Accurate estimates of blackbird populations are needed to develop and assess management programs to reduce blackbird damage to sunflower. Our objective was to use North American Breeding Bird Survey (BBS) indices to estimate the total blackbird population causing sunflower damage in North Dakota, South Dakota, and Minnesota

Homan, H. Jeffrey

Linz, George M.

Penry, Linda B.

English

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln USDA National Wildlife Research Center - Staff Publications U.S. Department of Agriculture: Animal and Plant Health Inspection Service February 2004 Linear Modeling of Blackbird Populations Breeding in Central North America H. Jeffrey Homan USDA/APHIS/WS National Wildlife Research Center Linda B. Penry USDA/APHIS/WS National Wildlife Research Center George M. Linz USDA/APHIS/WS National Wildlife Research Center, george.m.linz@aphis.usda.gov Follow this and additional works at: https://digitalcommons.unl.edu/icwdm_usdanwrc  Part of the Environmental Sciences Commons Homan, H. Jeffrey; Penry, Linda B.; and Linz, George M., "Linear Modeling of Blackbird Populations Breeding in Central North America" (2004). USDA National Wildlife Research Center - Staff Publications. 342. https://digitalcommons.unl.edu/icwdm_usdanwrc/342 This Article is brought to you for free and open access by the U.S. Department of Agriculture: Animal and Plant Health Inspection Service at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USDA National Wildlife Research Center - Staff Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Published in National Sunflower Association Research Forum Papers 2004.  The annual NSA Research Forum is highly regarded as the premier conference at which private and public researchers present the results of their latest sunflower research. For nearly a quarter of a century, the National Sunflower Association has produced the workshop as a forum for sunflower research to be shared with the public.  Papers from NSA Research Forums 2003 and later are online at: http://www.sunflowernsa.com/research/default.asp?contentID=70  NSA Contact Information National Sunflower Association  4023 State Street Bismarck, ND 58503-0690 (701) 328-5100  (888) 718-7033   Linear Modeling of Blackbird Populations Breeding in Central North America   H. Jeffrey Homan, Linda B. Penry, and George M. Linz  USDA, Wildlife Services, National Wildlife Research Center, Bismarck, ND   Introduction  Accurate estimates of blackbird populations are needed to develop and assess management programs to reduce blackbird damage to sunflower.  Our objective was to use North American Breeding Bird Survey (BBS) indices to estimate the total blackbird population causing sunflower damage in North Dakota, South Dakota, and Minnesota.   Photos by H. J. Homan               Species Information  Most of the damage is done by three species of blackbirds:  red-winged blackbirds, yellow-headed blackbirds, and common grackles.   Red-winged Blackbird Photo by Peter S. Weber  The most abundant passerine; widely distributed throughout North America.  A polygynous species that prefers to nest in shallow wetlands or dense upland vegetation.  Average territory size is 0.5 acre, but size varies greatly depending on habitat quality.  Primarily an insectivore during the reproductive season; a granivore during other phases of its life cycle.  Feeds on several types of agricultural crops, including sunflower, corn, and rice.  Considered a major pest by agricultural producers.  Smallest of the three major blackbird species, averaging between 2-3 oz; length 8.75 in.    Yellow-headeded Blackbird  Locally common throughout most of its range.  More limited in distribution than the red-winged blackbird.  Most abundant in the central region of North America where it breeds strictly in wetlands.  Polygynous.  Territories are usually much smaller than those of the red-winged blackbird.  Similar in feeding habits to the red-winged blackbird; less likely to rely on agricultural crops, foraging more often on weed seeds.  Length 9.5 in.  Male 3-4 oz and nearly twice as large as the female.   Photo by Craig Larcom 1 Photo by Lang Elliott Common Grackle  Abundant east of the Rockies.  Less likely to use wetlands as nesting sites than the other two species.  Frequently found nesting in groves of trees or shrubs, often forming loose aggregations in towns and near farmsteads.  Monogamous.  Joins with red-winged blackbirds and yellow-headed blackbirds in large mixed-species flocks during late summer.  At times, feeding behavior can make this species a serious agricultural pest, particularly in corn and sunflower.  Largest of the three blackbird species.  Males usually greater than 4 oz; females 3.5 oz.  Length 12.5 in.     