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.

5 thoughts on “Nest Predation: Be skeptical of conventional wisdom”

  1. I guess it depends if you call the city feral cats domesticated. At one time they were. I have seen much cat predation on birds in my neighborhood. In many cases, it is all they are catching to eat in winter. I never did see a cat climb a tree to access a nest though. Other birds are feasting on the young of their kind. I have seen this often and you pictured the robin. I have seen a robin nest raided repeatedly through the nesting season, trying to raise different clutches – unsuccessfully.

  2. “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.”

    Or maybe cats aren’t really much of nest predators, or maybe they weren’t in great abundance at the study areas represented in the analyses? Lack of nest predation by cats wouldn’t explain away predation on adult birds, or fledglings. Your quote below leads to a better hypothesis.

    “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.”

    Cats most likely kill a greater amount of birds where cats are in greater abundance. There is ample evidence that cats are prolific bird killers, just look at how cats have affected populations of birds and other animals when they have been introduced to islands. The question isn’t “are cats bird killers”, its “how do they affect birds and within what context”. After that is figured it out is just a matter of coming to a consensus on how to deal with the problem in a way that works, and is as ethically agreeable as possible.

    “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.”

    True, because it is easier to blame something other than ourselves. The cowbird/kirtland’s warbler story is a pretty good example of glancing over the underlying issues of how our land utilization practices have influenced the amount of habitat and its influence on the abundance of species. Cowbirds are becoming “problems” for other birds because we have created we have opened up new areas for them through forest fragmentation. With respect to kirtland’s warblers removing cowbirds certainly the warbler’s persist, but their numbers didn’t increase until the habitat they required returned. Habitat is a requirement for existence of any species. It is the reason that habitat and ecosystem rehabilitation is performed.

  3. I love this blog, but must disagree with this post. In a discussion of how many animals cats kill, focusing only on nesting birds is like discussing car accident fatalities while focusing only on people killed by cars crashing into their houses.

    Of course cats rarely kill nesting birds (except for ground nesting birds) since most cats don’t need to climb trees to kill birds. In some parts of the world where birds evolved to no longer fly, cats have completely wiped out entire bird species.

    Anyone who has outdoor cats has to know how many small animals they are killing, but most people just do not care. I’ve heard animal-loving friends say how horrified they are by the number of animals their cats kill. A friend who posted publicly that her cat is too slow to kill any bird had recently called crying about a baby mockingbird that her cat had killed. (She had previous complained about the mockingbird mother chasing her cat, so I explained that the bird probably was terrified the cat would kill her babies when they were on the ground, learning to fly.) My friend then said, “There are plenty of birds, though, aren’t there?” She still didn’t keep her cat inside even though another friend’s outdoor cat disappeared and we never found out what happened to her.

    Cats basically kill any animal they can, whether hungry or not. Well-fed cats actually kill more. We want to keep our yard safe for small animals, but cats have killed every reptile and amphibian who used to live here, and most of the birds. We’ve even seen them kill squirrels. If neighbors’ dogs came into people’s yards and killed their cats, there would be such an outcry, but almost no one cares if their cats are coming into people’s yards and killing other animals they love.

    My mother lived in an area where cats were not allowed. For years, she loved to see the Western Fence Lizards run on her wall. Then cats were allowed, and, within a month, she never saw another lizard again. (Western Fence Lizards being murdered will greatly affect humans as Lyme disease spreads since they have the capability to clean Ixodes Pacificus — “Lyme ticks” — of the Lyme spirochete Borrelia Burgdorferi.)

    This is humans’ fault and not the cats’ fault. We’re only just learning about the extent of deaths caused by cats, which could easily be avoided by keeping them inside, protecting them as well as the animals they kill, and by disposing of their feces (by putting it into garbage and not toilets). Cat feces carries toxoplasmosis (Toxoplasma Gondii), which if flushed or gotten into water, then flows to the ocean where it kills endangered sea otters.

    Click to access cats_toxo.pdf

    Toxoplasmosis affects many other animals, including causing miscarriages and birth defects if pregnant women are exposed. Toxoplasmosis is also implicated in schizophrenia and other mental illness when the parasite reaches the brain. The effect in some small animals is that they are then attracted to cat feces, making them more likely to become victims of cats. This is one of the many ways that nature functions when the natural system is out of whack because of what humans have done.

    Over 2.6 billion pounds of cat waste is created every year in the United States. Health officials say that most of this fecal matter contains Toxoplasma gondii, which can infect humans and lead to a compromised immune system, birth defects and mental illness.

