Deadly stats on bird mortality

HOW many birds die as a result of human activities each year?

Deadly stats on bird mortality

Canadian government scientists claim 269 million birds are killed by people, directly or indirectly, each year in Canada and two million nests are destroyed. The figures come from 10 papers published in a special edition of Avian Conservation & Ecology. Our birds, with some exceptions, are not the same as theirs and the climate is more severe there. However, Irish and Canadian birds face similar challenges.

Canada has 8.5m house cats and 1.4m to 4.2m feral cats. These kill between 100m and 350m birds each year, according to Peter Blancher of the National Wildlife Centre in Ottawa. Feral cats account for about 59% of kills. Cats, Blancher believes, ‘are probably the largest human-related source of bird mortality in Canada’.

The effects of power lines on the landscape, and their alleged threat to human health, are hot topics in Ireland. The impact of cables and pylons on birds will, no doubt, be raised at forthcoming oral hearings. Canada has 232,000km of transmission lines. Between 2.5m and 25.6m birds collide with them each year, an estimate so wide as to be virtually useless for practical purposes. Waterfowl, waders and grebes are the main victims. There may be significant impacts on trumpeter swan and whooping crane numbers but ‘collision mortality, at current levels, is not limiting population growth’, according to Sébastien Rioux and colleagues.

Surprisingly, buildings present more serious collision obstacles to birds than electricity cables. Craig Machtans and co-authors claim that 16m to 42 million birds die hitting windows in Canada annually. Houses account for 90% of the carnage, low-rise buildings for 10% and high-rise around 1%. Windfarm operations were monitored at 43 sites. On average, 8.2 birds were killed per turbine per year. The number for individual turbines varied from 0 to 27. Habitat lost to birds was 1.23 hectares per turbine, leading to a reduction of 5,700 nests for the country as a whole. ‘Population level impacts are unlikely, provided that highly sensitive or rare habitats, as well as the concentration areas for species at risk, are avoided’, say Ryan Zimmerling and co-researchers.

Experts on the impact of road vehicles on birds couldn’t come up with an overall mortality figure. Measurements taken on selected paved roads, outside urban areas during the breeding season, suggest that about 3,462 birds are killed per 100km.

As in Ireland, the barn owl is particularly vulnerable on Canadian roads. It’s estimated that 851 owls are killed annually on four-lane roads in their ‘last remaining range in Canada’.

Seabirds become entangled in fishing nets and oil spillages at rigs kill them. Mortality at sea can be high locally but is relatively low on a national scale, the scientists believe. They admit, however, that their estimates are based on ‘untested assumptions’.

Harvesting machines have been blamed for the demise of the corncrake in Ireland. Partridge and quail have also disappeared. Nests are destroyed and birds sitting on eggs are killed. Noise and disturbance prevent chicks from fledging. These, and the widespread use of lethal pesticides, have dramatically depleted populations of grassland birds. Similar problems are encountered in Canada. Fifteen species declined ‘significantly between 1980 and 1999’, says lead author Joerg Tews.

A summary paper by Anna Calvert, and six co-authors, combines the findings of the specialists in an overall assessment of Canadian bird mortality. ‘Land birds are the most affected group’, they claim, and ‘the highest industrial causes of mortality were the electrical power and agriculture sectors’. Most importantly, the estimates cover direct mortality only. The effects of habitat destruction climate change and site alteration must also be taken into account in assessing bird mortality.

Is the picture much different here? * Quantifying human-related Mortality of Birds in Canada. Travis Longcore & Paul Smith. Avian Conservation & Ecology. 2013.

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