Predation is nature’s disease control, including for Tweety & Sylvester

Too few cats around bird feeders means pine siskins die by thousands

         SACRAMENTO, California––When are free-roaming cats watching a bird feeder the birds’ best friend?

When infectious disease is at large among common bird feeder species,  spreading rapidly from bird to bird in the absence of predators to kill and consume the sick and disabled.

Many mid-sized predators can do the job,  before a feeder-spread disease drops birds by the hundreds,  thousands,  or tens of thousands,  but such candidates as hawks,  owls,  eagles,  foxes,  raccoons,  and fishers are much less well adapted to live and hunt in yards and on porches than cats.

Feral cats in particular do most of their bird-hunting after dark,  when only sick and injured birds are still on the ground beneath bird feeders.

Dr. Cat made house calls

The value of this prophylactic cat service to bird populations has seldom become more evident than among pine siskins in the winter and early spring of 2020-2021.

A seasonal shortage of pine cones in the Canadian range of the pine siskin has caused this small member of the finch family to congregate in rarely seen numbers around bird feeders along the U.S. west coast,  from Puget Sound all the way to southern California.

A scarcity of free-roaming cats,  gradually developing over the past thirty years,  may be among the major contributing factors behind outbreaks of salmonellosis among displaced pine siskins,  reported most often in communities with some of the oldest and largest neuter/return programs in the United States.

“Since December,  the California Department of Fish and Wildlife (CDFW) and wildlife rehabilitation centers have been inundated with calls from residents who are finding sick or dead finches at bird feeders,”  CDFW senior environmental scientist Krysta Rogers and communications officer Ken Paglia announced on February 8,  2021.

“Most reports have come from locations on California’s Central Coast,  the San Francisco Bay Area and Sierra Nevada communities,”  Rogers and Paglia said then.

How bird feeders kill birds

A March 8,  2021 update was considerably more specific.  Rogers and staff had by then logged more than 2,000 songbird death reports,  mostly of pine siskins,  “along with a small number of goldfinches,”  they added.

A closely related species,  goldfinches often feed alongside pine siskins.

The California Department of Fish and Wildlife “determined the birds are contracting salmonellosis,  a disease caused by salmonella bacteria,  by sharing bird feeders with other infected avian species,”  Rogers told media.

“When there is a disease circulating in the population,”  Rogers explained,  “bird feeders ensure the birds remain in really close contact with one another,  as well as other bird species,  as they share the feeder.”

“Bird feeders also keep the birds in one location for longer periods of time,”  Rogers added, “allowing feces to build up.  If there are sick birds in that group, they will shed the salmonella bacteria in their feces,  contaminating the area,  leading to more and more birds becoming sick and dying from the infection as they try to access the feeder or feed on the spilled seeds under the feeder.

“Remove the bird feeders”

“The most effective way to slow the spread of this infection and reduce mortality,”  Rogers emphasized,  “is to remove the bird feeders,  because that is primarily where this disease spreads.”

Rogers said nothing about cats,  but the numbers she shared did.

Note that each number of confirmed bird deaths probably represents thousands of dead birds whose remains were not retrieved and tested.

Sonoma County,  reporting 238 bird deaths from salmonellosis,  has had active feral cat neuter/return programs in the largest cities in the county since 1990.

Santa Clara County,  reporting 231 bird deaths from salmonellosis,  had enough active feral cat neuter/return programs,  sterilizing and vaccinating enough cats,  that in 1993-1994 it was the location of one of the first major studies of the efficacy of the neuter/return method.

Home of Project Bay Cat

Marin County,  reporting 173 bird deaths from salmonellosis,  has also had neuter/return programs for nearly 30 years,  with central coordination of the programs since 2005.

Contra Costa County,  reporting 150 bird deaths from salmonellosis,  had multiple active neuter/return programs by 1998.

San Mateo County,  reporting 143 bird deaths from salmonellosis,  had scattered independent neuter/return programs in the 1990s,  with central coordination of the largest programs since 2000.  The best-known San Mateo County neuter/return program,  Project Bay Cat,  began in 2004.

(See Ceasefire halts cat shootings in East Bay Regional Parks––for now.)

More dispersed in breeding habitat

Santa Cruz County,  reporting 115 bird deaths from salmonellosis,  has also had scattered neuter/return programs since the 1990s.

Alameda County,  reporting 109 bird deaths from salmonellosis,  has had neuter/return programs of significant size since 1994.

Sacramento County,  reporting 109 bird deaths from salmonellosis,  has had some neuter/return activity since the early 1990s,  with larger projects at least since 2004.

Placer County,  the most sparsely populated county among the nine reported salmonellosis hot spots,  has had 72 confirmed bird deaths from the disease.  Active neuter/return work has been underway in Placer County at least since 2012,  but very likely began much earlier.

Finished Rogers,  “Most salmonellosis outbreaks [among pine siskins] end when the pine siskins return to their breeding grounds,  typically late March or April.  In their breeding grounds,  pine siskins are more dispersed for activities like nest building and raising chicks,  so infection tends to be less common.”

