Exposure ranking: where many birds and much light coincide

A lights-out alert is only useful where many birds pass and many lights are on. This ranking multiplies the two, following the method with which Horton et al. (2019) ranked the cities of the United States.

The light is the artificial component of the zenith night-sky brightness according to the New World Atlas (Falchi et al., 2016), averaged over a disc of 10 km around the city centre. The natural sky sits at 0.174 mcd/m²: the «times the natural sky» column says how many times brighter that city's sky is than one without artificial light.

The birds come from the phase 3 model run over the weather archive from 2021 onwards at each city's coordinates: the mean density of the season and the mean of its ten busiest nights, which is when the alert matters. The index is normalised to 100 in the most exposed city.

Autumn

Spring

How to read it and what it is not

• The atlas measures the brightness of the sky seen from the ground, which includes light scattered by the atmosphere and light arriving from neighbouring towns. What matters to a bird flying at a thousand metres is the radiance the city emits upwards, which the VIIRS satellites measure. For ranking cities the two measures go hand in hand, but they are not the same quantity.
• The light data are from 2015. In ten years the switch to diode lighting has raised the blue component, to which birds are more sensitive, and changed the intensity in many cities.
• The 10 km disc is the same rule for every city, not the municipal boundary: in a large metropolitan area it clips, and in a small city it takes in dark countryside.
• The forecast densities are compressed in the tail: the intensity model is a mean regression, so its peak nights fall short of those observed by radar (predicted: 99th percentile of about 8 birds/km²; observed at the radars: about 30). For ranking cities it does not matter, because the bias is the same everywhere, but the figures must not be read as absolute densities.
• The light drives the order. Between the brightest and the dimmest city of the pilot there is a factor of 3-4, whereas in forecast birds there is barely a factor of 2. The ranking is mostly saying where more light is on, qualified by the passage; they are not two factors of similar weight.
• The index is not a measure of mortality. It says where the passage and the light coincide, which is the necessary condition; what happens also depends on the flight altitude, the fog, the type of luminaire and the presence of tall glazed buildings.
• The atlas licence forbids redistributing the files. This report publishes derived results citing both mandatory references; the raster is not uploaded anywhere. For a public version it is better to replace it with the VIIRS radiance, which is public domain and annual.

Citations

Falchi F. et al. (2016). The new world atlas of artificial night sky brightness. Science Advances 2(6):e1600377. — Falchi F. et al. (2016). Supplement to: The New World Atlas of Artificial Night Sky Brightness. GFZ Data Services, doi:10.5880/GFZ.1.4.2016.001. — Horton K. G. et al. (2019). Bright lights in the big cities: migratory birds' exposure to artificial light. Frontiers in Ecology and the Environment 17(4):209-214.