Population status of four endemic land bird species after an unsuccessful rodent eradication on Henderson Island

Abstract

SummaryInvasive rodents detrimentally affect native bird species on many islands worldwide, and rodent eradication is a useful tool to safeguard endemic and threatened species. However, especially on tropical islands, rodent eradications can fail for various reasons, and it is unclear whether the temporary reduction of a rodent population during an unsuccessful eradication operation has beneficial effects on native birds. Here we examine the response of four endemic land bird species on subtropical Henderson Island in the Pitcairn Island Group, South Pacific Ocean, following an unsuccessful rodent eradication in 2011. We conducted point counts at 25 sampling locations in 14 survey periods between 2011 and 2015, and modelled the abundance trends of all species using binomial mixture models accounting for observer and environmental variation in detection probability. Henderson Reed Warbler Acrocephalus taiti more than doubled in abundance (2015 population estimate: 7,194-28,776), and Henderson Fruit Dove Ptilinopus insularis increased slightly between 2011 and 2015 (2015 population estimate: 4,476–10,072), while we detected no change in abundance of the Henderson Lorikeet Vini stepheni (2015 population estimate: 554–3014). Henderson Crake Zapornia atra increased to pre-eradication levels following anticipated mortality during the operation (2015 population estimate: 4,960–20,783). A temporary reduction of rat predation pressure and rat competition for fruit may have benefitted the reed warbler and the fruit dove, respectively. However, a long drought may have naturally suppressed bird populations prior to the rat eradication operation in 2011, potentially confounding the effects of temporary rat reduction and natural recovery. We therefore cannot unequivocally ascribe the population recovery to the temporary reduction of the rat population. We encourage robust monitoring of island biodiversity both before and after any management operation to better understand responses of endemic species to failed or successful operations.

Population status of the endemic Pitcairn Reed Warbler Acrocephalus vaughani on Pitcairn Island, South Pacific

“…We used a distance of 50 m around transects to define the area in which we sampled reed warbler abundance, and our estimates would decrease substantially if that area was 100 m. However, past descriptions of reed warbler vocalisations and territory sizes (Williams 1960; Graves 1992; Brooke and Hartley 1995) render it unlikely that reed warblers would be detected >50 m from a transect, and we therefore consider our extrapolations realistic. The density of the Pitcairn Reed Warbler that we extrapolated is higher than that of the Henderson Reed Warbler (Graves 1992; Bond et al 2019), and much higher than closely related species that became extinct (Cibois et al 2008), but within the density range of extant island-endemic reed warbler species (Thibault and Cibois 2006). Our estimates are fairly imprecise due to the relatively small number of transects that could be accommodated on the island: we originally established 100 transects and conducted 400 surveys, which resulted in more precise estimates of similar order of magnitude, but because several of those transects were within 30 m of each other and could have led to individuals being counted on more than one transect, we discarded some of the data to reduce the risk of pseudo-replication.…”

“…As this area is not unambiguously defined, the abundance estimates cannot be transformed into density estimates without making an explicit assumption about the area that is being sampled. Following a similar assessment of Henderson Reed Warblers Acrocephalus taiti on Henderson Island (Bond et al 2019), we assumed that reed warblers within 50–100 m of each transect were potentially detectable on each transect on Pitcairn. This distance is a combination of both detection and movement distance, and should be appropriate for a small songbird with a quiet call that cannot be heard more than 50 m under ideal conditions (Graves 1992; Oppel et al 2014).…”

Records of Brown Booby Sula leucogaster in the Pitcairn Islands with additional observations during 2015–19

“…The Sir Peter Scott Memorial Expedition in 1991-92 stationed researchers on Henderson Island for more than a year, and thoroughly studied the birds (Brooke 1995a,b, Brooke & Jones 1995, Imber et al 1995, Trevelyan 1995. This was followed in the early 21st century by studies into the effect of introduced Pacific Rats Rattus exulans on Henderson's native fauna (Brooke et al 2011, Amos et al 2016, Bond et al 2019b, and of the abundance of various species (Oppel et al 2017, Bond et al 2019a). There are far fewer studies of the avifauna of Pitcairn (Williams 1960, Schofield & Bond 2016, Oeno (Brooke et al 2018) and Ducie, although all were reviewed by Brooke (1995b) and Bourne & David (1983).…”

“…Ducie and Oeno possess no endemic species of birds, whilst Pitcairn is home to the endemic Pitcairn Reed Warbler Acrocephalus vaughani (Brooke 1995b) (Graves 1992, Brooke 1995b, Oppel et al 2017, Bond et al 2019a. The avifauna of the group as a whole has been described largely in three works (Williams 1960, Brooke 1995b, Thibault & Cibois 2017, and one review (Bourne & David 1983).…”

A Consideration of Mesopredator Release and New Zealand's Predator Free Plan: A Literature Review

“…Other introduced predators include the common brushtail possum (Trichosurus vulpecula) native to Australia and the feral cat (Felis catus). Damage by invasive species to the New Zealand native fauna has been extensively researched over the past few decades; including damage to bat (Scrimgeour et al 2012), bird (Bond et al 2019), lizard (Monks et al 2019), and fish populations (O'Donnell et al 2017).…”