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2009 SOUTHEASTERN NATURALIST 8(1):83–92
Recovery of Nesting Bald Eagles in Texas
Sarah T. Saalfeld1,*, Warren C. Conway1, Ricky Maxey2, Chris Gregory3,
and Brent Ortego4
Abstract - One of the most successful conservation stories in United States’ history
resulted in Haliaeetus leucocephalus (Bald Eagle) being removed recently from the
federal endangered species list. Few studies, however, have documented regional
long-term recovery trends for Bald Eagles. We quantified Bald Eagle nesting density,
distribution, and productivity trends by using aerial surveys of nests located in
69 counties in eastern Texas from 1971–2005. The total number of occupied nests,
productive nests, and offspring produced increased exponentially during that time,
with the most dramatic increases occurring from 1995–2005. Since 1971, the total
number of occupied nests increased 13% per year, from 5 in 1971 to 157 in 2005,
and the total number of young produced also increased 13% per year, from 6 in 1971
to 195 in 2005. Apparent nest success estimates (50–100%) and mean brood size
(1–2 young/nest), however, remained relatively consistent from 1971 to 2005. By
1989, Bald Eagles in Texas exceeded recovery goals set by the Southeastern States
Bald Eagle Recovery Team (i.e., >0.9 young produced/occupied nest, >1.5 young
produced/successful nest, and >50% of nests successful in raising at least 1 young).
Continued regional increases in Bald Eagle nesting activity may warrant specific
attention, particularly as related to anthropogenic pressures and interactions in increasingly
fragmented and urbanized habitats.
Haliaeetus leucocephalus Linnaeus (Bald Eagle) populations have increased
dramatically after being placed on the endangered species list in
1973 (Buehler 2000). Populations have increased and expanded throughout
the United States, with nesting attempts documented in all of the lower 48
states (US Fish and Wildlife Service 2007). In Texas, nesting Bald Eagles
have increased since the early 1970s, when only 8 nests were documented
from the lower reaches of rivers within coastal areas (Ortego 2005). Bald
Eagles in Texas have recently reoccupied portions of their assumed historical
range, now nesting in high densities on tall trees near rivers within the
coastal region and near inland reservoirs (Ortego et al. 2006) where nesting
had not been previously documented (Campbell 1995).
Although the federal government removed the Bald Eagle from the endangered
species list (US Fish and Wildlife Service 2007), it remains on the
threatened list of Texas, and still receives federal protection from the 1940
Bald Eagle and Golden Eagle Protection Act and the Migratory Bird Treaty
Act of 1918. Therefore, continued monitoring of long-term changes in density,
1Arthur Temple College of Forestry and Agriculture, Stephen F. Austin State University,
Nacogdoches, TX 75962. 2Texas Parks and Wildlife Department, Karnack, TX
75661. 3Texas Parks and Wildlife Department, Livingston, TX 77351. 4Texas Parks
and Wildlife Department, Victoria, TX 77901. *Corresponding author - saalfeldst@
84 Southeastern Naturalist Vol. 8, No. 1
distribution, and productivity is important to evaluate current status as related
to recovery plans. According to the Southeastern States Bald Eagle Recovery
Team (US Fish and Wildlife Service 1989), along with documentation of
population vigor and supporting habitats, recovery within southeastern states,
including Texas, was defined to occur when the most recent 3 years of data
document >600 occupied breeding areas distributed in >75% of the historical
range within southeastern United States, ≥0.9 young produced per occupied
nest, ≥1.5 young produced per successful nest, and ≥50% of nests successfully
fl edged ≥1 young. By the late 1980s and early 1990s, it appeared that Bald Eagles
attained recovery in eastern Texas, but no comprehensive nor quantitative
formal documentation of this recovery exists. Therefore, our objectives were
to quantify and examine recovery trends of nesting Bald Eagles throughout
Texas and to determine how current nesting activity and productivity correspond
to regional recovery goals.
Personnel of Texas Parks and Wildlife Department collected aerial survey
data for Bald Eagle nests throughout eastern Texas from 1971–2005 (Ortego
et al. 2006). We conducted surveys annually from February–April to monitor
known nest locations and to locate new nests. Additionally, we located
nests reported from private individuals to verify existence and incorporate
into future surveys. We classified nests as occupied/unoccupied by presence/
absence of adult eagles near nests and/or presence of eggs and/or young in
nest. We revisited occupied nests observed prior to hatching to determine
productivity (i.e., number of eaglets observed within each occupied nest) in
April and May. We also aerially surveyed territories in which an active nest
was not located during the February–April survey to locate missing nests in
April and May. We defined a productive nest as one in which at least 1 eaglet
was observed during either survey. We determined nesting success using the
ratio of productive nests to occupied nests, where productivity was defined
as the mean number of young produced per occupied nest, and mean brood
size was defined as the mean number of young produced per productive nest.
