Species extinctions have defined the global biodiversity crisis, but extinction begins with loss in abundance of individuals that can result in compositional and functional changes of ecosystems. Using multiple and independent monitoring networks, we report population losses across much of the North American avifauna over 48 years, including once-common species and from most biomes. Integration of range-wide population trajectories and size estimates indicates a net loss approaching 3 billion birds, or 29% of 1970 abundance. A continent-wide weather radar network also reveals a similarly steep decline in biomass passage of migrating birds over a recent 10-year period. This loss of bird abundance signals an urgent need to address threats to avert future avifaunal collapse and associated loss of ecosystem integrity, function, and services.

https://science.sciencemag.org/content/sci/early/2019/09/18/science.aaw1313.full.pdf

Decline of the North American avifauna

Anthropogenic threats, such as collisions with man-made structures, vehicles, poisoning and predation by domestic pets, combine to kill billions of wildlife annually. Free-ranging domestic cats have been introduced globally and have contributed to multiple wildlife extinctions on islands. The magnitude of mortality they cause in mainland areas remains speculative, with large-scale estimates based on non-systematic analyses and little consideration of scientific data. Here we conduct a systematic review and quantitatively estimate mortality caused by cats in the United States. We estimate that free-ranging domestic cats kill 1.4–3.7 billion birds and 6.9–20.7 billion mammals annually. Un-owned cats, as opposed to owned pets, cause the majority of this mortality. Our findings suggest that free-ranging cats cause substantially greater wildlife mortality than previously thought and are likely the single greatest source of anthropogenic mortality for US birds and mammals. Scientifically sound conservation and policy intervention is needed to reduce this impact.

/pdf/the-impact-of-free-ranging-domestic-cats-on-wildlife-of-the-1fdc2wutig.pdf

The impact of free-ranging domestic cats on wildlife of the United States

Nature is under siege. In the last 10,000 y the human population has grown from 1 million to 7.8 billion. Much of Earth’s arable lands are already in agriculture (1), millions of acres of tropical forest are cleared each year (2, 3), atmospheric CO2 levels are at their highest concentrations in more than 3 million y (4), and climates are erratically and steadily changing from pole to pole, triggering unprecedented droughts, fires, and floods across continents. Indeed, most biologists agree that the world has entered its sixth mass extinction event, the first since the end of the Cretaceous Period 66 million y ago, when more than 80% of all species, including the nonavian dinosaurs, perished.

Ongoing losses have been clearly demonstrated for better-studied groups of organisms. Terrestrial vertebrate population sizes and ranges have contracted by one-third, and many mammals have experienced range declines of at least 80% over the last century (5). A 2019 assessment suggests that half of all amphibians are imperiled (2.5% of which have recently gone extinct) (6). Bird numbers across North America have fallen by 2.9 billion since 1970 (7). Prospects for the world’s coral reefs, beyond the middle of this century, could scarcely be more dire (8). A 2020 United Nations report estimated that more than a million species are in danger of extinction over the next few decades (9), but also see the more bridled assessments in refs. 10 and 11.

Although a flurry of reports has drawn attention to declines in insect abundance, biomass, species richness, and range sizes (e.g., refs. 12⇓⇓⇓⇓⇓–18; for reviews see refs. 19 and 20), whether the rates of declines for insects are on par with or exceed those for other groups remains unknown. There are still too … 

[↵][1]1To whom correspondence may be addressed. Email: david.wagner{at}uconn.edu.

 [1]: #xref-corresp-1-1

https://www.pnas.org/content/pnas/118/2/e2023989118.full.pdf

Insect decline in the Anthropocene: Death by a thousand cuts

Recent case studies showing substantial declines of insect abundances have raised alarm, but how widespread such patterns are remains unclear. We compiled data from 166 long-term surveys of insect assemblages across 1676 sites to investigate trends in insect abundances over time. Overall, we found considerable variation in trends even among adjacent sites but an average decline of terrestrial insect abundance by ~9% per decade and an increase of freshwater insect abundance by ~11% per decade. Both patterns were largely driven by strong trends in North America and some European regions. We found some associations with potential drivers (e.g., land-use drivers), and trends in protected areas tended to be weaker. Our findings provide a more nuanced view of spatiotemporal patterns of insect abundance trends than previously suggested.