Study Area  In the map below, the polygon encompasses an area that produces blackbirds responsible for most of the sunflower damage.  Seven different physiographic regions are found within this polygon.  Five of the seven regions have only BBS indices as measures of blackbird abundance.  Indices have been done annually since 1965 in most of the regions.    MontanaMinnesotaNorth DakotaSouth DakotaManitobaAlbertaSaskatchewan0 100 200 300 40050MilesPhysiographic RegionsGreat Lakes TransitionSpruce-Hardwoods ForestClosed Boreal ForestAspen ParklandsDissected Till PlainsDrift PrairieMissouri Plateau - GlaciatedMissouri Plateau - UnglaciatedBlack PrairiePolygon    Methods  Indices must be associated with some measure of density to predict population size.  Indices correlate nearly linearly with population density when conducted on conspicuous species in fairly open landscapes.  The landscapes and bird species met these requirements in our study.   We used density estimates of territorial male blackbirds from a random census of quarter sections conducted from 1996-1998 in the Drift Prairie and Glaciated Missouri Plateau of North Dakota.  To adjust for females, nonbreeding males, and males missed during the census, we multiplied the density of  2 territorial males per quarter section by 4.08.  The 4.08-value was derived from models of demographic age structure.  To obtain the total population size by physiographic region, we multiplied average density per quarter section by the area of the physiographic region within the polygon.  We used a Geographic Information System to estimate areas of the physiographic regions.  To establish a proportional relationship between the two measures of abundance, the density estimates from 1996-1998 were linked to BBS indices over the same period in the Drift Prairie and Glaciated Missouri Plateau.  Using this relationship, we calculated densities for the remaining five physiographic regions in which BBS indices were the only population measurements available.     Results & Discussion  The polygon covered 180,000 mi2.  Canada accounted for the largest amount of land area, occupying 47% of the total area within the polygon (Table 1).  Drift Prairie and Aspen Parkland were the dominate physiographic regions within the polygon.       Table 1.  Percentage of area for seven physiographic regions found within the polygon.   Physiographic Region1 North  Dakota South  Dakota Minnesota Canada Total GLT   0   0   1   0     1 NSH   0   0   1   1     2 CBF   0   0   0   4     4 AP   0   0   3 33   36 DP 16 10   0   7   33 GMP   8   1   0   0     9 BP   4   1   8   2   15 Total 28 12 13 47 100  1GLT - Great Lakes Transition, NSH - Northern Spruce Hardwoods, CBF - Closed Boreal Forest, AP - Aspen Parklands, DP - Drift Prairie, GMP - Glaciated Missouri Plateau, BP - Black Prairie     For every seven birds tallied on BBS routes, approximately one territorial male/0.4 mi2 was counted in the quarter section surveys.  This 7:1 ratio was similar to that found in other studies comparing BBS route indices to blackbird densities in open landscapes.    Nearly 40% of the total blackbird population produced within the polygon originated from North Dakota (Table 2).  North Dakota and Canada produced 70% of the red-winged blackbird population, which comprised the largest population of the three species.  Red-winged blackbirds outnumbered yellow-headed blackbirds and common grackles by greater than 2:1.  3  Table 2.  Estimates of blackbird populations1 in late-summer originating from the polygon.    Numbers (million) of Blackbirds by Species State/Country Red-winged blackbird Yellow-headed blackbird Common grackle Total North Dakota 15   8   8 31 South Dakota   6   5   3 14 Minnesota   6   1   5 12 Canada 13   4   3 20 Total 40 18 19 77  1The spring population was multiplied by 1.45 to allow for recruitment of young into the late-summer population.    More blackbirds are found in central North America than anywhere else on the Continent.  Given this fact, it is not surprising that so many blackbirds were within the relatively confined area of the polygon.  The total population of red-winged blackbirds, yellow-headed blackbirds, and common grackles in North America in late summer is probably near 500 million.  Thus, the polygon held approximately 15% of the total population.  The numerous wetlands in this region serve to attract blackbirds in such numbers.    There are several drawbacks to using indices to estimate densities of birds.  For example, detectability of a species may change as the landscape changes; however, we believe that the relative homogeneity of the landscapes in the polygon combined with the abundance and conspicuousness of the three species during the breeding season justifies use of this method.      4 