Salmonellosis outbreak hit Lynnwood first

On January 8,  2021,  exactly one month before Rogers issued the first warning of the salmonellosis outbreak hitting pine siskins,  the disease outbreak was observed by the Progressive Animal Welfare Society,  of Lynnwood,  Washington––which,  coincidentally, was circa 1990 among the very first established humane societies to introduce a neuter/return program.

“The large flocks [of pine siskins] we are seeing all over Western Washington are incredible to witness,”  the Progressive Animal Welfare Society acknowledged on Facebook.

However,  the posting continued,  the PAWS Wildlife Center had admitted 68 pine siskins in 60 days,  and was receiving multiple calls about sick siskins every day.

“Wash all nearby surfaces”

“Usually, we recommend removing feeders for a few weeks when a sick bird is found nearby and cleaning the area thoroughly,”  the Progressive Animal Welfare Society advised.

“However, the flocks are so large and cases so frequent right now, “  PAWS continued,  “that we recommend removing your feeders even before you detect a sick bird, until the irruptive migrants move on.”

The Progressive Animal Welfare Society also recommended that bird-feeding individuals should “Wash all nearby surfaces,”  with a 10% bleach solution,  “and rake the ground around the feeder,”  wearing gloves whenever possible and washing hands often,  since “humans can contract salmonella too.”

Mycoplasma gallisepticum

Observed Program for Monitoring Emerging Diseases wildlife disease moderator Pablo Beldominico, “Bird feeders also facilitate the spread of mycoplasmosis among birds that use them.”

Indeed the feeder disease mycoplasma gallisepticum should have impressed upon bird-feeding enthusiasts a valuable lesson about the ecological importance of the free-roaming cats they frequently curse,  and often trap and kill,  shoot,  or poison,  when they see cats with birds in their mouths,  or find the remains of birds they believe were killed by cats.

Alley Cat Allies president and founder Becky Robinson,  with Louise Holton,  who later founded the Maryland organization Alley Cat Rescue,  together introduced the neuter/return technique to Washington D.C. and surrounding suburbs in 1990.

Outdoor cat population crashed

Data collected by then-Calvert Animal Rescue League executive director Phil Arkow showed that in 1992 Maryland shelters killed 85,600 homeless cats.

Similar data is not available for Washington D.C. itself,  nor for the Virginia side of Washington D.C.,  but the Maryland data is likely generally indicative of the abundance and ecological significance of free-homing cats in the region at the time.

Within five years the Maryland shelter cat death toll dropped to 58,000.  By 2000 it was down to circa 30,000,  and as of 2016 had declined to about 14,200.

Outbreaks of mycoplasma gallisepticum meanwhile exploded around local bird feeders––possibly because cats were no longer killing sick birds before they could spread the infection.

Originally known as factory-farmed turkey disease

Before the mid-1990s,   mycoplasma gallisepticum was most closely identified with outbreaks on factory-style turkey farms,  which had arrived in the outer Washington D.C. area 10-20 years earlier.

Between the early 1990s and the mid-2000s,  mycoplasma gallisepticum spread up and down the east coast and across the U.S. as a bird feeder disease.

Each major outbreak followed several years after the debut of big neuter/return programs in the vicinity.

Edward Howe Forbush

Antipathy toward free-roaming cats as bird predators has been whetted by birding organizations since the 1916 publication of The Domestic Cat: Bird Killer, Mouser and Destroyer of Wild Life;  Means of Utilizing and Controlling It,  by Edward Howe Forbush,  the 19th and early 20th century Massachusetts state ornithologist whose inveterate hatred of cats was based largely on his own scientific ineptitude.

Among other conspicuous errors, Forbush wrongly blamed cats who were never there in the first place for the decline of roseate terns on Muskeget Island,  later established to have been caused by gull predation;  claimed birds do not prey on other birds;  conflated the Quebecois slang term for raccoons,  chat sauvage,  and descriptions of raccoon behavior,  with second hand anecdotal accounts of cat behavior;  and conflated bobcats with domestic cats.

Forbush nonetheless furnished the quasi-scientific basis for more than half a century of concerted efforts by hunters and birders to add cats to state lists of legally hunted species.

The Forbush tract continues to be the usually unacknowledged template for the Nature Conservancy,  National Audubon Society,  and American Bird Conservancy policies of opposition to neuter/return feral cat control.

(See How a birder’s mistakes incited 100 years of cat hatred.)

Predation vs. disease

But an accurate understanding of the ecological role of predation includes recognizing that except on isolated islands,  predation is rarely if ever a significant cause of lasting prey species population declines.

This is for two reasons.

First,  almost all animal predation is “compensatory,”  meaning that the predators kill and eat mainly the sick and injured,  the aged and infirm,  and unattended young,  none of whom are likely to contribute any more to the reproductive success of their species than they already have.

Most animals killed by predators would not survive for much longer anyhow.  Especially if the victim animal is suffering from a contagious disease,  like pine siskins afflicted with salmonellosis,  predation tends to help the species far more than would the temporary survival of an individual who might infect many others.

Only “additive” predation actually reduces the long term abundance of the prey species,  and then only under rare circumstances.

“Additive” predation,  including by parasites and disease-carrying bacteria,  cuts into the successful breeding population of the prey species,  but seldom occurs for long,  because if the prey population declines,  the predators,  including pathogens,   starve out long before the prey or host species disappears entirely.

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