. However, it is not known if nests located in artificially-lit areas result in lowered reproductive success, which could occur as a result of increased predation and/or increased distractive behaviours of attending birds.

Bird Migration

Many bird species (seabirds, migratory shorebirds and migratory land birds) are attracted to artificial light when migrating at night, especially if that light source is very bright. Typical sources of this light pollution are oil rigs at sea, lighthouses, large (including tall) buildings, refineries and infrastructure that are over-illuminated or produce a lot of light clutter, and homes that are located in or adjacent to wildlife corridors that would otherwise be very dark at night (e.g. remnant forest and woodland corridors). This effect can disrupt the orientation in night-migrating diurnal birds, especially when the sky is overcast. Disorientated flight can lead to mortality from exhaustion as a result of substantially-increased flight times, and from direct collision with the light structure.?

An estimated 365 million and 988 million songbirds are killed each year through collisions with glass in the United States while migrating at night. To a large extent, this is because they are attracted to the internal and external lighting of buildings and, in the process, collide with glass that is invisible to them.

NATIONAL LIGHT POLLUTION GUIDELINES

The Australian Government released its Draft National Light Pollution Guidelines for Wildlife in 2019. These guidelines recognise that natural darkness has the same conservation value as clean water, air and soil, and should be protected through good-quality lighting design. In promoting this overall principle, it prescribes the following management principles to reduce light pollution:

1. start with natural darkness and only add light for specific purposes;
2. use adaptive light controls to manage light timing, intensity and colour;
3. light only the intended object or area – keep lights close to the ground, directed and shielded;
4. use appropriate lighting;
5. use non-reflective, dark-coloured surfaces; and
6. use lights with reduced or filtered-out blue, violet and ultraviolet wavelengths.

There are also two Australian lighting standards that provide for human safety, but also limit unnecessary light pollution. These are:

1. Australian Standard DR AS/NZS 1158.3.1:2018 Lighting for roads and public spaces pedestrian area (Category P) lighting. This provides minimum light performance and design standards for pedestrian areas.
2. Australian Standard AS/NZS 4282:2019 Control of the obtrusive effects of outdoor lighting. This provides for consideration of environmental concerns, and is the regulation that is of most use to ecological consultants who wish to recommend ways to minimise light pollution impacts on wildlife.

The Australasian Dark Sky Alliance (ADSA), a non-government organisation dedicated to reducing light pollution has also recently set up a certification scheme that recognises luminaires that meet the requirements of the Australian and New Zealand lighting standards and the Draft National Pollution Guidelines. There are three categories of ADSA certification:

1. ADSA-approved: a luminaire delivering appropriate levels of performance for use in dark sky-friendly lighting designs.
2. ADSA-prized: a higher-level of luminaire management and performance, providing even greater control over sky glow and associated light pollution, as well as glare and other human factors.
3. ADSA-prized Wildlife Sensitive: for luminaires potentially impacting areas where sensitivities of the local wildlife take priority.

Therefore, recognition and managment of light pollution in Australia (and in other parts of the world) is likely to be strengthened over time through the implementation of appropriate legislation and regulation.