Long‐term declines of European insectivorous bird populations and potential causes

Abstract

Evidence of declines in insect populations has recently received considerable scientific and societal attention. However, the lack of long‐term insect monitoring makes it difficult to assess whether declines are geographically widespread. By contrast, bird populations are well monitored and often used as indicators of environmental change. We compared the population trends of European insectivorous birds with those of other birds to assess whether patterns in bird population trends were consistent with declines of insects. We further examined whether declines were evident for insectivores with different habitats, foraging strata, and other ecological preferences. Bird population trends were estimated for Europe (1990–2015) and Denmark (1990–2016). On average, insectivores declined over the study period (13% across Europe and 28% in Denmark), whereas omnivores had stable populations. Seedeaters also declined (28% across Europe; 34% in Denmark), but this assessment was based on fewer species than for other groups. The effects of insectivory were stronger for farmland species (especially grassland species), for ground feeders, and for cold‐adapted species. Insectivory was associated with long‐distance migration, which was also linked to population declines. However, many insectivores had stable populations, especially habitat generalists. Our findings suggest that the decline of insectivores is primarily associated with agricultural intensification and loss of grassland habitat. The loss of both seed and insect specialists indicates an overall trend toward bird communities dominated by diet generalists.

Contrasting latitudinal patterns in diversity and stability in a high‐latitude species‐rich moth community

“…The y axis was truncated at the lower limit to allow better visualization of patterns across latitude (excluding seven and three traps with very low number of species and individuals in the NB zone, respectively) [Colour figure can be viewed at wileyonlinelibrary.com] and affect ecosystems services. Specifically, declines in insect abundance (regardless of their causes) appearto drive declines in bird population trends, with pronounced effects on insectivores and long-distance migrant species(Bowler, Heldbjerg, Fox, Jong, & Böhning-Gaese, 2019;Hallmann, Foppen, Turnhout, Kroon, & Jongejans, 2014).Our results highlight that changes in richness and abundance can be decoupled from each other(Schipper et al, 2016). In a similar vein,Antão et al (2019) reported how temperature-related trends in the number of individuals and number of species were not only decoupled, but exhibited geographically contrasting patterns during several decades of climate change.…”

Are insects declining and at what rate? An analysis of standardised, systematic catches of aphid and moth abundances across Great Britain

“…Bowler et al . () showed that declines in insectivorous birds was most strongly felt in agricultural habitats, particularly grasslands, that caused bird communities to be dominated by diet generalists. On the North American continent, similar patterns are apparent (Fitzgerald et al ., ; Michel et al ., ).…”

Landscape drivers and effectiveness of pest control by insectivorous birds in a landscape-dominant woody crop

“…Our results might be showing the serious consequences of current low insectivorous bird densities on the pest control ecosystem service carried out in olive orchards. Since alarming bird declines have been reported in North America (Rosenberg et al, 2019) and Europe in the last decades (Bowler, Heldbjerg, Fox, de Jong, & Böhning-Gaese, 2019; Inger et al, 2015), avian insectivory function in olive orchards is expected to be aggravated in the olive farms in the near future, since olive matrix is clearly suboptimal for avian insectivores, and suboptimal habitats become empty earlier that optimal ones. According to our results, urgent actions should be taken to reverse this situation, such as policy changes aimed to the recuperation of landscape heterogeneity and semi-natural patches in landscapes homogenized by monoculture, and active restoration of bird assemblages (e.g.…”