Statement of cooperation and/or descriptive note

Population Size and Trends for Nesting Ospreys in 

Northwestern Mexico: Region-wide Surveys, 1977, 

1992/1993 and 2006 

Charles J. Henny, Daniel W. Anderson, Aradit Castellanos Vera and Jean-Luc E. Cartron 

Open-File Report 2007–1298 

U.S. Department of the Interior 

U.S. Geological Survey 
ii Report Title

U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia: 2007 

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Suggested citation:


Henny, C.J., Anderson, D.W., Vera, A.C., and Carton, J-L. E., 2007, Population Size and Trends for Nesting Ospreys in 


Northwestern Mexico: Region-wide Surveys, 1977, 1992/1993 and 2006: Reston, Virginia, U.S. Geological Survey, 


Open-File Report 2007-1298, p. 15.


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U.S. Government. 

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reproduce any copyrighted materials contained within this report. 
Heading 1 iii 

Contents


Abstract ...........................................................................................................................................................1


Introduction.....................................................................................................................................................1


Methods...........................................................................................................................................................3


Results........................ .....................................................................................................................................5


Northwest Baja California, Lower California (L.C.) .........................................................................5


Southwest Baja California, L.C. ..........................................................................................................5


Northeast Baja California, L.C. ..........................................................................................................5


Southeast Baja California, L.C. ...........................................................................................................8


Midriff Islands........................................................................................................................................8


Coastal Sonora .....................................................................................................................................8


Coastal Sinaloa ...................................................................................................................................10


Nesting Sites........................................................................................................................................10


Discussion and Conclusions ......................................................................................................................11


Acknowledgments .......................................................................................................................................13


References ....................................................................................................................................................14


Figures 

Figure 1. The Baja California and Gulf of California study area for ospreys. .......................................2


Tables 

Table 1. Number of occupied osprey nests (nesting pairs) seen from the air and ground. .............4 

Table 2. Distribution and abundance of occupied osprey nests at the “time of the aerial survey” 

on the Pacific side of Baja California in 1977, 1992 and 2006. ...............................................6 

Table 3. Distribution and abundance of occupied osprey nests at the “time of the aerial survey” 

on the Gulf side of Baja California and Midriff Islands in 1977, 1992 and 2006. ................7 

Table 4. Distribution and abundance of occupied osprey nests at the “time of the aerial survey” 

in coastal Sonora and Sinaloa in 1977, 1993 and 2006. ...........................................................9 

Table 5. Types of nest sites selected by ospreys in the study area, 1977, 1992/1993 and 2006. .....10 

Table 6. A summary of estimated number of occupied osprey nests ± 95% C.I. in the study area, 

1977, 1992/1993, and 2006. .........................................................................................................11 
iv Report Title

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Population Size and Trends for Nesting Ospreys in 

Northwestern Mexico: Region-wide Surveys, 1977, 

1992/1993 and 2006 

Charles J. Henny1, Daniel W. Anderson2, Aradit Castellanos Vera3 and Jean-Luc E. Cartron4 

Abstract 

We used a double-sampling technique (air plus ground survey) in 2006, with partial double coverage, to estimate the present size of the osprey (Pandion haliaetus) nesting population in northwestern Mexico. With the exception of Natividad, Cedros, 

and San Benito Islands along the Pacific Coast of Baja California, all three excluded from our coverage in 2006 due to fog, this 

survey was a repeat of previous surveys conducted by us with the same protocol in 1977 and 1992/1993 (Baja California surveyed in 1992, Sonora and Sinaloa 1993), allowing for estimates of regional population trends. Population estimates at the “time 

of aerial survey” include those nesting, but missed from the air. The population estimate for our coverage area in 2006 was 1,343 

nesting pairs, or an 81% increase since 1977, but only a 3% increase since 1992/1993. The population on the Gulf side of Baja 

California generally remained stable during the three surveys (255, 236 and 252 pairs, respectively). The overall Midriff Islands 

population remained similar from 1992/1993 (308 pairs) to 2006 (289 pairs), but with notable population changes on the largest 

two islands (Isla Angel de la Guarda: 45 to 105 pairs [+ 60 pairs]; Isla Tiburon: 164 to 109 pairs [- 55 pairs, or -34%]). The estimated osprey population on the Sonora mainland decreased in a manner similar to adjacent Isla Tiburon, i.e., by 26%, from 214 

pairs in 1993 to 158 pairs in 2006. In contrast, the population in Sinaloa, which had increased by 150% between 1977 and 1993, 

grew again by 58% between 1993 and 2006, from 180 to 285 pairs. Our survey confirmed previously described patterns of rapid 

population changes at a local level, coupled with apparent shifts in spatial distribution. The large ground nesting population that 

until recently nested on two islands in San Ignacio Lagoon was no longer present on the islands in 2006, but an equivalent number of pairs were found to the north and south of the lagoon, nesting in small towns and along adjoining power-lines, with no 

overall change in population size for that general area (198 pairs in 1992; 199 in 2006). Use of artificial nesting structures was 

4.3% in 1977 and 6.2% in 1992/1993, but jumped to 26.4% in 2006. Use of power poles poses a risk of electrocution to ospreys 

as well as causes power outages and fires; modification of power poles to safely accommodate osprey nests has been successful 

in many countries. 