Calculations of nesting success, productivity, and mean brood size included
only nests in which productivity was determined.
We used simple linear regression (PROC REG; SAS Institute 1999)
to examine relationship(s) between year and total number of (1) occupied
nests, (2) productive nests, and (3) young produced, as well as (4) nest success,
(5) productivity, and (6) mean brood size. Once we determined that
the total number of (1) occupied nests, (2) productive nests, and (3) young
produced was exponentially related to year (i.e., highest R2 value), we transformed
these metrics using a natural logarithmic (ln) transformation prior
to performing linear regression. However, all untransformed values are
presented. We also used correlation analysis (PROC CORR; SAS Institute
1999) to examine the relationship between total number of young produced
and productive nests observed each year, as well as between total number
of occupied nests, productive nests and young produced. Finally, to quantify
2009 S.T. Saalfeld, W.C. Conway, R. Maxey, C. Gregory, and B. Ortego 85
change in rates of increase in total number of (1) occupied nests, (2) productive
nests, and (3) young produced per year, we used simple linear regression
(PROC REG; SAS Institute 1999) to fit a linear model to untransformed data
during 3 predefined time periods: 1971–1982 (early), 1983–1994 (middle),
and 1995–2005 (late). We then used analysis of variance (ANOVA; SAS Institute
1999) to compare regression slope coefficients between early, middle,
and late periods for each nesting parameter.
Some biases could occur when using aerial survey data (Nesbitt et al.
1998). Specifically, underestimation of occupancy and production may occur
if nests could not be relocated or productivity determined, either within
or among years. To estimate this potential bias, we determined percentage
of (1) known nests not relocated during aerial surveys in subsequent years
and (2) known occupied nests in which productivity could not be determined
within the same year. Although difficult to quantify, underestimates of nesting
activity and productivity may also occur if eagles abandoned nests or
nestlings left nests prior to observation(s). However, these estimates are assumed
to be consistent between and among years. Additionally, increases in
nesting activity observed could be a result of increased knowledge of known
territories among years and increased reporting of nests from the public.
From 1971–2005, we made 2211 observations of 482 Bald Eagle nests
located in 248 individual breeding territories. Bald Eagle nests were found in
69 counties within the Coastal Prairies, Edwards Plateau, Oaks and Prairies,
Osage Plains, Pecos and Staked Plains, Rolling Red Plains, and West Gulf
Coastal Plain physiographic regions (modified from Texas Parks and Wildlife
Department ) of Texas (Fig. 1). Bald Eagle nesting activity (i.e.,
total number of occupied nests: R2 = 0.996, productive nests: R2 = 0.993,
and total number of young produced: R2 = 0.990) increased exponentially
from 1971–2005 (Appendix 1, Fig. 2). Year was positively associated with
and accounted for significant variation in the total number of occupied nests,
productive nests, and young produced (Table 1). However, year was not
associated with nest success, productivity, nor mean brood size (Table 1),
indicating that individual nest metrics have remained fairly constant in time.
As expected, the total number of young produced each year from all Bald
Eagle nests throughout Texas was positively correlated with the total number
of productive nests observed each year (R = 0.998, P < 0.001). Additionally,
the total number of occupied nests throughout Texas was positively correlated
with total number of productive nests (R = 0.997, P < 0.001) and total
number of young produced (R = 0.994, P < 0.001) each year.
Specifically, the total number of occupied nests (annual rate = 0.127
± 0.004), productive nests (annual rate = 0.127 ± 0.004), and young produced
(annual rate = 0.130 ± 0.005) increased each year from 1971–2005
(Table 1 and Appendix 1). For all metrics tested, regression slopes differed
(P < 0.001) among time periods (early: 1971–1982, middle: 1983–1994, and
late: 1995–2005). Specifically, regression slopes were greater (P < 0.001) for
86 Southeastern Naturalist Vol. 8, No. 1
Table 1. Simple linear regression models comparing year to measures of Bald Eagle nesting
activity and productivity collected from aerial surveys of nests in Texas, 1971–2005.
