Patterns of Vagrancy

 

Over the years, the Vermont Bird Records Committee (BRC) has been performing a vital service to birders by compiling records of vagrant birds within the state. By doing so they have helped preserve and grow our institutional memory about patterns of occurrence of rare species, and have laid the foundation for evaluating changes in their distribution, seasonality and occurrence.

In these pages, I present analyses of the records compiled by the BRC for species that I, personally, find interesting. I have begun with Nelson's sharp-tailed sparrow and black vulture, but hope to include occasional pieces on other rare species as time goes by. My hope is that Vermont birders will enjoy these reports and, since luck is nine tenths preparation, will benefit by being more able to anticipate and find future vagrants.

It is important to note that the analyses presented below reflect my own personal thoughts and conclusions, and are not necessarily those of the BRC.

1. NELSON'S SHARP-TAILED SPARROWS IN VERMONT

Prior to 1998, what we now know as Nelson’s sharp-tailed sparrow (Ammodramus nelsoni) was considered a race of the saltmarsh sharp-tailed sparrow (Ammodramus caudacutus). However, mitochondrial DNA and behavioral studies led the AOU to split the former into two separate species: Nelson’s and saltmarsh. Nelson’s sharp-tailed sparrow includes three races: A.n. nelsoni, which breeds in North Dakota and the prairie provinces of Canada; A. n. alterus, which breeds on the southern shores of Hudson’s Bay; and A.n. subvirgatus, which breeds in Atlantic coastal areas from the St. Lawrence south to Maine. While A. n. subvirgatus is separable in the field from the two inland races, it is not clear whether A.n. nelsoni and A.n. alterus can be confidently separated in the field.

Eight of the nineteen sharp-tailed sparrow sightings in Vermont occurred prior to the taxonomic split and they were recorded as either “sharp-tailed sparrows” or “saltmarsh sharp-tailed sparrows of the Nelson’s race”. In this analysis it is assumed that all of these birds were, in fact, what we now recognize as Nelson’s sharp-tailed sparrow. While it is possible that one or more of these birds were saltmarsh sharp-tailed sparrows, it is highly unlikely because of the lack of their preferred saltmarsh habitat and the fact that they have not been recorded inland in any of the other New England states.

Vermont Records

Between 1916, when the first occurrence of Nelson’s sharp-tailed sparrow occurred, and November of 2007, a total of 19 individuals (see table below) were recorded in Vermont (all of these have been accepted by the Vermont Bird Records Committee except for the four 2007 records which are awaiting review.)

Seasonality of sightings

All accepted records or records that are currently under review occurred during the fall migration season. No birds have yet been recorded in winter, spring or summer. As shown in the graph below, Nelson’s sharp-tailed sparrows apparently migrate through Vermont in the fall within a remarkably narrow seasonal “window”. All 19 birds occurred during a 4-week window from September 21 to October 20, and 18 were recorded during the first three weeks of October. Thus, the optimal time to search for this species is between October 1 and October 21. The lack of birds in the spring in New England poses an intriguing question for which, as yet, no answer exists. However, as most A. n. nelsoni winter on the Gulf Coast, perhaps they return to their inland breeding areas via a more westerly route.

Timing of Nelson's sharp tailed-sparrow sightings in Vermont.


Localities

The majority of the Vermont sightings of Nelson’s sharp-tailed sparrows are from three areas: Rutland County, the Lake Champlain counties (Chittenden, Shelburne, Addison), and southeast Windham County. Between them these three account for 17 of the recorded individuals. All sightings have been in lowland areas of these counties (<600 feet above sea level).

Races

Only seven individual sparrows were identified to race. Of these, one was tentatively assigned to A.n.nelsoni , another to A.n.alterus and the remaining five to nelsoni/alterus (i.e. A.n.subvirgatus was excluded). Thus, it appears that at least the majority of the birds migrating through Vermont in the fall are of the two inland breeding races (nelsoni and alterus). As already stated, separation of nelsoni and alterus in the field is problematic and the racial proportions of the Vermont birds cannot be determined at this time.

Habitat

Only six of the 19 sightings reported detailed habitat information. All of these indicate that the birds were found in meadows dominated by tall and dense grasses, sedges and weeds. Plant species mentioned include, goldenrod, purple loosetrife, carex spp., typha, briars, and canary grass. Five of the records stated that the ground was damp or marshy. Thus, the preferred habitat seems to be fields or meadows that have become invaded and dominated by rank weed or marshland species. In southeast Windham County, the plants in the areas where the sparrows were found were typically up to about thigh-high, dense and difficult to walk through.

