22 April 2009

Status of Hybrid Birds Currently Unknown for Niobrara River Valley

[Smith Falls, Niobrara River Valley, June 2007]

A primary aspect given for attributing unique values to the Niobrara River Valley is its recognition as a biological crossroads. This designation relies on a readily apparent setting of different habitats with distinct associations of flora and fauna.

One of the special features mentioned, is a zone of hybridization for a few species of birds with typically eastern and western distributions, according to statements given by conservation groups and a government website.

The Niobrara Valley is recognized as a biological crossroads for wildlife by the Nebraska Wildlife Federation.

The National Audubon Society and its local chapter in Nebraska, designates the Niobrara Valley Preserve - along the river north of Johnstown - as an Important Birding Area, giving pertinent details in an ornithological summary. Therein is mentioned that eastern and western forms of birds breed, referring to Baltimore and Bullock's orioles, lazuli and indigo buntings, rose-breasted and black-headed grosbeaks. The summary also says that eastern and western wood-pewee's may also hybridize.

The Northern Prairie Lands Trusts website, on their projects and news page, says the Valentine, NE area and the Middle Niobrara River Valley Biologically Unique Landscape, is a zone of hybridization for some eastern and western bird species.

The National Park Service, on a webpage with details about the Niobrara Scenic River, which it administers, says: "Hybridization of eastern and western associated species, such as indigo and lazuli buntings, yellow-shafted and red-shafted flickers, and Baltimore and Bullock's orioles are vivid testament of the biological uniqueness of the Scenic River."

Studies of Hybrid Birds

Hybrid birds have been documented as occurring along the profound Niobrara River, as well as elsewhere south to north on the central Plains, based upon several seminal studies that provide particulars for several species.

An initiative to study hybrid birds on the Great Plains started in 1955, according to comments made by Charles G. Sibley and David A. West in their 1959 article on towhees.

The following species have been known to hybridize, according to just a few articles issued by preeminent ornithological scientific journals. There are some additional details on post A.D. 2000 sightings, that should be presented with this consideration, as the more recent co-mingling of species should be considered along with the original studies.

Baltimore Oriole x Bullock's Oriole

The hybrid zone in the central Great Plains - spanning a 150-200 mile distance west to east - includes the Niobrara Valley from northwest of Bassett to at least the western edge of Cherry county, in the article by Charles G. Sibley and Lester L. Short, Jr., published in 1964, and based on more than 600 specimens collected between 1954-1957. William Youngworth noted both species at Fort Niobrara NWR in the mid-1950s. When Short published an additional paper on the distribution of species on the Great Plains in 1961, the Niobrara River in Keya Paha county was given as a region where these two species both occurred. He also included the eastern portion of the river valley in Cherry County.

Spotted Towhee x Eastern Towhee

Most of the 487 specimens evaluated for this research were obtained in Nebraska during 1954-1957. Determination of the "hybrid index" was based on a "back-spotting index" with Spencer, Bassett and Valentine particular places where birds were collected for eventual, focused evaluation. The article in the Auk, extensively discussed biotic features of the Niobrara Valley. The Spotted Towhee "influence" was said to extend eastward into Holt County.

There are no known modern records available for the Niobrara Valley that indicate where both species occur at the same locality, lending credence to species' hybrids.

Rose-breasted Grosbeak x Black-headed Grosbeak

"Pure" species - based mostly upon measurements of certain characteristics of specimens - were noted in Cherry county and eastward, with hybrids found in the Holt County area. This evaluation was conducted from 1955-1957 by researchers from Cornell University in Nebraska, as well as South Dakota and Colorado. A figure showing the zone where hybrid occurs, extends the furthest west along the Niobrara Valley, to a short distance west of Bassett.

William Youngworth noted both of these grosbeaks at Fort Niobrara in his 1955 paper on birds of the Quicourt valley, which is a historic name given for the Niobrara river. Lester Short, Jr., then noted them along the Niobrara River in Keya Paha county during the same year. Recent records indicate both species occur in the immediate vicinity of Valentine, at Anderson Bridge WMA in central cherry County, as well at private property along the river south of Nenzel.