Linear Modeling of Blackbird Populations Breeding in Central North America

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University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
USDA National Wildlife Research Center - Staff 
Publications 
U.S. Department of Agriculture: Animal and 
Plant Health Inspection Service 
February 2004 
Linear Modeling of Blackbird Populations Breeding in Central 
North America 
H. Jeffrey Homan 
USDA/APHIS/WS National Wildlife Research Center 
Linda B. Penry 
USDA/APHIS/WS National Wildlife Research Center 
George M. Linz 
USDA/APHIS/WS National Wildlife Research Center, george.m.linz@aphis.usda.gov 
Follow this and additional works at: https://digitalcommons.unl.edu/icwdm_usdanwrc 
 Part of the Environmental Sciences Commons 
Homan, H. Jeffrey; Penry, Linda B.; and Linz, George M., "Linear Modeling of Blackbird Populations 
Breeding in Central North America" (2004). USDA National Wildlife Research Center - Staff Publications. 
342. 
https://digitalcommons.unl.edu/icwdm_usdanwrc/342 
This Article is brought to you for free and open access by the U.S. Department of Agriculture: Animal and Plant Health 
Inspection Service at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USDA 
National Wildlife Research Center - Staff Publications by an authorized administrator of DigitalCommons@University 
of Nebraska - Lincoln. 
Published in National Sunflower Association Research Forum Papers 2004. 
 
The annual NSA Research Forum is highly regarded as the premier conference at 
which private and public researchers present the results of their latest sunflower 
research. For nearly a quarter of a century, the National Sunflower Association 
has produced the workshop as a forum for sunflower research to be shared with 
the public. 
 
Papers from NSA Research Forums 2003 and later are online at: 
http://www.sunflowernsa.com/research/default.asp?contentID=70 
 
NSA Contact Information 
National Sunflower Association  
4023 State Street 
Bismarck, ND 58503-0690 
(701) 328-5100  
(888) 718-7033 
 
 
Linear Modeling of Blackbird Populations Breeding in Central North America 
 
 
H. Jeffrey Homan, Linda B. Penry, and George M. Linz 
 
USDA, Wildlife Services, National Wildlife Research Center, Bismarck, ND 
 
 
Introduction 
 
Accurate estimates of blackbird populations are needed to develop and assess management programs to 
reduce blackbird damage to sunflower.  Our objective was to use North American Breeding Bird Survey 
(BBS) indices to estimate the total blackbird population causing sunflower damage in North Dakota, 
South Dakota, and Minnesota. 
 
 
Photos by H. J. Homan 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Species Information 
 
Most of the damage is done by three species of blackbirds:  red-winged blackbirds, yellow-headed 
blackbirds, and common grackles. 
 
 
Red-winged Blackbird 
Photo by Peter S. Weber 
 
The most abundant passerine; widely distributed throughout North America.  A 
polygynous species that prefers to nest in shallow wetlands or dense upland 
vegetation.  Average territory size is 0.5 acre, but size varies greatly depending on 
habitat quality.  Primarily an insectivore during the reproductive season; a granivore 
during other phases of its life cycle.  Feeds on several types of agricultural crops, 
including sunflower, corn, and rice.  Considered a major pest by agricultural 
producers.  Smallest of the three major blackbird species, averaging between 2-3 
oz; length 8.75 in. 
 
 
 
Yellow-headeded Blackbird 
 
Locally common throughout most of its range.  More limited in distribution than the 
red-winged blackbird.  Most abundant in the central region of North America where it 
breeds strictly in wetlands.  Polygynous.  Territories are usually much smaller than 
those of the red-winged blackbird.  Similar in feeding habits to the red-winged 
blackbird; less likely to rely on agricultural crops, foraging more often on weed seeds.  
Length 9.5 in.  Male 3-4 oz and nearly twice as large as the female.   Photo by Craig Larcom 
1 
Photo by Lang Elliott 
Common Grackle 
 
Abundant east of the Rockies.  Less likely to use wetlands as nesting sites than the other two 
species.  Frequently found nesting in groves of trees or shrubs, often forming loose aggregations in 
towns and near farmsteads.  Monogamous.  Joins with red-winged blackbirds and yellow-headed 
blackbirds in large mixed-species flocks during late summer.  At times, feeding behavior can make 
this species a serious agricultural pest, particularly in corn and sunflower.  Largest of the three 
blackbird species.  Males usually greater than 4 oz; females 3.5 oz.  Length 12.5 in.  
 
 
 
Study Area 
 
In the map below, the polygon encompasses an area that produces blackbirds responsible for most of the 
sunflower damage.  Seven different physiographic regions are found within this polygon.  Five of the 
seven regions have only BBS indices as measures of blackbird abundance.  Indices have been done 
annually since 1965 in most of the regions.   
 
