*  (MOVIE) *

 

 A  Collusion of Harriers

The winter of 2012-2013 brings an unusual number of Northern Harriers to Virginia's Shenandoah Valley. We see them in strong numbers and in expeditions; not just individuals quartering separately but pairs and trios sectioning the fields at staggered altitudes and in close proximity. A juvenile Harrier crosses a pasture flying low, straight, as if to a destination. A second bird, an adult male, follows 100 feet behind and 100' above. They recede from view in that echelon formation. On a friend's farm two Harriers circle low over her stand of native warm-season grasses, again an adult and a hatch-year bird. They tangle and the adult drives the young bird into the tree line, but two more approach from different directions to hunt the 30-acre grass stand, and the gray bird gives up his swaggering. A minute later they all leave. Out my kitchen window a juvenile sits on  a fence post scanning the winter-brown berry patch. Egad, three posts away sits another!  Then a third juvenile makes a run up the row of raspberry canes and both the perched birds turn to watch.

The close proximity of these hounds of the grasslands is not likely the result of chance; there are hundreds -- OK, thousands -- of acres of pasture in the Shenandoah Valley near Buffalo Gap
and if the Harriers so desired each could have a private hunting reserve. Instead they sometimes cluster, to the extent that if you see one they have trained you to look for another nearby.

So what's going on here? You don't suppose this is... cooperative hunting? The two perched juveniles watching a third make a  pass nearby behave similarly to a squad of Harris' Hawks I saw in Cochise County, Arizona; some perching to watch as others maneuvered. Harris' Hawks are accomplished cooperators, purposefully chasing and flushing while teammates wait to strike. But perhaps there is also a less coordinated, if still intentional, mode of gang hunting. Short-eared Owls do it, wheel and scan for grassland rodents in grumpy proximity,  tangling when their circles overlap. But  chummy or not, they bark at one another and cluster-hunt and you have to suspect there is a net benefit. They could as easily hunt singly. Perhaps it's not essential that everybody agree on an attack plan to bring off an ambush -- something less elaborate than the old  "You wait here while I circle around and flush him your way" trick might work occasionally. It may be sufficient that the adult Harrier who was dogging the young bird in echelon will nab a rodent fleeing the lead bird but unaware of the second. It may be that adjacent-hunting raptors would just as soon make a meal of one another as snatch a vole, but the vole is an easier snack and likely more rattled by two Harriers than by one. It may be that experienced adults clepto-hunt by keeping an eye on juveniles. Other raptors and scavengers, too -- a Common Raven for example -- might be caught on camera confiscating a young Harrier's catch.

Or the explanation may be as simple as young Harriers in casual collusion as a means of making it through their first winter.

Update: Researcher Amy Johnson sends her photos of a pair and a trio of Harriers hunting together.

*(MOVIE)*

Buffalo Gap Dickcissels, 2012

Distinctive Calls of Male Dickcissels

If you have Dickcissels in your pastures or if you pass their field with your window down there will be little doubt of it, for the males take their bright colors and loud voices to their advertisement perches and sing and counter-sing, hardly taking a breath break.   They arrive as a group and seize the grassland acoustics. Breeding males are loud and obvious and they generate a territorial din. This year the Dickcissels arrived at Wheatlands Farm at Buffalo Gap in Virginia’s Shenandoah Valley on June 1  and for the next 66 days they dominated the rap.

In our last post we noted the connection Kenn Kaufman sees between recent eastern breeding appearances of the Dickcissel and the stresses from drought and fire the bird faces in its traditional prairie nesting range.  In 2011 drought choked the southern prairies, engulfing Texas plus a 200-mile margin, and arguably accounted for the Dickcissel irruption east of the Allegheny Mountains as the birds sought alternate breeding opportunities. This year the drought expanded northward, toasting crops and pastures across Nebraska, Iowa, and  Wisconsin and compromising grassland nesting over an enlarged dry zone.  And in 2012 the Dickcissel chorus at Buffalo Gap grew by many voices; upward of a dozen males sang on this farm of 160 acres of grassland, more than double the 2011 numbers. That level of Dickcissel breeding density is characteristic of the grassland sweet spots in Missouri and Kansas but is uncommon in the East. (Check Lang Elliot's comments and recordings at www.birdtunes.com)

The U.S. Drought Monitor, maintained at the University of Nebraska in association with NOAA and USDA, observes regarding the 2012 episode,

 " The drought has inflicted, and is expected to continue to inflict, catastrophic economic ramifications for the affected states." 

