An all-too familiar pest, the seagull poses both a health risk and aviation threat due to the high incidence of birdstrikes.
‘Seagulls’ are one of the key pest birds in Australia. Management can be more challenging than for other pest birds, as seagulls in Australia – the silver gull, Chroicocephalus novaehollandiae and the Pacific gull, Larus pacificus – are both native species. Here we focus on the main pest gull, the silver gull.
The silver gull is found throughout coastal Australia and inland areas where there are significant water bodies. Although its natural diet consists of fish, crustaceans, worms and insects, it is a very adaptable bird and has become a successful scavenger in urban areas, getting food from garbage dumps, shopping centres and unsuspecting tourists!
Much of their pest status is the same as for other pest birds – noise, mess, transmission of diseases and parasites. It’s just that because they are so much bigger than other pest birds, there is a lot more noise and a lot more mess! Also, by virtue of their size, they present a significant issue to aviation traffic, particularly around airports and can be quite aggressive in ‘begging’ outdoor diners for food.
The silver gull is a large bird up 45 cm long with a wingspan of up to 94 cm. However, it is much smaller than its Northern Hemisphere counterparts, the herring gull and lesser black-backed gull, which both having a wingspan of up to 125 cm. The silver gull was previously classed in the same Larus genus as these Northern Hemisphere gulls but was recently moved into the Chroicocephalus genus.
Silver gulls breed continuously between August and November, although generally, mated pairs will produce two broods a year, each consisting of between 2-4 eggs. In their natural environment they will build their nests on the ground or on cliffs in isolated areas and offshore islands. In urban areas, roofs and ledges become the preferred nesting sites, which generates the noise and mess that contributes to its pest status.
Silver gull populations have increased over the decades since the second world war, which gives an indication of the main influence on population dynamics and also the main route to management. Silver gull numbers have increased along with urban development and the availability of food. The key is to prevent the birds from accessing waste food – for pest managers this means ensuring clients have rubbish in sealed bins and that access to rubbish and food preparation areas is denied through the use of bird netting.
The amount of food silver gulls obtain from outdoor diners can be significant. Not only does this support the gull population, but humans actively feeding the birds can lead to dangerous encounters as the birds start demanding more. Education and clear warning notices in troublesome areas warning against feeding the birds and advising to place rubbish in bins is an important element in reducing gull problems.
Open garbage tips are somewhat more problematic. Whilst scaring devices have some impact, silver gulls seem able to learn to ignore scaring devices once they realise they are not associated with an actual threat. Regular rotating of techniques is likely to provide the best results.
As a native bird, culling is not an option unless there are exceptional circumstances. But even then, it would only likely have a short-term effect as without reducing food availability, other gulls will move into the area and the population will rebound. Preventing silver gulls nesting on buildings relies on suitable ledge protection products such as spikes and optical gels.
Silver gulls represent two significant pest issues that are generally not an issue for other pest birds. Firstly, there’s the aviation threat. For example, between 2001 and 2011 gulls have been involved in at least 281 birdstrikes at Sydney airport (CAA data). Population reduction has been achieved by netting over nearby water bodies, improving airfield drainage, more frequent mowing of the airfield and insecticide spraying (to remove potential food) and reducing exposure to nearby household rubbish. However, as many of the cities’ major airports are in coastal locations, the threat of birdstrikes is constant.
The second pest issue is from significant gull populations around drinking water reservoirs. Their droppings and regurgitated food can generate a significant increase in the bacteria that cause gastro illnesses. In such situations, rotating scare techniques are generally the only option.