Rarity or Otherwise

Thus far, the majority of Vermont Nelson’s sharp-tailed sparrows have been recorded from three main areas (Lake Champlain, Rutland County and southeast Windham County). Does this mean that the species is restricted as a migrant to these areas, or is it more widespread, but under recorded? Obviously, the answer to these questions cannot be known with certainty. However, it is more likely that the species is under recorded and occurs more widely in the Vermont lowlands than the current records would suggest. It is a relatively secretive species, rarely calls, may not be as easily lured out of hiding as other sparrows, and inhabits dense wet vegetation that is not pleasant to walk through. All of these factors may contribute to the species being under recorded. It is also worth noting that Nelson’s sharp-tails in Vermont in the last decade or so have all been found where birders expect to find them (i.e., where the species has been previously found) – Dead Creek, Shelburne Bay and Brattleboro. This may indicate that if habitat is present the species may occur anywhere with some regularity. Overall, it may be inaccurate to refer to this species as a Vermont “rarity”; it might be more accurate to describe it as “uncommon but regular”. More systematic searching in suitable habitat and at the appropriate time of year might prove that the species is much more widespread than previously thought.

Records of Nelson's sharp-tailed sparrows in Vermont
Locality Date Number of individuals Race
Rutland, Rutland County 10/8/19161?
Clarendon, Rutland County 10/8/19161?
Rutland, Rutland County 10/2/19171?
Rutland, Rutland County 10/10/19181?
Rutland, Rutland County 9/21/19191?
Newbury, Orange County 10/4/19421?
Newbury, Orange County 10/19/19461?
Winooski River, Chittenden County 10/3/19851?
Shelburne Bay, Shelburne County10/3/20041nelsoni
Shelburne Bay, Shelburne County 10/16/20041alterus
Shelburne Bay, Shelburne County 10/18/20041nelsoni or alterus
Brattleboro, Windham County10/20/20041nelsoni or alterus
Dead Creek, Addison County 10/1/20062?
Brattleboro, Windham County10/4/20061nelsoni or alterus
Dead Creek, Addison County 9/30/20071?
Brattleboro, Windham County10/1/20071nelsoni or alterus
Brattleboro, Windham County10/11/20071nelsoni or alterus
Dead Creek, Addison County10/21/20071?


2. BLACK VULTURES IN VERMONT

Between 1884 and 2006 a total of eight sightings of black vultures had been accepted by the Vermont Bird Records Committee (VBRC). Since 2006, a further three sightings have been submitted to the VBRC and are under consideration.

Over the 123-year period since the first sighting, the seasonality of sightings and their annual rate have altered remarkably. These changes reflect, to a great extent, changes that have been occurring in the seasonal occurrence and distribution of black vultures elsewhere in New England. Over the last century the species has been extending its range northwards in Connecticut (where they first nested in 2002) and Massachusetts. Prior to 1955 there had been about 20 sightings in Massachusetts, but between 1955 and 1993 there were over 30. The species has become annual in some parts of Massachusetts, particularly in the Connecticut River Valley where it occurs in small numbers (though up to 31 individuals have been recorded in Sheffield) as far north as the Northampton/Deerfield area.

The reasons for the northward range extension are not clear. It is interesting that it has occurred only on the east coast; in the western states there is no evidence of the species extending its range northward from the Mexican border, where it has traditionally occurred.

The accepted Vermont records and the three sightings submitted for 2006/2007 are shown below:

Records of Black Vultures in Vermont
Date Locality
July, 1884 Woodbury, Washington
July, 1912Pawlet, Rutland
June, 1954Royalton, Windsor
September, 1983Vernon, Windham
September, 1999Colchester, Chittenden
May, 2001Shelburne, Chittenden
June, 2002White River Junction, Windsor
March, 2006Dummerston, Windham
May, 2006West Haven, Rutland
April, 2007Vernon, Windham
April, 2007Williston, Chittenden


In the 99 years between 1884 and 1983, there were only 4 sightings of black vultures in the state. However, there were 7 sightings in the 9 years between 1999 and 2007. Thus the annual rate of sightings has accelerated markedly in the last decade (it has been recorded in 4 of the last 7 years).

As might be expected for a species that is undergoing a northward range extension, many of the sightings in Vermont have been in the southern part of the state, with six of the 11 records being from Rutland and further south. Also, given the distribution of the species in Massachusetts, the fact that 4 of the 10 Vermont sightings are either on or close to the Connecticut River is not unexpected.

Although the data are few, there may also have been some interesting shifts in the seasonality of occurrence: The 5 sightings between 1884 and 1999 all occurred either in mid or late summer. However, since then, the species has been sighted most often in early spring or summer (late March through June). It is possible that the early historical pattern may indicate post-breeding season dispersal of birds from south of New England, whereas the more recent patterns may indicate pre-breeding prospecting by younger birds. It is interesting that breeding in Connecticut was preceded by an increased incidence of spring sightings. Therefore, it is possible that breeding by black vultures may occur in Vermont in the foreseeable future.