Action: Control predators not on islands for songbirds
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- A before-and-after study from New Zealand found that a reintroduced population of New Zealand robins Petroica australis declined without predator control and increased with rat poisoning. Two UK studies, one non-experimental, found increased populations of some species following control of bird and mammal predators.
- One replicated, controlled study from New Zealand found lower New Zealand robin survival in areas where rodent bait was broadcast, but no difference with controls when dispensers were used.
- Six studies from New Zealand, Australia, UK found increased nest success or survival (in one case with artificial nests) following bird and mammal predator control.
- One randomised, replicated and controlled study from the USA found no difference in nest survival in a site with mammalian predator removal.
Supporting evidence from individual studies
A replicated, controlled, paired sites study in Fiordland, New Zealand, in 1990-3 (O'Donnell et al. 1996) found that mohua (yellowhead) Mohoua ochrocephala nests produced significantly more chicks in a site where stoats Mustela erminea were trapped than in a site without trapping for the first breeding season (1990-1) (80% fledging success, 2.1 fledglings/breeding group for ten groups vs. 36%, 1.1 fledglings/breeding group for 14 groups). In subsequent years, as the stoat population fell, success increased in both areas and the difference between sites became non-significant (87-90% fledging success, 2.6-2.7 fledglings/breeding group for 19 groups at the trapped sites vs. 66-75%, 1.9-2.5 fledglings/breeding group for 15 groups at the untrapped site). A total of 62 stoats were removed from the trapped site in 1990-1.
A replicated, controlled study in three southern beech Nothofagus stands on South Island, New Zealand between August and November 1996 (Brown 1997) found that survival of South Island robins Petroica australis australis was not significantly higher when brodifacoum bait was dispensed from bait feeders (29/30 birds surviving, 97%) than in a control site (18/21, 86%), but was significantly lower when the bait was broadcast (12/23, 52%). This study is described in ‘Distribute poison bait using dispensers’.
A randomised, replicated and controlled trial conducted over two breeding seasons in North Dakota, USA (Dion et al. 1999), found no significant difference in the daily survival rates of songbird nests at eight sites where medium-sized mammalian nest predators had been removed compared with eight control sites. Species removed were red fox Vulpes vulpes, striped skunk Mephitis mephitis, raccoon Procyon lotor and American badger Taxidea taxus. Observations from artificial nests suggest that compensatory predation by smaller mammal species (such as ground squirrels Spermophilus spp.) may have counteracted any effects of target predator removal.
A controlled before-and-after study in November-December 1999 in a 240 ha eucalypt forest in New South Wales, Australia (Fulton & Ford 2001), found that 104 artificial nests survived for significantly longer following the removal of pied currawongs Strepera graculina from an experimental grid (average survival of 1.9 days before cull vs. 3.0 days afterwards). There was no change in a nearby control grid, without currawong removal (3.1 days before vs. 3.6 afterwards). Before the cull, survival was significantly higher on the control grid, but there was no difference following the cull. Three pairs of currawongs were culled, with three more nests having either fledged or failed by the time of the cull in early December 1999.
A study at three farmland sites in central England in 1992-1998 (Stoate & Szczur 2001) found that nest survival rates of four songbirds were negatively related to the breeding density of carrion crows following the control of nest predators. These species were Eurasian blackbird Turdus merula, song thrush T. philomelos, dunnock Prunella modularis and yellowhammer Emberiza citrinella. Non-significant negative relationships were also found for whitethroat Sylvia communis and chaffinch Fringilla coelebs nesting success and predator densities. Brown rats Rattus norvegicus, red foxes Vulpes vulpes, stoats Mustela erminea, weasels M. nivalis, carrion crows Corvus corone and magpies Pica pica were controlled through trapping and shooting. Between 151 and 951 nests of each species were studied.
A small replicated, controlled study from May-June in 1992-1998 in 1 experimental (3 km²) and four unmanaged arable farms in Leicestershire, England (Stoate 2002) found that the abundance of nationally declining songbird species and species of conservation concern significantly increased through time in the sites at which predators were controlled. Although there was no overall difference in bird abundance, species richness or diversity between the experimental and control sites, numbers of nationally declining species rose by 102% (except for skylark Alauda arvensis and yellowhammer Emberiza citrinella). Nationally stable species rose (insignificantly) by 47% (with 8 species exhibiting net increases, especially greenfinch Carduelis chloris 68%, and 4 species exhibiting net decreases). The author concluded that controlling nest predators (from April-July each year), as part of an integrated management package, provided the greatest benefits to species of conservation concern, but did not affect species diversity at the farm scale.
