Despite their proven predilection for carbohydrates, when the weather changes so does the diet of inner-city ibis.
By quantifying bird density, bin density, food dumping and picnicking in Sydney’s parks, our research group from the University of Wollongong, the Royal Botanic Gardens and the Australian Museum discovered that the Australian White Ibis, known fondly as the Bin Chicken, is engaged in complex foraging decisions mediated by rainfall.
All of Sydney’s parks supported good populations of Ibis, but Belmore Park was the stand-out in terms of fast food volume. This park gets its fair share of people sharing their lunches, but also has a high bin-density supplemented by the excess from nearby restaurants.
In dry weather, Australian White Ibis congregate in Belmore Park (adjacent to Sydney’s Central Station) at a density that is 10 times higher than other parks, including Hyde Park and the Domain.
But after rainfall, ibis numbers dropped in Belmore - a trend that was backed up by seven years of additional survey data. No such decline was apparent in the other parks, some of which saw their visiting Ibis populations increase after rain.
The Eureka moment was when an anonymous member of the research team emptied his home-brew sterilizer on his lawn and observed a handful of earthworms wriggling out of the soil. Building on this discovery, UOW honours student, Matt Chard took up the challenge and systematically dosed 1m2 quadrats of city parks with the new-found skin-irritant, both before and after rainfall.
The carboy-cleanser turned out to be the perfect brew, demonstrating that worms were 80% more abundant after rain than in dry periods. More importantly, Matt uncovered six times as many worms in the Domain as in Belmore Park.
We found that in between rummaging in bins and feeding on sandwiches, ibis actually spend a large proportion of their day picking around for natural foods. We suspect that the persistently high density of carb-loving Ibis in Belmore Park has simultaneously led to the depletion of its resident worm population, due to intense foraging pressure.
So when the rain comes, some of the birds, perhaps those in protein deficit, use the rainfall cue in combination with their foraging memory to spread out from the food court and into greener pastures. There, they exploit the worm windfall that moves up in the soil profile. Indeed, the rate of worm consumption by Ibis was four times higher in wet weather than dry weather.
Humans have complex interactions with wildlife, which are both direct and indirect, and these are accentuated in landscapes where human impacts are highest.
Richard Major, Principal Research Scientist
P.S. No earthworms suffered long-term harm from this research. All worms were counted, rinsed in water and returned to irritant free ground where they soon reburied – unless they were discovered by an opportunistic ibis.