Reproductive success of the wood warbler Phylloscopus sibilatrix varies across Europe

Abstract

Differences in population trends across a species' breeding range are ultimately linked to variation in demographic rates. In small songbirds, demographic rates related to fecundity typically have strong effects on population trends. Populations of a forest songbird, the wood warbler Phylloscopus sibilatrix, have been declining in many but not all regions of the European breeding range. We investigated if clutch size, hatching rate, nest survival and number of fledglings vary across Europe, and if nest survival is related to differences in the regionally dominant nest predator class (birds versus mammals). From 2009 to 2020, we monitored 1896 nests and used cameras at a subsample of 645 nests in six study regions: the United Kingdom (mid-Wales, Dartmoor, the New Forest), Germany (Hessen), Switzerland (Jura mountains) and Poland (Białowieża National Park). Number of fledglings was lowest in the New Forest (1.43 ± CI 0.23), intermediate in Jura (2.41 ± 0.31) and Białowieża (2.26 ± 0.24) and highest in mid-Wales (3.02 ± 0.48) and Dartmoor (2.92 ± 0.32). The reason for low reproductive success in the New Forest, Jura and Białowieża was low nest survival, and large clutch sizes in Białowieża did not compensate for high nest losses. High reproductive success in mid-Wales and Dartmoor was due to high nest survival and large clutch sizes. Overall predation rates were similar everywhere despite variation between the regions in the dominant nest predator class. Unsuccessful nests in mid-Wales were mainly predated by birds; in Dartmoor, the New Forest, Hessen and Jura similarly by birds and mammals; and in Białowieża exclusively by mammals. Regional reproductive success does not match the population trends recently reported for the wood warbler in the six study regions (i.e. high reproduction ≠ positive trend). Annual survival may be a decisive factor, but it is difficult to quantify for a nomadic species such as the wood warbler that rarely returns to the same breeding locations.

Wood warbler population dynamics in response to mast seeding regimes in Europe

“…Similarly, the relatively stable observed and modeled population trends in BNP from 2012 onwards, despite a moderate increase in mast seeding frequency, points to additional factors influencing abundance. An indication of the involvement of changes in immigration is a contraction of the wood warbler breeding range in the northwest over the past 20-30 years, for example in the UK, Denmark, and Finland (Keller et al, 2020), which seems to be unrelated to reproductive output (Maag et al, 2022).…”

“…Our simulations suggest that wood warbler populations are at risk due to recent and projected increases in mast events (e.g., every second year instead of every third or fourth year). Although higher reproductive output in springs following nonmast years could hypothetically compensate for the increased occurrence of mast years, this is an unlikely scenario considering that nest predation rates in wood warblers are relatively high (Maag et al, 2022). In addition, the magnitude of the absolute percent difference in mean reproductive output across study areas ((2.19 − 2.57)/2.19 × 100 = 17%) is much lower than that of mast frequency ((0.23 − 0.48)/0.23 × 100 = 109%), suggesting that possible adaptations (e.g., increases in clutch size) cannot compensate for the increase in mast frequencies.…”

Habitat detection, habitat choice copying or mating benefits: What drives conspecific attraction in a nomadic songbird?

“…protection of offspring against predation) improbable as drivers of conspecific attraction because the social conditions at settlement are almost certainly not representative of the conditions that will be present by the time there are young to protect. Second, nest predation in this species often occurs at night and by predators that Wood Warblers cannot defend against (Grendelmeier et al, 2018 ; Maag et al, 2022 ; Maziarz et al, 2019 ), which makes the putative anti‐predation benefits of aggregation (i.e. group vigilance or defence) biologically unlikely.…”

Accounting for predator species identity reveals variable relationships between nest predation rate and habitat in a temperate forest songbird

“…To adjust to spatial variation in habitat availability and quality, animals exhibit varying degrees of habitat association (Mayr, 1963), and predators that mainly forage in woodlands in some parts of their distribution can adapt foraging behavior in response to habitat loss elsewhere (Evans et al, 2009; Wesołowski & Fuller, 2012). However, it is questionable if adaptations of foraging patterns by predators to habitat loss leads to decreased nest success in wood warblers and other ground‐nesting birds, as nest failure rates of wood warblers in Western Europe are similar to those in Eastern Europe, where deforestation is less severe (Maag et al, 2022). As ground‐nesting birds, wood warblers are well adapted to high nest predation rates, but when there are other potential pressures on a population, high nest predation rates may become detrimental to avian populations (Newton, 1998).…”

“…Among the most important wood warbler nest predators are Eurasian jay Garrulus glandarius , common buzzard Buteo buteo , Eurasian sparrowhawk Accipiter nisus , pine marten Martes martes , red fox Vulpes vulpes , and European badger Meles meles (Bellamy et al, 2018 ; Grendelmeier et al, 2018 ; Mallord et al, 2012 ; Maziarz et al, 2019 ). Wood warbler nest predation rates are similar across its range, but avian predation is more frequent in the United Kingdom, while mammals are more important nest predators in mainland Europe (Maag et al, 2022 ). Previous studies have investigated relationships between wood warbler nest survival and habitat structures including nest concealment or shrub cover at the scale of the nest site or territory (Bellamy et al, 2018 ; Grendelmeier et al, 2015 ; Maziarz et al, 2019 ).…”