Population decline of a long‐distance migratory passerine at the edge of its range: nest predation, nest replacement and immigration

Abstract

The relative importance of predators and resources (i.e. food) for the dynamics of migratory bird populations is poorly known. Resource availability may be more likely in resource poor environments, but given that nest failure in most systems is due mainly to predation, predator effects may predominate. We document a rapid decline of an isolated eastern kingbird Tyrannus tyrannus population breeding in the Great Basin Desert of eastern Oregon, USA, and evaluate whether it was driven by limited food resources (water availability ~ food), nest predation, or first‐year or adult return rate (RRJ and RRA, respectively) that reflect nonbreeding season events. Most nests failed (~68% of nests) due mainly to nest predation (> 90% of failures); nestling starvation was rare. Bioyear precipitation (October–April), breeding season precipitation, and river flow all varied widely but none could account for annual variation in either nest success (NS) or fledging success of successful nest (FSSN). Neither RRJ nor RRA varied with year, any measure of water resources, or reproductive success. Annual population growth rate (ln[Nt+1/Nt]) was independent of year, all measures of water availability, RRJ and RRA, and NS of replacement nests, but was high following years of high NS of initial nests; FSSN was also higher in years of high NS. High rates of nest predation, due likely to the expansion of the local American crow Corvus brachyrhynchos population, appeared to drive the population decline. Empirical data and population simulations indicated that replacement nests contributed little to λ, while simulations revealed that, given site‐specific vital rates, the decline would have been even greater without immigration (λ = 0.812) than the observed (λ = 0.914). Long distance dispersal may thus be critical for persistence of kingbirds in the naturally highly fragmented habitat that it occupies in the Great Basin Desert.

Traditional egg-harvesting policies for the endemic Moluccan ScrubfowlEulipoa wallaceion Haruku Island, Indonesia, and its conservation implications

“…2011, Rigby and Haukos 2015, Murphy et al . 2020). It is imperative to regulate or even restrict the number of Moluccan Scrubfowl eggs that can be harvested to prevent potential population declines in the long term.…”

“…In contrast, egg conservation is not a top priority for the community due to the influence of traditional beliefs regarding the origin of the eggs at the Cape Maleo nesting grounds and their economic value to the community. By collecting bird eggs, the number of eggs incubated to produce chicks decreases, resulting in slower recruitment of new members into the population (Etterson et al 2011, Rigby and Haukos 2015, Murphy et al 2020. It is imperative to regulate or even restrict the number of Moluccan Scrubfowl eggs that can be harvested to prevent potential population declines in the long term.…”

Changing patterns of nest predation and predator communities along a tropical elevation gradient

“…Nest predation is widely recognized as a strong selective force shaping population dynamics (Dyson et al, 2020;Murphy et al, 2020;Pienkowski, 1984), life history (LaManna & Martin, 2017;Martin, 1995;Skutch, 1985), evolution (Lima & Dill, 1990;Warkentin, 1995;Zanette et al, 2011), behaviour (Fontaine et al, 2007;Ghalambor et al, 2013;Lima & Bednekoff, 1999), and ecology of birds (Fontaine & Martin, 2006;Hua et al, 2014;Slagvold, 1984). In a recent review, Menezes and Marini (2017) tallied 256 species of nest predators across a wide variety of neotropical ecosystems and nesting species, which suggests that neotropical nest predator communities are extremely diverse.…”