Many birders and birdwatchers in western and central Nebraska have noticed the unfortunate absence in recent years of a distinctive, entertaining and familiar bird species, the Black-billed Magpie. Magpies standout among birds with their striking black and white plumage. Their blackish wings and long tail show greenish iridescence in appropriate lighting. Magpies often make their presence known in an area with their “ka ka ka“ call before they are seen. Not long ago in the late 1990s, Black-billed Magpies were found over most of the state with the exception of the extreme east and southeast. Black-billed Magpies occurred as far east as western Lancaster and Saunders County during this time. In other areas, especially the west, magpies were fairly common and could be reliably seen with little effort.
Since about 2000, Black-billed Magpies have declined sharply. They are no longer found in Lancaster, Saunders and many other counties in eastern and central Nebraska. Even in strongholds in western Nebraska where the species was formerly common it can now be difficult to find. Data from both the Breeding Bird Survey(BBS) and Christmas Bird Counts (CBC) illustrate the decline. First is a graphic showing BBS abundance in Nebraska. The graphic shows a general long-term decline since 1966.
CBC data are perhaps a bit more useful at illustrating the more recent decline. The graphic below shows the number of Black-billed Magpies detected per party hour for each individual CBC in which this species has been detected regularly (the gray line and points also show composite data for the state). Trend lines were generated using locally weighted scatterplot (LOESS) smoothing. The number of Black-billed Magpies recorded per party hour has declined since 1985, but especially since 2000, at all of the CBCs shown. This include the Scottsbluff (pink line and points), Crawford (green line and points) and Lake McConaughy CBCs (orange line and points) in western Nebraska, which are areas where you would expect to see some of the highest numbers of magpies in Nebraska. All the trend lines converge to near zero magpies per party hour between 2010 and 2015.
So what is going on? Why have magpies declined so sharply in Nebraska? Declines in Nebraska coincide with the arrival of West Nile Virus to Nebraska and the Great Plains. Now, after years of research and monitoring, it is well known that corvids (crows, jays, magpies) are susceptible to West Nile Virus and the novel disease is especially lethal to magpies. But, is this virus the only cause of the magpie’s decline? In other areas of the United States, such as northern Minnesota and the Intermountain West, magpies are doing fine and are even increasing in some areas. Hasn’t West Nile Virus spread across most of North America? Thus, shouldn’t we be seeing a decline in magpies throughout their range?
These are really good questions. After doing some digging, I believe the explanation may be found in the following graphic.
This graphic shows West Nile Virus incidence in humans. The darkest shaded area in the graphic above shows areas with the highest rates of West Nile Virus incidence in humans. This area approximately corresponds to the area where Black-billed Magpies have experienced the greatest declines, as shown in the the graphic below which was generated by the USGS using BBS data. Areas in red show where magpies have declined the most while blue and aqua color show areas where abundance has increased.
The pattern of heterogeneous West Nile Virus incidence in humans in the United States appears to be associated with variations in temperature and precipitation that affect breeding habitats of Culex species of mosquitos, which transmit the virus. Conceivably, the variables that influence West Nile Virus’s incidence in humans also affects Black-billed Mapgies. Unfortunately, for most magpies incidence equals death.
Obviously some of the ideas presented in this post are speculative and should be taken with a grain of salt. Other variables may be at play. However, other species, such as Black-capped Chickadees, that suffered declines when West Nile Virus swept across the state have slowly recovered. The same cannot be said about the Black-billed Magpie and its future in Nebraska is uncertain. It is possible that, over time, mapgies could develop resistance to West Nile Virus. This could lead to an increase in numbers and possibly a re-occupation of their former range. However, if current trends continue, it is conceivably the species could be found only in far western areas of the state or extirpated from Nebraska altogether. The latter scenario would be very unfortunate.
For more information about the status of the Black-billed Magpie in Nebraska, check out this report appropriately titled “Status review of the Black-billed Magpie in Nebraska”