Dependent variable R2 P-value Estimate SE P-value EstimateA SE P-value
Ln (occupied nests) 0.96 <0.01 -250.06 8.59 <0.01 0.13 0.00 <0.01
Ln (productive nests) 0.96 <0.01 -250.26 8.49 <0.01 0.13 0.00 <0.01
Ln (young produced) 0.96 <0.01 -255.14 9.16 <0.01 0.13 0.01 <0.01
Nesting successB 0.01 0.55 3.66 4.71 0.44 0.00 0.00 0.55
ProductivityC <0.01 0.95 0.61 9.76 0.95 0.00 0.01 0.95
Mean brood sizeD 0.02 0.44 -3.76 6.86 0.59 0.00 0.00 0.44
AAnnual percent increase in dependent variable.
BNesting success was defined as the ratio of productive nests to occupied nests.
CProductivity was defined as the mean number of young produced per occupied nest.
DMean brood size was defined as the mean number of young produced per productive nest.
Figure 1. Location and number of Bald Eagle nesting territories per Texas physiographic
region (modified from Texas Parks and Wildlife ) located from aerial
surveys of nests, 1971–2005. Shaded areas correspond to counties in which ≥1 nest
2009 S.T. Saalfeld, W.C. Conway, R. Maxey, C. Gregory, and B. Ortego 87
the late period than the early and middle periods and for the middle period
when compared to the early period (total number of occupied nests: early
rate = 0.269, middle rate = 3.210, late rate = 11.809, F2,29 = 223.99; productive
nests: early rate = 0.224, middle rate = 2.587, late rate = 9.064, F2,29 =
194.59; and young produced: early rate = 0.413, middle rate = 4.066, late
rate = 15.945, F2,29 = 111.05; Fig. 2), indicating that rates of these parameters
have increased more dramatically during the last 11 years.
The percentage of known nests not relocated during aerial surveys in subsequent
years for all years combined was 4.4%. Similarly, the percentage of
known occupied nests in which productivity could not be determined within
the same year for all years combined was 9.5%.
Figure 2. Total number of occupied nests (A), productive nests (B), and young produced
(C) for Bald Eagles located from aerial surveys of nests in Texas, 1971–2005,
as well as fitted exponential models and rates of increase from 1971–1982, 1983–
1994, and 1995–2005.
88 Southeastern Naturalist Vol. 8, No. 1
Nesting activity (i.e., number of occupied and productive nests and young
produced) of Bald Eagles in Texas increased exponentially between 1971 and
2005 with the most dramatic increases occurring within the last 11 years (i.e.,
1995–2005; see Fig. 2). Individual productivity rates (i.e., nesting success,
productivity, and mean brood size) remained consistent throughout this study.
One of the main causes attributed to Bald Eagle declines in the mid-1900s was
pesticide use, mainly DDT, causing egg shell thinning and ultimately decreasing
productivity (Buehler 2000). Since the ban on use of DDT in 1972, recovery
of Bald Eagle hatching rates and productivity has occurred throughout most of
North America (Buehler 2000, Campbell 1995, Grier 1982), although at differing
rates regionally (see Buehler 2000, Dykstra et al. 2005). In Texas, Bald
Eagles may (1) have recovered from the effects of DDT prior to the time this
study was initiated (i.e., 1971) or (2) not have been as severely impacted by
DDT as in other regions as indicated by high productivity rates observed in this
study (i.e., range 1.20–1.45 young/occupied nest from 1971–2005; mean for
all years = 1.27 young/occupied nest). As suggested by Buehler et al. (1991),
productivity rates >0.74 young/occupied nest are indicative of an expanding
population. Therefore, we hypothesize that the main cause for Bald Eagle
recovery post-DDT in Texas can be attributed to increased population growth
from consistently, high productivity rather than increased individual nesting
success and/or productivity. Moreover, high fl edgling survival rates and
recruitment, strong fidelity to natal nesting areas for breeding, and creation of
suitable nesting habitat (i.e., man-made inland reservoirs) may have also facilitated
increased population growth and post-DDT recovery of Bald Eagles in
Texas. In a previous study on Bald Eagles in Texas conducted from 1985–1991,
Mabie et al. (1994) estimated a fl edgling survival rate of 97%, a recruitment
rate of 33.3%, and a fidelity rate to natal nesting areas for breeding of 20%, all
of which may positively affect population growth. In addition, creation of additional
suitable nesting habitat in the form of inland reservoirs (see below)
also occurred during the same time period (i.e., 1960s–1970s) as the ban on use
of DDT. By increasing suitable nesting habitat, the Bald Eagle population of
Texas was able to expand and grow without habitat limitations.