A before-and-after study between 1996 and 1998 at a farmland site in eastern England (Donald et al. 2002) found that daily survival rates of Eurasian skylark Alauda arvensis nests in non-rotational set-aside areas were significantly higher (96% daily survival for 168 nests) following the introduction of intensive control of mammalian predators than when predator control was either ‘light’ (95.6% survival for 51 nests) or absent (91% survival for 192 nests). There was no significant difference between light control and no control. These differences resulted in average overall survival rates of 40.7%, 23.3% and 12.3% for heavy, light and no control, respectively. The main species targeted were mustelids, hedgehogs Erinaceus europaeus and red foxes Vulpes vulpes. This study also discusses the impact of set-aside plots, described in ‘Provide or retain set-aside areas in farmland’.
A before-and-after study in a forest remnant in the south of New Zealand’s North Island between September 1999 and September 2004 (Armstrong et al. 2006) found that the population of 40 New Zealand robins Petroica australis (refered to as North Island robins P. longipes) reintroduced to the study site in March 1999 declined to 18 individuals by September 1999 and then to 11 birds by September 2000. Following the control of introduced predators (black rat Rattus rattus and brush-tailed possum Trichosurus vulpecula) with brodifacoum baits between mid-2000 and March 2002, rat populations (measured with tracking tunnels) fell and the robin population increased to 19 birds. Poisoning was stopped in March 2002, the rat population increased and the robin population decreased to eight birds by September 2004.
A before-and-after study on a mixed farmland-woodland site in central England (Stoate & Szczur 2006) found that the fledging success of spotted flycatcher Muscicapa striata nests was significantly higher when predators (grey squirrels Sciurus carolinensis, brown rats Rattus norvegicus, red foxes Vulpes vulpes, black-billed magpies Pica pica and carrion crows Corvus corone) were controlled (77% for 11 nests in 1997-2001) than when there was no control (16% for 28 in 2002-4).
A before-and-after study on a mixed farm in central England (White et al. 2008) between 1992 and 2007 (a continuation of the data series used in Stoate & Szczur 2001), found that controlling predator (carrion crow Corvus corone, magpie Pica pica, red fox Vulpes vulpes and other mammals) populations appeared to increase blackbird Turdus merula breeding population. However, the authors caution that the study is not experimental and that other explanations for the trends seen cannot be eliminated.
- O'Donnell C.F.J., Dilks P.J. & Elliott G.P. (1996) Control of a stoat (Mustela erminea) population irruption to enhance mohua (Mohoua ochrocephala) breeding success in New Zealand. New Zealand Journal of Zoology, 23, 279-286
- Brown K.P. (1997) Impact of brodifacoum poisoning operations on South Island robins Petroica australis australis in a New Zealand Nothofagus forest. Bird Conservation International, 7, 399-407
- Dion N., Hobson K.A. & Larivière S. (1999) Effects of removing duck-nest predators on nesting success of grassland songbirds. Canadian Journal of Zoology, 77, 1801-1806
- Fulton G.R. & Ford H.A. (2001) The pied currawong's role in avian nest predation: A predator removal experiment. Pacific Conservation Biology, 7, 154-160
- Stoate C. & Szczur J. (2001) Could game management have a role in the conservation of farmland passerines? A case study from a Leicestershire farm. Bird Study, 48, 279-292
- Stoate C. (2002) Multifunctional use of a natural resource on farmland: wild pheasant (Phasianus colchicus) management and the conservation of farmland birds. Biodiversity and Conservation, 11, 561-573
- Donald P.F., Evans A.D., Muirhead L.B., Buckingham D.L., Kirby W.B. & Schmitt S.I.A. (2002) Survival rates, causes of failure and productivity of skylark Alauda arvensis nests on lowland farmland. Ibis, 144, 652-664
- Armstrong D.P., Raeburn E.H., Lewis R.M. & Ravine D. (2006) Estimating the viability of a reintroduced New Zealand robin population as a function of predator control. The Journal of Wildlife Management, 70, 1020-1027
- Stoate C. & Szczur J. (2006) Potential influence of habitat and predation on local breeding success and population in spotted flycatchers Muscicapa striata. Bird Study, 53, 328-330
- White P.J.C., Stoate C., Szczur J. & Norris K. (2008) Investigating the effects of predator removal and habitat management on nest success and breeding population size of a farmland passerine: a case study. Ibis, 150, 178-190