Data from: Parallel declines in abundance of insects and insectivorous birds in Denmark over 22 years

Farmers in most western countries have increased use of fertilizer and
pesticides with impact on wild animals and plants, including the abundance
of insects and their predators. I used 1,375 surveys of insects killed on
car windscreens as a measure of insect abundance during 1997–2017 at two
transects in Denmark. I cross‐validated this method against three other
methods for sampling insect abundance, and I investigated the effects of
this measure of insect abundance on the abundance of breeding
insectivorous birds. The abundance of flying insects was quantified using
a windscreen resulting in reductions of 80% and 97% at two transects of
1.2 km and 25 km, respectively, according to general additive mixed model.
Insect abundance increased with time of day, temperature, and June date,
but decreased with wind resulting in a reduction by 54%. The abundance of
insects killed on a car windscreen was strongly positively correlated with
the abundance of insects caught in sweep nets and on sticky plates in the
same study areas and at the same time as when insects were sampled using
windscreens. The decline in abundance of insects on windscreens predicted
the rate at which barn swallows Hirundo rustica fed their nestlings, even
when controlling statistically for time of day, weather, and age and
number of nestlings. The abundance of breeding pairs of three species of
aerially insectivorous birds was positively correlated with the abundance
of insects killed on windscreens at the same time in the same study area.
This suggests a link between two trophic levels as affected by the
temporal reduction in the abundance of flying insects. These findings are
consistent with recent dramatic declines in insect abundance in Europe and
North America with consequences for the rate of food provisioning of barn
swallow offspring, the abundance of aerially insectivorous birds and
bottom‐up trophic cascades.

大多数西方国家的农民已增加化肥和农药的使用，这对野生动植物产生了影响，包括昆虫及其捕食者的数量。1997-2017年间，我在丹麦的两条样带（transects）上开展了1375次汽车挡风玻璃昆虫死亡调查，以此作为昆虫数量的衡量指标。我将该方法与另外三种昆虫数量采样方法进行交叉验证（cross-validated），并研究了这一昆虫数量指标对繁殖期食虫鸟类数量的影响。根据广义加性混合模型（general additive mixed model），通过挡风玻璃法量化的飞行昆虫数量在两条分别为1.2公里和25公里的样带上分别减少了80%和97%。昆虫数量随一天中的时段、温度及六月日期的推移而增加，但随风速增大而减少，降幅达54%。在相同研究区域及相同采样时间下，汽车挡风玻璃上的昆虫死亡数量与扫网法和粘板法捕获的昆虫数量呈强正相关。即使在统计上控制了时段、天气及雏鸟的年龄和数量等变量，挡风玻璃昆虫数量的下降仍能预测家燕（Hirundo rustica）的雏鸟喂食率。在相同研究区域及相同时间下，三种空中食虫鸟类的繁殖对数量与挡风玻璃上的昆虫死亡数量呈正相关。这表明，飞行昆虫数量的时间性减少对两个营养级（trophic levels）之间的关联产生了影响。这些发现与欧洲及北美近期昆虫数量急剧下降的现象一致，而这一现象对家燕后代的食物供给率、空中食虫鸟类数量及自下而上的营养级联（trophic cascades）均产生了影响。