Although not directly comparable because of regional area dissimilarities
(Coastal Prairies = 4,658,340 ha; Oaks and Prairies = 10,738,000 ha; West Gulf
Coastal Plain = 6,481,950 ha), we found differing recovery trends throughout
the 3 physiographic regions where we located the majority of eagle nests (i.e.,
Coastal Prairies, Oaks and Prairies, West Gulf Coastal Plain; see Fig. 1). Within
the West Gulf Coastal Plain region, we found a greater increase in the number
of occupied Bald Eagle nests (1 to 75 from 1986–2004) than the other 2 physiographic
regions (Coastal Prairies = 5 to 40 from 1971–2005, Oaks and Prairies =
1 to 40 from 1980–2005; Fig. 3), which ultimately resulted in more Bald Eagle
nests within the West Gulf Coastal Plain region by 2005, despite a longer history
of nesting within the Coastal Prairies and Oaks and Prairies regions. One
possible explanation may be that these differences are caused by habitat availability
for nesting Bald Eagles within each physiographic region. Bald Eagles
2009 S.T. Saalfeld, W.C. Conway, R. Maxey, C. Gregory, and B. Ortego 89
within all 3 physiographic regions nest within suitable riparian habitat adjacent
to rivers and creeks; however, man-made inland reservoirs host the majority of
nesting eagles within the Oaks and Prairies and West Gulf Coastal Plain regions
(B. Ortego, unpubl. data). Essentially, these reservoirs, along with the maturation
of forested areas surrounding them, create additional nesting habitat within
these regions where nesting has not been previously documented (Campbell
1995). Additionally, the West Gulf Coastal Plain region is the only region with
large tracts of public land (i.e., National Forest Lands) managed for nesting
Bald Eagles. Essentially, these areas provide the structural habitat necessary
for nest placement near quality foraging grounds and protection from human
disturbance. As Bald Eagle populations increased from 1990–2005, nesting
expanded throughout the Coastal Prairies region and into the Oaks and Prairies
and West Gulf Coastal Plain regions where suitable habitat was found in forested
areas near rivers, creeks, and man-made inland reservoirs.
Figure 3. Total number of occupied nests (A), productive nests (B), and young produced
(C) for Bald Eagles for 3 physiographic regions (Coastal Prairies, Oaks and Prairies,
and West Gulf Coastal Plain) located from aerial surveys of nests in Texas, 1971–2005.
90 Southeastern Naturalist Vol. 8, No. 1
Nesting Bald Eagles in Texas have exceeded productivity goals (i.e., >0.9
young/occupied nest, >1.5 young/productive nest, and >50% of nests successful
in raising ≥1 young) set by the Southeastern States Bald Eagle Recovery
Team (US Fish and Wildlife Service 1989) since 1989 (Appendix 1).
Similar results were also observed in Florida (Nesbitt et al. 1998) and Oklahoma
(Jenkins and Sherrod 2005), where increasing nesting numbers of Bald
Eagles have occurred since the mid-1980s and early 1990s. In Oklahoma, it
was estimated that productivity has reached and/or exceeded recovery goals
set by the Northern States Bald Eagle Recovery Plan (>1 young fl edged per
productive nest) since 1991 (Jenkins and Sherrod 2005).
Despite their rapid population increase in eastern Texas, specific threats to
current populations still exist. Although additional suitable nesting habitat has
been created in the form of inland reservoirs and maturation of forested lands
surrounding a few of these reservoirs, disease, habitat loss, disturbance, and
human tolerance are still major concerns for nesting populations (see Buehler
2000, US Fish and Wildlife Service 1989). Currently, nesting Bald Eagles in
Texas appear to have an adequate amount of habitat for nesting as indicated
by the exponential growth in the total number of occupied nests and consistent
productivity. However, if sufficient nesting locations become limiting, the
number of nesting pairs and young produced may plateau at some point in the
near future. Two major habitat requirements necessary for nesting by Bald Eagles
are suitable nest trees near productive foraging areas (i.e., large bodies of
water and/or permanent fl owing creeks and rivers; Garrett et al. 1993, Livingston
et al. 1990, Murphy 1965) and freedom from human disturbance (Andrew
and Mosher 1982, Garrett et al. 1993, Murphy 1965, Thompson and McGarigal
2002). However, because of increased recreation and development pressure,
these habitat characteristics are becoming increasingly rare not only in Texas,
but throughout the lower 48 states (McGarigal et al. 1991). To continue protecting
this vulnerable species and allow nesting populations to increase further,
specific measures should be taken to identify, conserve, and limit human disturbance
around potential nesting habitat throughout Texas. Some potential
nesting habitat is likely to be located on private land; therefore, it will also be
important to continue public education about the needs of eagles and the importance
of conserving nesting habitat regardless of ownership.
