VU
Yellow-billed Magpie Pica nutalli



Justification

Justification of Red List Category
This species is currently listed as Vulnerable. Between 2003-2008, the species underwent a rapid population reduction owing to mortality caused by West Nile Virus and whilst declines have slowed, they are still occurring at a sufficient rate to meet the threshold for listing as Vulnerable.

Population justification
Partners in Flight (2020) currently estimate the population of Pica nutalli to total 400,000 mature individuals, with the population continuing to decline following the severe impacts of the West Nile Virus.

Trend justification
The species suffered high levels of mortality and a severe population decline owing to an outbreak of West Nile virus (Airola et al. 2007, Crosbie et al. 2008). Following the documented arrival of the virus in California in summer 2003 (Reisen et al. 2004), data have suggested a decline of 42-49% from 2004 to 2006 (Crosbie et al. 2008). Christmas Bird Count (CBC) data from the Lower Sacramento Valley suggest that numbers of this species declined by 48% between 2004/2005 and 2005/2006, with surveyed numbers in 2005/2006 having declined by 38% compared to the previous 10-year average when accounting for the effects of bad weather (Airola et al. 2007). It was thought that the population appeared to reach a low in 2007-08 before showing signs of recovery, however the evidence suggests declines are ongoing. albeit at a lower rate, and the population is still greatly depleted compared to the data collected in the late 1950s (W. Koenig in litt. 2012, Kilpatrick and Wheeler 2019). 

Year by year records from the Breeding Bird Survey (Pardieck et al. 2018) suggest that the species has continued to decline since the end of the West Nile Virus outbreak, although at a lower rate. This data shows that between 2008 and 2019, the average annual decline was 5.64%, which equates to a decline of ~53% across the past three generations (12.9 years). The Audubon Christmas Bird Count suggest a much lower rate of annual decline, 3.12%, which equates to a ~34% reduction across the past three generations (Meehan et al. 2018). To account for the relatively large band of the trend estimate, it is here tentatively assumed that the true rate of decline is in between both extremes and as such the past decline is here placed in the band 30-49% over three generations.

The rate of decline appears to have slowed down considerably in recent years. Between 2016 and 2019, the annual decline amounted to -2.06% per year, although this value is non-significant (Pardieck et al. 2018). Extrapolating over three generations, the rate of past and future decline equates to 24% over three generations.

Distribution and population

This species is endemic to California, USA, occurring west of the Sierra Nevada mountains (del Hoyo et al. 2009, Marzluff and Sharpe 2018). The species's population, estimated at c.180,000 individuals in 2003, is thought to have been reduced by 49% by 2006 (del Hoyo et al. 2009), owing to the impacts of West Nile Virus. Following a low in 2007-2008, the population now appears to have stopped its decline and may have recovered somewhat (W. Koenig in litt. 2016), even though observational data suggest a moderate ongoing decline (Sauer et al. 2017).

Ecology

This species inhabits oak savanna, with large trees scattered among broad expanses of open grassland and pasture (del Hoyo et al. 2009, Marzluff and Sharpe 2018). Over recent decades, it has been increasing in suburban settings, notably in the Sacramento Valley. It forages in cultivated fields and orchards. This omnivorous species feeds on a range of items, including invertebrates, small mammals, bird eggs and nestlings, carrion, food discarded by humans, grains, fruits, nuts and other seeds. Nest-building takes place from December through to March, with egg-laying from March to May (del Hoyo et al. 2009).

Threats

Habitat loss through urban and agricultural developments, as well as the impacts of Sudden Oak Death, summer droughts and poisons used to kill ground squirrels were thought to be potential threats to the species (see Marzluff and Sharpe 2018). These may have caused minor local declines, but the species was still abundant in other areas (Marzluff and Sharpe 2018), such that these threats were thought to have no/negligible impacts on the species as a whole and the population was deemed to be stable, although Airola et al. (2007) suggest the species may have even increased between 1980/81 and 2001/02. However, since the mid-2000s the population has been in very rapid decline as West Nile Virus (WNV) sweeps through the population and it is thought to have suffered the largest, range-wide impact from WNV of any bird species (Kilpatrick and Wheeler 2019). It was first noted in 2003 (Reisen et al. 2004) and although there was suggestions of a potential recovery after 2007-08 (W. Koenig et al. 2012), recent evidence concludes that the species is continuing to decline, possibly due to the species's limited range and the fact that WNV transmission has occurred throughout its range in every year since 2004, despite the peak of the outbreak being a number of years ago (Kilpatrick and Wheeler 2019).

Conservation actions

Conservation Actions Underway
This species has been the subject of monitoring through citizen science surveys. The species is included on the 'Watch List' of the State of North America's Birds as a species of high conservation concern (NABCI 2016).

Conservation Actions Proposed
Continue to monitor the species's population trend through regular surveys. Protect areas of suitable habitat.

Acknowledgements

Text account compilers
Hermes, C., Everest, J.

Contributors
Ashpole, J, Butchart, S., Derhé, M., Ekstrom, J., Koenig, W., Meehan, T., Sharpe, C.J., Symes, A., Taylor, J. & Westrip, J.R.S.


Recommended citation
BirdLife International (2022) Species factsheet: Pica nutalli. Downloaded from http://www.birdlife.org on 19/08/2022. Recommended citation for factsheets for more than one species: BirdLife International (2022) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 19/08/2022.