So, the drought sequence will likely continue, seemingly a component of global climate change.  If 2013 is comparably dry in the prairie states and if the  Dickcissels again seek nesting relief  in the East, there will be little doubt of the Kaufman correlation. Would a record search show that eastward Dickcissel irruptions in earlier years were also the result of western weather stresses? At this point we are on notice of a likelihood that the Dickcissel will be needing welcome in such grasslands as remain in the East (after the massive twentieth-century abandonment of eastern farmland). In seeking breeding refuge to the east of its principal historic range, is the Dickcissel an early indicator species of climate change?

The Dickcissel has problems in the east which it did not have in its pre-Colombian prairie range where the main perils to nesting were bison hooves and fires; worse, of course, in dry years. Arriving in Virginia's Shenandoah Valley on June 1st of this year, the Dickcissels set up shop a month or more behind the Bobolinks and Meadowlarks. Eastern farmers cut hay in late May and early June.  Hay made in early June on this farm this year would have contained Dickcissel nests; some other nests as well, but many of the other grassland obligates have brought off a brood by then and could better afford the loss than could the Dickcissels.

Most of the eastern grasslands available for breeding are those which farmers maintain for pasture or hay and the Dickcissels arrive at the wrong moment in the farming schedule. If they choose hay ground the nests go through the cutter. If they select lightly-grazed pasture, that is likely to be clipped for weed control in early July. A Dickcissel's best hope is to find a spot inaccessible to cutting equipment -- a wet or rocky place -- large enough to accommodate a territory.

That is another problem.  Grassland birds including Dickcissels and Bobolinks are, in the terms of researchers, "area sensitive"; they need tracts of suitable habitat large enough to accommodate not just one territory, but several, perhaps many.  You don't often find one pair of Dickcissels or Bobolinks (like many grassland birds, both are polygamous and sometimes polyandrous so the term "pair" applies loosely), you find a nesting cohort which requires a large tract of grassland; research suggests a minimum of 60 acres for Bobolinks. Dickcissels migrate together, claim adjacent territories, winter together, and perhaps know one another as individuals across their lives.  A migrating group in search of accommodations doesn't just shed one or two pairs at each promising weed patch; the entire group stops when it finds a tract suitable to the tribal need. When nesting is finished, they decamp for Venezuela en masse as abruptly as they arrived.  At Wheatlands Farm they departed on August 5th this year.

In 2012 this farm gave up a cutting of hay on 45 acres of alfalfa and orchard grass to accommodate the Dickcissels’ relaxed schedule.  By the coincidence of a mild previous winter there was enough hay left over for the coming winter, assuming average weather, meaning no back-to-back three-foot snows. Such hospitality would normally be beyond the means of many farms, this one included, even with all hearts in the right place and all awarenesses attuned. Few farms can offer the largesse Dickcissels require of the eastern yeoman -- on the order of ten large round bales of hay per nest. By that crude estimate, a farm would sacrifice 250 cow days of winter feed in hosting one Dickcissel nest. But -- you don't get one nest; you get some nests or none.

The Dickcissels at Buffalo Gap outline a conservation challenge. The bird is persecuted with surfactants and other chemicals on its wintering grounds in Venezuela where grain farmers consider it a pest, as we once viewed the Carolina Parakeet. The Dickcissel is under pressure from drought and fire on its traditional North American grassland breeding grounds. So, we surmise, it has turned eastward for a chance to recruit and to sustain viable numbers. Here it runs into an adverse farming schedule in its principal nesting habitat. We must think of a way to accommodate all parties.

Buffalo Gap's Dickcissels, 2012

If you have Dickcissels in your pastures or if you pass their field with your window down there will be little doubt of it, for the males take their bright colors and loud voices to their advertisement perches and sing and counter-sing, hardly taking a breath break.   They arrive as a group and seize the grassland acoustics. Breeding males are loud and obvious and they generate a territorial din. This year the Dickcissels arrived at Wheatlands Farm at Buffalo Gap in Virginia’s Shenandoah Valley on June 1  and for the next 66 days they dominated the rap.