Indigo Bunting x Lazuli Bunting

The first instance of hybrids of these two species was recorded by Youngworth, when a oddly marked male specimen was collected June 1, 1932. He noted in his brief article published in the Wilson Bulletin: "The country bordering the swift Niobrara River in Cherry County is ideal for the summer home of grosbeaks and buntings." These two bunting species were also noted as occurring at Fort Niobrara NWR in 1955, by the same bird watcher.

The article by Sibley and Short published in 1959, was based upon specimens gathered during 1955-1957. Valentine was recognized as an especially interesting locale, with "pure" species present at the same immediate locale as hybrids. Hybrids were also noted at Spencer.

The most recent study along the Niobrara, looked at these two species near Niobrara, at Valentine and south of Chadron in 1969. Hybridization was noted in the vicinity of Chadron.

"We conclude: (1) That massive convergence and introgression is not occurring among the Passerina of the Great Plains. (2) The species appear to have diverged to a point where hybrids and mixed-pairs are at a selective disadvantage. (3) The two forms minimize ecological competition through the maintenance of non-overlapping, interspecific territories. - Stephen T. Emlen, James D. Rising and William T. Thompson

There have been additional, recent concurrence elsewhere in the region, particularly the sandhills, but not in the confines of the river valley.

Northern Flicker - red-shafted and yellow-shafted forms

Extensive details have been published on hybridization among flickers with different colored featheration, but as the two forms - red-shafted and yellow-shafted - are now designated as a single species, the mingling of these two species which have different color characteristics is not a valid example of hybridization. This occurrence does, nonetheless, have its own distinctive biotic interest.

Two sets of additional species have also been included as having the potential for there being hybrids in the river valley.

Western Wood Pewee x Eastern Wood Pewee

There is no actual scientific research paper that gives any particulars that these two species actually hybridize in northern Nebraska. Particular locales where both have been known to occur is shown in recent years by sightings in Cherry County at Anderson Bridge WMA, the Valentine City Park, and at Fort Niobrara NWR.

Scarlet Tanager x Western Tanager

There is no known documentation of inter-specific breeding along the Niobrara by these two species. They both occurred at Fort Niobrara NWR in 1955, according to Youngworth in his article on species of the vicinity.

Current Status

It has been more than five decades — with the exception of the buntings where it has been forty years — since rigorous and authoritative studies have been done on the occurrence and distribution of hybrid birds in the central Niobrara River Valley. There is no known source of current information on this topic, based upon contacts with several regional authorities familiar with bird occurrence in the Nebraska, as well as several detailed searches of web-based information.

There are no modern details on bird hybrids and also a readily apparent lack of knowledge of the current distribution of most birds - not only species which may hybridize - along the Niobrara River in northern Nebraska. Despite surveys such as the detailed work during the early 1980s for the Niobrara Valley Preserve, when The Nature Conservancy bought thousands of acres of what had been ranch property.

Numerous site surveys personally conducted in the valley since 2000 have been done at many privately and publicly owned land tracts to get some sense of bird species occurrence, and here are a few other occasional reports from easily accessible places such as Fort Niobrara NWR. None of this information discusses to any useful extent the essential aspects of bird hybridization, although it does indicate to a somewhat limited extent, places where species do co-mingle.

Considering the intense attention being given to how climate change is supposedly influencing the temporal occurrence of many bird species across their normal range of distribution, a "crossroads" where several species mix is a prime region where there should be a detailed evaluation of any flux or alteration in range. Yet, there is no known research being done to evaluate any changes in the occurrence and distribution of avian species along the unique Niobrara River valley where an obvious, and well known mix of several types of plant communities create a floristic crossroads of habitat.

There is a obvious and prevalent need for detailed research to determine the status of bird species recognized for their hybridization in the Niobrara Valley. Also essential is an understanding of the current occurrence and distribution of other species of concern which are changing due to changes in conditions suitable for their existence.

Until there is actual, record-based and current gathering of information on the inter-mingling of different bird species along the Niobrara River, there is no basis to define the valley as a biological crossroads for hybrid birds. Any statement about this needs to be given in the context as being based upon historical conditions. Any claims otherwise are erroneous.

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