Article and images by Dr Julie O'Connor, senior conservation partnerships officer, Sunshine Coast Council
The silver stealers
“Though always seeming so spotless, so graceful and beautiful, the silver gull is a scavenger, and an egg-stealer” Truth, Brisbane Sept 20 1936.
And “stealers” they are. When Kees Hulsman studied the robbing behaviour of silver gulls on One Tree Island in the Capricorn Bunker group in the 1970s, he documented a range of strategies employed by these birds to persuade several tern species to give up their fish catch. For example, their harassment of crested terns at nesting areas included:
- jumping from the ground to attempt to grab a fish from a tern flying a little above it
- walking or running at a tern to snatch a fish
- attacking the tern in flight from behind or below and grabbing the fish
- hovering over the tern on the ground and diving on the fish or the tern
- hovering over the tern on the ground then pursuing it when it fled
- chasing the tern until it swallowed or dropped its fish, and
- hovering above a tern on the ground without getting the opportunity to dive or chase its victim.
Crested terns, of course, have their own set of defences, and the gulls’ success rates varied from zero to a little over 25%.
However, not all their feeding habits have been viewed as stealing. When journalist Charles Barrett visited western Victoria in 1935 to write an article for The Herald, one farmer praised the gulls for their habit of following his plough to pick up the “takeall” grubs, an activity that he credited with saving his crop from the voracious grubs. In the same area, he also described gulls begging scraps from farmhouses, sneaking food from pig sties and scavenging food from factories. Not stealing exactly - more like enthusiastic and innovative scavenging.
The silver gull is Australia’s most common gull, but it also occurs in New Zealand and New Caledonia. They usually, but not exclusively, nest in large colonies on offshore islands. In his 1935 article, Barrett described the goings on in large nesting colonies of silver gulls on islets in Lakes Corangamite and Colac in western Victoria. An estimated 50,000 gulls were using the lake system with 5,000 nesting birds observed on just one group of islets.
Life can be tough in many bird breeding colonies. In Barrett’s words, death stalked the rookeries he saw on the islets of Lake Corangamite, due in part to the predation of nests by foxes and rats on those islets that were accessible from the shore via natural stepping stones.
The serene simplicity of the silver gull’s plumage appears to be at no greater odds with its personality than when it is nesting in a large busy breeding colony. As with many other colony breeding birds (gentoo and rockhopper penguins immediately come to mind), quarrels are frequent, and any bird walking or settling within reach of another is likely to be pecked and berated until it moves further away.
Silver gull nests are little more than a scrape in the ground, usually lined with seaweed and other vegetation. Both parents share incubation of the 1-3 eggs and share feeding duties of their developing brood. When resources are plentiful, they will often raise a second brood in a season.
One of my most vivid memories as a child holidaying with my mother’s family at Wollongong was the huge numbers of silver gulls ready to snatch up the nearest chip or sandwich dropped by beachgoers. A study undertaken in the region between 1988 and 1991 confirmed that the large silver gull numbers were more than an embellished childhood memory. Researchers found that gull numbers had grown both in the number of colonies and their sizes, with Big Island just off the coast of Wollongong supporting 43,000-50,000 breeding pairs. The authors attributed the expansion of the gull population to their ability to so effectively exploit anthropogenic food sources. Having adapted very well to human habitat and perfected the art of scavenging, the silver gull is definitely one of Australia’s urban winners.
- Hulsman, K. 1976. The robbing behaviour of terns and gulls, Emu, 76: 143-149
- Smith, G. C. and Carlisle, N. 1992. Silver gull breeding at two colonies in the Sydney-Wollongong region, Australia, Wildlife Research, 19: 429-41.