    Yet we have no choice about exposure, just as we have no choice to protect wild animals from cats. It would be nice to be able to walk in our yard and not step in cat feces, but as the numbers of humans owning cats and feeding feral cats increases, so do the exposures, even in our own yard.

    Why are the “rights” of cats and cat owners more important than the rights of animals killed by cats and the people who don’t want to be exposed to cat parasites?

    Yes, nature is very wise and that is why cats evolved where predators kept them under control. But humans usurp that natural wisdom and wreak havoc wherever they go. Cats are now responsible for killing billions of animals a year, including exterminating at least 33 species of birds.

    An individual cat can kill 800 small animals a year. A friend in England says she thinks that’s an underestimate, after seeing her neighbor’s cat killing 2 birds in an hour. I’ve heard too many people actually brag about the animals their cats kill. I care for the cats who are killed by being in danger outside, but I also care for all the animals cats kill and object to being exposed to parasites against our will.

    According to the research, published today (Jan. 29) in the journal Nature Communications, cats kill between 1.4 billion and 3.7 billion birds and between 6.9 billion and 20.7 billion small mammals, such as meadow voles and chipmunks.

    Though it’s hard to know exactly how many birds live in the United States, the staggering number of bird deaths may account for as much as 15 percent of the total bird population, said study co-author Pete Marra, an animal ecologist with the Smithsonian Conservation Biology Institute.

    It’s up to humans to protect the cats they love, but also their responsibility to protect all the animals that their cat will be allowed to kill. If you love animals and own a cat, please protect those animals who are already dealing with other assaults by humans with loss of habitat, water contamination, poisoning, etc.

  4. I have never owned a cat, never lived with a cat, never been friends with a cat. It is unlikely that I will ever have a cat. I don’t particularly like cats. BUT . . .

    I think the comments so far have illustrated what the article said: Some people militantly hold cats responsible for more damage to bird populations than evidence supports. The article didn’t deny that cats kill birds, and the article urged cat owners to keep their cats indoors. If that’s too much of a “defense” of cats, then we’ve moved beyond reason.

    The important question is: Do cats significantly suppress bird populations? I know of no case where cats have extirpated a bird species. The claim in the comment above that cats have caused the extinction of 33 bird species does not reflect what the cited article actually says, which is, “Free-ranging cats on islands have caused or contributed to 33 (14%) of the modern bird, mammal and reptile extinctions” Notice the “or contributed to” and “bird, mammal and reptile.” The cited article doesn’t say cats caused the extinction of any bird. If someone can document a cat-caused extinction, I would appreciate a citation to the evidence. Flightless birds on islands (New Zealand, Hawaii, Mauritius, Falklands, etc.) were doomed by rats, pigs, and humans before cats arrived. Cats may have participated, but the birds would be just as extinct if no cats had been present.

  5. It’s not true about Aotearoa/New Zealand. Most of the flightless bird species were still alive when the Maori lived there for a thousand years with the Polynesian rat they brought. Pigs don’t kill birds. It was Europeans with cats and dogs who did the most damage and continue to. I personally know New Zealanders whose dogs have killed penguins, and those with cats describe many more species killed.

    The real proof is known by people who let their cats have access to the outside. They may cry and then lie, like my friend, but they do know, because cats bring many of their victims home. What most people don’t know is that cats kill many more animals than they bring home.

    It doesn’t take much to eliminate bird species from an area. When a cat kills the baby birds who are trying to learn to fly, and that’s it for that species in an area. The birds already have to deal with native predators, cars, and direct human attacks, but cats are far more efficient. The nesting season is the most dangerous.

    Another effect is that native predators also suffer because their usual prey is gone.

    Further proof is if you have no reptiles or amphibians where you live, which is what it’s like now in a lot of areas. I’ve seen the changes, as others have. These small animals don’t have the advantage of flight. When they are killed, that’s it, and we all suffer their loss. I have no idea how many decades it might be before we could again see lizards, frogs, salamanders, etc. where they once were if the cats were no longer killing them. We heard frogs in our yard twenty years ago, but the people with outdoor cats increased, as did the feces all over our yard. The places with the densest population of humans and cats have the least species. And don’t forget the toxoplasmosis killing the sea otters, as well as infecting humans.

    Again, it’s safest for the cats to keep them safely inside. But most people insist the cats want outside. I question how much they don’t want to clean litter boxes.

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