Meta-analysis reveals declines in terrestrial but increases in freshwater insect abundances.

Handbook of North American Birds Edited by Ralph S. Palmer. Vol. 1: Loons through Flamingos. Pp. vii + 567 + 6 plates. (New Haven, Conn., and London: Yale University Press, 1962.) 105s.

/pdf/birds-of-north-america-2ei7jfe040.pdf

Birds of North America

Predation by house cats (Felis catus) is one of the largest human-related sources of mortality for wild birds in the United States and elsewhere, and has been implicated in extinctions and population declines of several species. However, relatively little is known about this topic in Canada. The objectives of this study were to provide plausible estimates for the number of birds killed by house cats in Canada, identify information that would help improve those estimates, and identify species potentially vulnerable to population impacts. In total, cats are estimated to kill between 100 and 350 million birds per year in Canada (> 95% of estimates were in this range), with the majority likely to be killed by feral cats. This range of estimates is based on surveys indicating that Canadians own about 8.5 million pet cats, a rough approximation of 1.4 to 4.2 million feral cats, and literature values of predation rates from studies conducted elsewhere. Reliability of the total kill estimate would be improved most by better knowledge of feral cat numbers and diet in Canada, though any data on birds killed by cats in Canada would be helpful. These estimates suggest that 2-7% of birds in southern Canada are killed by cats per year. Even at the low end, predation by house cats is probably the largest human-related source of bird mortality in Canada. Many species of birds are potentially vulnerable to at least local population impacts in southern Canada, by virtue of nesting or feeding on or near ground level, and habitat choices that bring them into contact with human-dominated landscapes where cats are abundant. Because cat predation is likely to remain a primary source of bird mortality in Canada for some time, this issue needs more scientific attention in Canada. RESUME. La predation par les chats domestiques (Felis catus) est une des causes de mortalite liees aux humains les plus importantes chez les oiseaux sauvages aux Etats-Unis et ailleurs; elle a egalement ete mise en cause dans l’extinction et le declin de populations de plusieurs especes. Toutefois, la situation au Canada est plutot meconnue. Cette etude visait a obtenir des estimations plausibles du nombre d’oiseaux tues par les chats domestiques au Canada, a determiner les informations additionnelles pouvant contribuer a ameliorer ces estimations et enfin, a identifier les especes dont les populations pourraient etre touchees. Dans l’ensemble, nous avons estime que les chats tuaient entre 100 et 350 millions d’oiseaux par annee au Canada (> 95 % des estimations se trouvaient dans cette plage), les chats sauvages tuant vraisemblablement la majorite de ces oiseaux. Cette plage d’estimations est fondee sur des releves indiquant que les Canadiens possedent 8,5 millions de chats domestiques environ, une approximation grossiere du nombre de chats sauvages se situant entre 1,4 et 4,2 millions et des taux de predation tires d’autres recherches menees ailleurs qu’au Canada. Une meilleure connaissance du nombre de chats sauvages et de leur regime alimentaire au Canada permettrait d’ameliorer grandement la fiabilite de l’estimation du nombre d’oiseaux tues, bien que n’importe quelle donnee sur les oiseaux tues par les chats au Canada y contribuerait egalement. Nos estimations indiquent que de 2 a 7 % des oiseaux dans le sud du Canada sont tues par les chats chaque annee. Meme en considerant l’estimation la plus prudente, la predation par les chats domestiques est probablement la cause de mortalite d’oiseaux liee aux humains la plus importante au Canada. De nombreuses especes d’oiseaux sont potentiellement vulnerables a des effets sur leurs populations au moins localement dans le sud du Canada, en raison de leur nidification ou de leur alimentation au sol ou pres du niveau du sol, et des choix d’habitats qui les amenent dans des paysages domines par les humains ou les chats sont abondants. Etant donne que la predation par les chats demeurera vraisemblablement une source importante de mortalite d’oiseaux au Canada dans un avenir rapproche, cet enjeu merite davantage d’attention scientifique au Canada.

