Birds and Bugs and Plants
Birds eat bugs, and bugs eat plants. This is called a trophic cascade. So it might follow that if there were fewer birds, there would be more bugs and more damage to plants. Scientists, however, have had trouble documenting this. Some studies have reported more leaf damage, and some studies have not.
A study at Hubbard Brook in the New Hampshire mountains set out to exhaustively monitor the trophic cascade by setting up study plots at low, mid, and high elevations and over multiple years (2004 to 2006). The plant they studied is a large shrub called the striped maple (Acer pensylanicum).
In some study plots the striped maple was fenced in so birds could not eat the bugs inside. In other plots researchers removed bugs from the maples. A third set of maples were left as they were.
The bugs in the study are mostly caterpillars, prime food for birds to feed their young. Below are pictures of the most common birds observed foraging in the study plots.
Black-throated Blue Warbler
Black-throated Green Warbler
The study did find that there were differences between the years. It found that in 2004 there was substantially more damage to leaves due to a caterpillar outbreak. There were also differences in the elevation; the highest elevation plants grew more, presumably because of more light (due to a shorter tree canopy and more gaps in the canopy).
But keeping birds away from the plants did not result in more bugs on the plants. It seems that the bugs are so well camouflaged that the birds have trouble finding them.
For the plants, only about 5% of the leaves were damaged by insects, regardless of the plant's location. Removing bugs from plants did not result in the plant growing more. Plant growth seems to be governed more by the environment and climate than by insect's eating them.
This article summarizes the information in this scientific paper:
Schwenk, W. S., Strong, Allan M. and Sillett, Terence Scott. Effects of bird predation on arthropod abundance and tree growth across an elevational gradient, Journal of Avian Biology, 41 (4) 367-377. 2010.
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