Declining small woodland birds: is removing noisy miners the answer?

Date: 01, Jun, 2020
Author(s):   Beggs, R.
Publisher: Australian National University

PhD Thesis

Overabundant native species are a growing problem globally, in large part due to anthropogenic landscape modification. They are organisms whose abundance exceeds the carrying capacity of a given social-ecological system. The carrying capacity may be ecological or cultural. The cultural carrying capacity is the upper population density of an overabundant species accepted by human society due to non-ecological impacts such as nuisance or disease risk. Native organisms that exceed a cultural or ecological carrying capacity may require management interventions to reset ecological or cultural equilibrium. An overabundant native species with major ecological impacts in Australia is the noisy miner, Manorina melanocephala, an endemic, sedentary, colonial species with a preference for fragmented woodland landscapes. Noisy miners aggressively exclude all smaller woodland birds from colonized territory. Many small woodland birds are in serious decline due to habitat loss and noisy miners are an additional threat that could drive some to extinction. Noisy miners now dominate remnant woodland in eastern Australia at a sub-continental scale. In 2014 their aggressive behaviour was listed as a Key Threatening Process under federal conservation legislation. Some ecologists recommend culling as the best management option to prevent further declines of small woodland birds. Evidence that culling noisy miners benefits small woodland birds is limited.
To assess the feasibility of culling as a management intervention applicable at a broad scale to improve ecosystem function, I conducted a controlled and costed experimental cull of noisy miners in woodland patches in an agricultural landscape of south east Australia. I monitored foraging and harassment rates of small woodland birds before and after the cull. The purpose here was fourfold: to assess the amount of harassment carried out by noisy miners; to see if small woodland birds suffered less overall harassment after the cull; to indicate if there was any compensatory harassment by other aggressive species; and to see if removing noisy miners improved foraging opportunities for small woodland birds.
Successful breeding is essential for recovery of declining species. I therefore assessed post-cull changes in breeding potential of small woodland birds. In this landscape, nest predation is the principal cause of breeding failure and birds are the principal nest predators. Small woodland birds make few breeding attempts in sites colonized by noisy miners, however, due to aggressive disruption of nesting by noisy miners. I therefore conducted pre- and post-cull artificial nest predation experiments. I aimed to show the proportion of nest predation carried out by noisy miners and to indicate any compensatory nest predatory responses by other species.
My principal finding was the unexpected immediate recolonisation of treatment sites by noisy miners. Although noisy miner abundance in treatment sites post-cull was 25% lower than in control sites, abundance in all sites remained three to four times higher than ecological impact thresholds. Nonetheless, the cull disrupted intraspecific relations of this socially complex species, so I expected some effect on the responses of small woodland birds. Foraging rates doubled but I recorded no change in harassment rates. In my nest predation study, noisy miners were responsible for 18% of nest predation events where the predator was identified. I recorded predation by five other bird species but I detected no significant change in artificial nest predation rates post-cull. I conclude that in highly modified agricultural landscapes such as this, patch-scale culling is not an effective management option due to rapid recolonisation.
A second element of this thesis is a cultural history of the noisy miner. Through exploration of historical references I chart the shift in cultural attitudes to the species in parallel with its changed ecological role. As an antidote to environmental amnesia, this chapter provides an understanding of the social-ecological changes that have occurred in south east Australia since European settlement. These changes have fostered the transformation of a natural ecosystem process, interspecific competition, into a Key Threatening Process.