/pdf/estimated-number-of-birds-killed-by-house-cats-felis-catus-3rkd4oef8b.pdf

Estimated Number of Birds Killed by House Cats ( Felis catus ) in Canada

Following the example of the North American Waterfowl Management Plan, deriving numerical population estimates and conservation targets for priority landbird species is considered a desirable, if not necessary, element of the Partners in Flight planning process. Methodology for deriving such estimates remains in its infancy, however, and the use of numerical population targets remains controversial within the conservation and academic communities. By allowing a set of simple assumptions regarding species' detectability, relative abundance data from Breeding Bird Survey (BBS) routes may be extrapolated to derive first approximations of current, total species populations, both rangewide and within Bird Conservation Regions. Preliminary comparisons with independently derived abundance estimates (e.g., Breeding Bird Atlas) suggest that these population estimates are within acceptable limits of accuracy for many species. If restoring populations to early BBS levels (late 1960s) is desirable, trend data may be used to calculate the proportion of a species' population lost during this 35-year period, and an appropriate population target may be set. For example, in the Lower Great lakes/St. Lawrence Plain, BBS data indicate a current (1990-1999) population of about 14,000 Redheaded Woodpeckers (Melanerpes erythrocephalus) and a loss of >50 percent since 1966. A reasonable conservation objective, therefore, may be to double the Red-headed Woodpecker population in this region over some future time period. We encourage the use of numerical population estimates and conservation targets in implementing conservation objectives for priority landbird species, and we encourage further research that leads to refinement of our methodology and our estimates.

https://www.fs.fed.us/psw/publications/documents/psw_gtr191/psw_gtr191_0057-0067_rosenberg.pdf

Setting numerical population objectives for priority landbird species

We studied the breeding ecology and behavior of Western Kingbirds in relation to abundance of flying insects in an area of sympatry with Cassin's Kingbirds in southeastern Arizona. Day—to—day variation in abundance of flying insects was related to date and temperature, while annual differences appeared to reflect precipitation during the previous year. Western Kingbirds nesting in study sites with relatively high insect biomass had earlier clutch initiation dates, larger clutch sizes, higher nestling growth rates, shorter time between foraging flights, and shorter time between nestling feedings, compared to birds nesting where food supply was lower. Annual differences in insect biomass were also reflected in kingbird clutch initiation dates, clutch sizes, and nestling growth rats. Fledgling survival was related to rate of mass gain as a nestling. Indications were that food was not superabundant, yet no detectable effects of kingbird density on clutch size or nestling growth rate existed. Kingbirds did not appear to reduce significantly the local abundance of flying insects, which calls in question the importance of competition for food in determining local habitat separation.

Effect of Food Supply on the Breeding Biology of Western Kingbirds

Understanding the susceptibility of highly mobile taxa such as migratory birds to global change requires information on geographic patterns of occurrence across the annual cycle. Neotropical migrants that breed in North America and winter in Central America occur in high concentrations on their non-breeding grounds where they spend the majority of the year and where habitat loss has been associated with population declines. Here, we use eBird data to model weekly patterns of abundance and occurrence for 21 forest passerine species that winter in Central America. We estimate species’ distributional dynamics across the annual cycle, which we use to determine how species are currently associated with public protected areas and projected changes in climate and land-use. The effects of global change on the non-breeding grounds is characterized by decreasing precipitation, especially during the summer, and the conversion of forest to cropland, grassland, or peri-urban. The effects of global change on the breeding grounds are characterized by increasing winter precipitation, higher temperatures, and the conversion of forest to peri-urban. During spring and autumn migration, species are projected to encounter higher temperatures, forests that have been converted to peri-urban, and increased precipitation during spring migration. Based on current distributional dynamics, susceptibility to global change is characterized by the loss of forested habitats on the non-breeding grounds, warming temperatures during migration and on the breeding grounds, and declining summer rainfall on the non-breeding grounds. Public protected areas with low and medium protection status are more prevalent on the non-breeding grounds, suggesting that management opportunities currently exist to mitigate near-term non-breeding habitat losses. These efforts would affect more individuals of more species during a longer period of the annual cycle, which may create additional opportunities for species to respond to changes in habitat or phenology that are likely to develop under climate change.

Peter J. Blancher

Papers

Decline of the North American avifauna

Estimated Number of Birds Killed by House Cats ( Felis catus ) in Canada

Setting numerical population objectives for priority landbird species

Effect of Food Supply on the Breeding Biology of Western Kingbirds

Global change and the distributional dynamics of migratory bird populations wintering in Central America