Montana
Minnesota
North Dakota
South Dakota
Manitoba
Alberta
Saskatchewan
0 100 200 300 40050
Miles
Physiographic Regions
Great Lakes Transition
Spruce-Hardwoods Forest
Closed Boreal Forest
Aspen Parklands
Dissected Till Plains
Drift Prairie
Missouri Plateau - Glaciated
Missouri Plateau - Unglaciated
Black Prairie
Polygon
 
 
 
 
Methods 
 
Indices must be associated with some measure of density to predict population size.  Indices correlate 
nearly linearly with population density when conducted on conspicuous species in fairly open landscapes.  
The landscapes and bird species met these requirements in our study.  
 
We used density estimates of territorial male blackbirds from a random census of quarter sections 
conducted from 1996-1998 in the Drift Prairie and Glaciated Missouri Plateau of North Dakota.  To adjust 
for females, nonbreeding males, and males missed during the census, we multiplied the density of 
 2 
territorial males per quarter section by 4.08.  The 4.08-value was derived from models of demographic 
age structure.  To obtain the total population size by physiographic region, we multiplied average density 
per quarter section by the area of the physiographic region within the polygon.  We used a Geographic 
Information System to estimate areas of the physiographic regions. 
 
To establish a proportional relationship between the two measures of abundance, the density estimates 
from 1996-1998 were linked to BBS indices over the same period in the Drift Prairie and Glaciated 
Missouri Plateau.  Using this relationship, we calculated densities for the remaining five physiographic 
regions in which BBS indices were the only population measurements available.   
 
 
Results & Discussion 
 
The polygon covered 180,000 mi2.  Canada accounted for the largest amount of land area, occupying 
47% of the total area within the polygon (Table 1).  Drift Prairie and Aspen Parkland were the dominate 
physiographic regions within the polygon.    
 
 
 Table 1.  Percentage of area for seven physiographic regions found within the polygon.  
 
Physiographic 
Region1 
North  
Dakota 
South  
Dakota Minnesota Canada Total 
GLT   0   0   1   0     1 
NSH   0   0   1   1     2 
CBF   0   0   0   4     4 
AP   0   0   3 33   36 
DP 16 10   0   7   33 
GMP   8   1   0   0     9 
BP   4   1   8   2   15 
Total 28 12 13 47 100 
 
1GLT - Great Lakes Transition, NSH - Northern Spruce Hardwoods, CBF - Closed Boreal Forest, AP - Aspen Parklands, 
DP - Drift Prairie, GMP - Glaciated Missouri Plateau, BP - Black Prairie 
 
  
 
For every seven birds tallied on BBS routes, approximately one territorial male/0.4 mi2 was counted in the 
quarter section surveys.  This 7:1 ratio was similar to that found in other studies comparing BBS route 
indices to blackbird densities in open landscapes.   
 
Nearly 40% of the total blackbird population produced within the polygon originated from North Dakota 
(Table 2).  North Dakota and Canada produced 70% of the red-winged blackbird population, which 
comprised the largest population of the three species.  Red-winged blackbirds outnumbered yellow-
headed blackbirds and common grackles by greater than 2:1. 
 3 
 Table 2.  Estimates of blackbird populations1 in late-summer originating from the polygon. 
 
 
 Numbers (million) of Blackbirds by Species 
State/Country Red-winged blackbird 
Yellow-headed 
blackbird 
Common 
grackle Total 
North Dakota 15   8   8 31 
South Dakota   6   5   3 14 
Minnesota   6   1   5 12 
Canada 13   4   3 20 
Total 40 18 19 77 
 1The spring population was multiplied by 1.45 to allow for recruitment of young into the late-summer population. 
 
 
 
More blackbirds are found in central North America than anywhere else on the Continent.  Given this fact, 
it is not surprising that so many blackbirds were within the relatively confined area of the polygon.  The 
total population of red-winged blackbirds, yellow-headed blackbirds, and common grackles in North 
America in late summer is probably near 500 million.  Thus, the polygon held approximately 15% of the 
total population.  The numerous wetlands in this region serve to attract blackbirds in such numbers.   
 
There are several drawbacks to using indices to estimate densities of birds.  For example, detectability of 
a species may change as the landscape changes; however, we believe that the relative homogeneity of 
the landscapes in the polygon combined with the abundance and conspicuousness of the three species 
during the breeding season justifies use of this method.   
 
 
 4 


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Linear Modeling of Blackbird Populations Breeding in Central North America

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