In our last post we noted the connection Kenn Kaufman sees between recent eastern breeding appearances of the Dickcissel and the stresses from drought and fire the bird faces in its traditional prairie nesting range.  In 2011 drought choked the southern prairies, engulfing Texas plus a 200-mile margin, and arguably accounted for the Dickcissel irruption east of the Allegheny Mountains as the birds sought alternate breeding opportunities. This year the drought expanded northward, toasting crops and pastures across Nebraska, Iowa, and  Wisconsin and compromising grassland nesting over an enlarged dry zone.  And in 2012 the Dickcissel chorus at Buffalo Gap grew by many voices; upward of a dozen males sang on this farm of 160 acres of grassland, more than double the 2011 numbers. That level of Dickcissel breeding density is characteristic of the grassland sweet spots in Missouri and Kansas but is uncommon in the East. (Check Lang Elliot's comments and recordings at www.birdtunes.com)

The U.S. Drought Monitor, maintained at the University of Nebraska in association with NOAA and USDA, observes regarding the 2012 episode,

 " The drought has inflicted, and is expected to continue to inflict, catastrophic economic ramifications for the affected states." 

So, the drought sequence will likely continue, seemingly a component of global climate change.  If 2013 is comparably dry in the prairie states and if the  Dickcissels again seek nesting relief  in the East, there will be little doubt of the Kaufman correlation. Would a record search show that eastward Dickcissel irruptions in earlier years were also the result of western weather stresses? At this point we are on notice of a likelihood that the Dickcissel will be needing welcome in such grasslands as remain in the East (after the massive twentieth-century abandonment of eastern farmland). In seeking breeding refuge to the east of its principal historic range, is the Dickcissel an early indicator species of climate change?

The Dickcissel has problems in the east which it did not have in its pre-Colombian prairie range where the main perils to nesting were bison hooves and fires; worse, of course, in dry years. Arriving in Virginia's Shenandoah Valley on June 1st of this year, the Dickcissels set up shop a month or more behind the Bobolinks and Meadowlarks. Eastern farmers cut hay in late May and early June.  Hay made in early June on this farm this year would have contained Dickcissel nests; some other nests as well, but many of the other grassland obligates have brought off a brood by then and could better afford the loss than could the Dickcissels.

Most of the eastern grasslands available for breeding are those which farmers maintain for pasture or hay and the Dickcissels arrive at the wrong moment in the farming schedule. If they choose hay ground the nests go through the cutter. If they select lightly-grazed pasture, that is likely to be clipped for weed control in early July. A Dickcissel's best hope is to find a spot inaccessible to cutting equipment -- a wet or rocky place -- large enough to accommodate a territory.

That is another problem.  Grassland birds including Dickcissels and Bobolinks are, in the terms of researchers, "area sensitive"; they need tracts of suitable habitat large enough to accommodate not just one territory, but several, perhaps many.  You don't often find one pair of Dickcissels or Bobolinks (like many grassland birds, both are polygamous and sometimes polyandrous so the term "pair" applies loosely), you find a nesting cohort which requires a large tract of grassland; research suggests a minimum of 60 acres for Bobolinks. Dickcissels migrate together, claim adjacent territories, winter together, and perhaps know one another as individuals across their lives.  A migrating group in search of accommodations doesn't just shed one or two pairs at each promising weed patch; the entire group stops when it finds a tract suitable to the tribal need. When nesting is finished, they decamp for Venezuela en masse as abruptly as they arrived.  At Wheatlands Farm they departed on August 5th this year.

In 2012 this farm gave up a cutting of hay on 45 acres of alfalfa and orchard grass to accommodate the Dickcissels’ relaxed schedule.  By the coincidence of a mild previous winter there was enough hay left over for the coming winter, assuming average weather, meaning no back-to-back three-foot snows. Such hospitality would normally be beyond the means of many farms, this one included, even with all hearts in the right place and all awarenesses attuned. Few farms can offer the largesse Dickcissels require of the eastern yeoman -- on the order of ten large round bales of hay per nest. By that crude estimate, a farm would sacrifice 250 cow days of winter feed in hosting one Dickcissel nest. But -- you don't get one nest; you get some nests or none.

The Dickcissels at Buffalo Gap outline a conservation challenge. The bird is persecuted with surfactants and other chemicals on its wintering grounds in Venezuela where grain farmers consider it a pest, as we once viewed the Carolina Parakeet. The Dickcissel is under pressure from drought and fire on its traditional North American grassland breeding grounds. So, we surmise, it has turned eastward for a chance to recruit and to sustain viable numbers. Here it runs into an adverse farming schedule in its principle nesting habitat. We must think of a way to accommodate all parties.
 

Dispatches from Buffalo Gap