Nest Predation: Be skeptical of conventional wisdom

We have fond memories of the good old days when we could read the newspaper without questioning everything we read.  That was over 15 years ago, before we became engaged in our effort to save our urban forest from being needlessly destroyed because it is predominantly non-native.  Since then we have learned the uncomfortable lesson that it is necessary to be skeptical about every conventional belief about nature.  Today we will examine two such beliefs related to how birds are killed in nature.

Robin and chicks.  Courtesy SF Forest Alliance
Robin and chicks. Courtesy SF Forest Alliance

Cats are presumed to be the primary predator of birds

To illustrate how pervasive the belief is that cats kill birds, we start with an internet search, “cats kill birds.”  Here’s a selection of articles available on the internet that make that claim:

  • “Cats kill 3.7 billion birds annually”
  • “Outdoor cats kill between 1.4 billion and 3.7 billion birds a year”
  • “Cats kill more than one billion birds each year”
  • “Cats are Birds No. 1 Enemy, Study Says”

We have examined the specific claim about the number of birds killed by cats in an earlier post, so we won’t repeat it here.  Instead, we will tell you about a meta-analysis of 8 studies of nest predators of song birds in North America.  These studies used video cameras to identify the predators of 242 depredation events, that is, nests in which the eggs were destroyed or nestlings killed.  These studies were conducted all over North America in different vegetation types, such as forests, shrublands, and grasslands.   These studies report that these were the predators of the nests:

  • 88 mammals
  • 86 snakes
  • 52 birds
  • 16 insects

Only one of the 88 mammals was a domestic cat.  The detailed list of all 242 predators is fascinating reading, which we recommend to you.

We understand that nest predation is not the only cause of bird mortality.  However, most bird death occurs in the first year of life according to the Cornell Laboratory of Ornithology, so clearly nest predation is an important factor in bird mortality.  And if video cameras find virtually no evidence that cats are nest predators, then we must wonder if cats are the bird killers they are made out to be.

Like so many other assumptions of nature lovers, we wonder if people are misled by their personal experiences.  In this case, most people live in urban areas and there are probably more cats in urban areas, so it seems probable that people are more likely to witness bird-death-by-cat than snakes, for example.  But empirical studies suggest that we should not extrapolate from that personal experience to conclude that cats are responsible for most bird mortality.  We will reserve judgment on that question, although we encourage cat owners to keep their cats indoors.

Are cowbirds another scapegoat for bird death?

We have also reported earlier that cowbirds are scapegoated for declining populations of song birds.  Cowbirds are nest parasites, which means they lay their eggs in the nests of other species of birds.  Their chicks are often bigger than the chicks of other species so they out-compete them in the nest, which implies that cowbirds could reduce the reproductive success of other species of birds.  In fact, a study of a 30-year attempt to eradicate cowbirds did not find evidence that killing over 125,000 cowbirds increased the population of a rare songbird.

Yellow-billed Cuckoo.  NPS photo
Yellow-billed Cuckoo. NPS photo

Today we tell our readers about a study of another nest parasite, the cuckoo, which could explain why killing nest parasites does not benefit other species of birds.  The study of cuckoos was conducted over a 16-year period.  It did not find evidence that cuckoos were reducing the population of other species of birds.  The study hypothesizes that cuckoo chicks emit a foul-smelling substance that repels nest predators, thereby protecting its nest mates as well as the cuckoo from nest predators.  So, the disadvantage of the cuckoo chick competing for food with its nest-mates is counteracted by the protection the cuckoo chick confers on the nest.

Lessons we have learned

In the past 15 years, we have learned to be skeptical.  Here are a few lessons we have learned from questioning everything we read and hear about nature:

  • People seem to have a knee-jerk need to scapegoat someone or something without thinking carefully about the underlying causes of the problems we observe in nature.
  • When we hear a particular animal being blamed for a problem in nature, we turn to the scientific literature for verification to determine if there is any empirical evidence that supports that assessment.  We frequently find no evidence to support the conventional wisdom.  Sometimes we find evidence that contradicts the assumptions.
  • Even then, we must keep in mind that science is always moving forward.  Science only hypothesizes and every hypothesis must be repeatedly tested.  Hypotheses are often overturned as we learn more.
  • We do not think it is ethical to kill one animal based on the assumption that it will benefit another animal.  Aside from the presumption of deciding which animal is worthy of living, we think these projects are often mistaken in the assumption that a particular animal will benefit.  We believe that nature is far wiser than we are.