We thank all of the individuals who assisted with data collection from Texas
Parks and Wildlife Department and US Forest Service during the time this research
was conducted. We also thank John Steele for initial efforts on this research, as well
as Tawnya Brummett, Ron Mize, Patricia Johnson, Todd Johnson, and Bill Bartush
from the US Forest Service, Jeffrey Reid from the US Fish Wildlife Service, and
technicians Thad Choate and Kellon Harris, who assisted with this project. We also
thank the landowners who contributed information on nesting eagles and who continue
to protect and manage their land for nesting eagles. Fred LeBlanc of The Woodlands
Operating Corporation, the Arthur Temple College of Forestry and Agriculture
at Stephen F. Austin State University, the Texas Parks and Wildlife Department, the
US Forest Service, and the US Fish and Wildlife Service provided financial and logistical
support for this research.
2009 S.T. Saalfeld, W.C. Conway, R. Maxey, C. Gregory, and B. Ortego 91
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92 Southeastern Naturalist Vol. 8, No. 1
Appendix 1. Year, nesting activity, and productivity of Bald Eagles collected from aerial surveys
of nests in Texas, 1971–2005.
Occupied Productive Young Nesting brood
Year nests nests produced successA ProductivityB sizeC
1971 5 3 6 0.60 1.20 2.00
1972 3 3 3 1.00 1.00 1.00
1973 D 4 2 3 0.67 1.00 1.50
1974 3 3 2 0.67 0.67 1.00
1975 3 2 5 1.00 1.67 1.67
1976 2 3 3 1.00 1.50 1.50
1977 D 4 2 5 1.00 1.67 1.67
1978 D 4 3 5 1.00 1.67 1.67
1979 D 4 3 5 1.00 1.67 1.67
1980 D 5 3 6 1.00 2.00 2.00
1981 6 5 7 0.83 1.17 1.40
1982 D 8 6 10 0.86 1.43 1.67
Mean, 1971–1982 4.25 3.17 5.00 0.89 1.39 1.56
1983 8 4 7 0.50 0.88 1.75
1984 11 9 15 0.82 1.36 1.67
1985 15 8 10 0.53 0.67 1.25
1986 17 15 24 0.88 1.41 1.60
1987 D 19 12 22 0.67 1.22 1.83
1988 D 21 15 22 0.75 1.10 1.47
1989 D 26 19 29 0.76 1.16 1.53
1990 27 19 29 0.70 1.07 1.53
1991 36 24 38 0.67 1.06 1.58
1992 D 39 27 41 0.71 1.08 1.52
1993 D 39 32 53 0.84 1.39 1.66
1994 D 42 34 53 0.85 1.33 1.56
Mean, 1983–1994 25.00 18.17 28.58 0.72 1.14 1.58
1995 42 30 50 0.71 1.19 1.67
1996 49 33 51 0.67 1.04 1.55
1997 D 55 36 55 0.75 1.15 1.53
1998 D 64 47 71 0.81 1.22 1.51
1999 D 66 48 73 0.75 1.14 1.52
2000 D 81 63 99 0.82 1.29 1.57
2001 D 104 70 107 0.72 1.10 1.53
2002D 115 92 151 0.89 1.47 1.64
2003 D 123 93 148 0.96 1.53 1.59
2004 D 144 109 190 0.89 1.56 1.74
2005 D 157 112 195 0.82 1.45 1.74
Mean, 1995–2005 90.91 66.64 108.18 0.80 1.28 1.60
Mean, all years 38.60 28.26 45.51 0.80 1.27 1.58
ANesting success was defined as the ratio of productive nests to occupied nests.
BProductivity was defined as the mean number of young produced per occupied nest.
CMean brood size was defined as the mean number of young produced per productive nest.
DNot all productivity outcomes were determined for this year. Nesting success, productivity,
and mean brood size include only nests with known productivity.