Abundance of insects and aerial insectivorous birds in relation to pesticide and fertilizer use

Background The abundance of insects has decreased considerably during recent decades, resulting in current abundance showing 70–80% reductions in more than 15 studies across temperate climate zones. Dramatic reductions in the abundance of insects are likely to have consequences for other taxa at higher trophic levels such as predators and parasites. Pesticides, fertilizers and agricultural land use are likely candidates accounting for such reductions in the abundance of insects. Methods Here we surveyed the abundance of flying insects, and the reduction in the abundance of insects as a consequence of intensive reduction in agricultural practice linked to fertilizer use and pesticide use. Finally we demonstrated consistency in abundance of birds among study sites. Results We demonstrated that the use of fertilizers and pesticides had reduced the abundance of insects, with consequences for the abundance of insectivorous bird species such as Barn Swallows (Hirundo rustica), House Martins (Delichon urbicum) and Swifts (Apus apus). Juvenile Barn Swallows were negatively affected by the reduced abundance of insects and hence the reproductive success of insectivorous bird species. These effects imply that the abundance of insects could be reduced by the availability of insect food. Conclusions These effects of intensive agriculture on insect food abundance are likely to have negative impacts on populations of insects and their avian predators. This hypothesis was validated by a reduction in the abundance of insects, linked to an increase in the abundance of fertilizers and a general change in farming practice.

The effects of rainfall and arthropod abundance on breeding season of insectivorous birds, in a semi-arid neotropical environment

Rainfall in tropical semi-arid areas may act as a reliable cue for timing bird reproduction, since it precedes future food supply. With this in mind, we set-up a study to test the reproductive response of insectivorous bird to arthropod abundance and rainfall patterns. Sampling occurred in a seasonally dry Neotropical forest, in north-eastern Brazil, between October, 2015 and October 2016, at 14-day intervals. We used brood patch to assess reproductive periodicity of insectivorous birds (eight species, 475 captures, 121 patch records). We sampled arthropods to quantify abundance, using biomass and number of individuals (1755 individuals, 15 Orders). Rainfall temporal distribution was analyzed using daily precipitation data. We used a cross-correlation function to test for correlation and time-lags between the covariates under study. Both the number of reproductively-active birds and arthropod abundance were higher in time periods close to the rainy season. Increase in arthropod biomass in the aerial stratum preceded the period of greatest rainfall by one (14 days, r = 0.44) to three sampling periods (0.47). In contrast, the highest proportion of individuals with brood patches occurred after the main rainfall peak, with the strongest relationship occurring after two (0.52) to four (0.50) time lags. Finally, the proportion of individuals with brood patches was positively correlated with aerial stratum arthropod biomass when five time lags were considered (0.55). Our results support the hypothesis of a temporal process involving rainfall, arthropods and reproduction of insectivorous birds in the wet/dry tropics. However, rainfall did not appear to act as a cue for the timing of reproduction, since records indicated higher arthropod biomass before the main rainfall peak. At least occasionally in the study area, insectivorous bird reproduction peaks after food abundance.