Introduction 

The Baja California and Gulf of California region harbors 

a large resident osprey (Pandion haliaetus) population (Henny 

and Anderson 1979, 2004, Cartron et al. in press). An early 

qualitative assessment of that population was provided by 

Grinnell (1928), but only for the Pacific and Gulf sides of Baja 

California and adjacent islands. For decades thereafter, the status and distribution of the osprey along the eastern side of the 

Gulf of California remained essentially unknown, with only a 

few reports of the species’ occurrence on offshore islands (see 

Cartron et al. 2006, in press). 

Based on aerial and ground surveys conducted in 1977, 

Henny and Anderson (1979) provided the first osprey population estimates for the entire Baja California and Gulf of 

California region and for each of seven subregions, including 

coastal Sonora and coastal Sinaloa along the eastern side of 

the Gulf of California (Figure 1). At the time, Henny and 

Anderson (1979) envisioned that the regional osprey population could be monitored on a long-term basis to serve as 

1

 U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, 3200 SW Jefferson Way, Corvallis, OR 97331 U.S.A.


2

 Department of Wildlife, Fish, and Conservation Biology, University of California, Davis, CA 95616 U.S.A.


3

 Centro de Investigaciones Biológicas del Noroeste, S.C. Apdo. Postal No. 128 CP 2300, La Paz, Baja California Sur, Mexico


4 Department of Biology, University of New Mexico, Albuquerque, NM 87131 U.S.A.

2 Report Title

Figure 1. The Baja California and Gulf of California study area for ospreys 
Heading 1 3 

an indicator of ecosystem health and track anthropogenic 

impacts. Henny and Anderson (2004) repeated their regionwide survey in 1992/1993 (survey split into two years) and 

documented an important increase in osprey numbers. Beginning in the 1980s, monitoring studies were also conducted 

on a local scale (e.g., Carmona et al. 1994, Castellanos and 

Ortega 1995, Cartron 2000, Cartron et al. 2006, RodríguezEstrella et al. 2006). Some of these local studies documented 

sharp fluctuations in the reproductive success of osprey pairs 

and rapid and pronounced changes in the number of nesting 

pairs, often with no clear underlying explanation. 

In 2006 we conducted a third survey of the osprey 

population in the Baja California and Gulf of California 

region. The main objective of this survey was to document 

any population trend region wide and for each of the seven 

subregions, using 1977 and 1992/1993 numbers for comparisons. Additional goals of our survey were to assess whether 

some of the pronounced patterns described at a local level 

could be detected on a larger scale, whether they could be tied 

to shifts in the distribution of nesting pairs, and whether they 

were indicative of wide, naturally-occurring versus human 

caused changes in the environment of ospreys (see Cartron et 

al. in press). 

Methods 

During the aerial survey (total area surveyed from air 

in 2006, except several islands mentioned later which were 

covered with fog), we made a single pass along the shore to 

census rocky or sandy cliffs adjacent to the shoreline or flat 

terrain with no cacti. In areas where large cactus or mangrove forests existed near the shoreline, we flew transects 

at increasing distances landward from the shore looking for 

osprey nests until no more were observed. Emphasis was also 

placed on locating nests on power poles and other structures 

in towns. Generally, we did not find nesting pairs more than 

2 km landward from the shoreline, but when nests were found 

on power poles, we extended the survey landward a few extra 

kilometers. 

We located osprey nests in 2006 from a twin engine 

fixed-wing aircraft with excellent visibility and a Global 

Positions System (GPS) (Partenavia PN68TC), i.e. the same 

type of plane used in 1992/1993. The survey was flown at an 

altitude of 60-100 m between 23 March and 1 April. The 

GPS allowed us to record the location of each occupied nest 

in 1992/1993 and 2006. No GPS was available during our 

1977 survey. The surveys in 1977 and 1992/1993 were flown 

at about the same time (24 March to 1 April, and 20 March 

to 2 April). A pilot and two observers (same as in 1977 and 

1992/1993, CJH and DWA) were present in the plane, and 

about 80 h of flying time were logged during each survey. 

We classified nests observed during the aerial survey 

as occupied if an adult was present on or in the immediate 

vicinity of the nest, or if young/eggs were seen in the nest. 

An attempt was made to schedule the aerial survey during 

the peak of the nesting cycle, although the season is not well 

synchronized in southern latitudes (but see nesting chronology below). Occupied nests would have been missed if a bird 

was not at or near a nest when it was surveyed, the nest was 

abandoned before the area was surveyed, the nest was initiated after the area was surveyed, or we failed to see the nest. 

Unoccupied nests were also recorded. 

Because it is costly and time-consuming to conduct 

the ground or boat survey portion of the double survey, we 

surveyed all of the study area by air but covered < 10% of the 

population from the ground, to develop a partial double-survey 

population estimate and its associated variance. The ground 

studies were generally made by boat or from a pickup truck. 

The ground studies were conducted within a few days to 2 

wks of the interval for the aerial survey, and made it possible 

to compare numbers of occupied nests at the time of census in 

various areas seen from air, ground, and both air and ground. 

Comparing data from both counts allowed us to obtain a 

visibility rate for adjusting aerial counts to the total nesting 

population at the time of the survey by use of a modification 

of the Petersen Estimator (see Henny and Anderson 2004). 

We sampled a finite population of size N (N unknown) by use 

of the two methods. The data were then recorded so that we 

knew the number of elements s a, observed by method 1 (aerial 

survey), the number of elements sg, observed by method 2 

(ground survey), and m, the number of elements observed by 

both methods. Then, 

Nˆ = s a sg / m 

is a reasonably good estimator of N. In this approach we 

assumed statistical independence of sa and s

g

. 

In sampling osprey nests, it was also necessary to assume 

N was not changing during the time between the ground and 

air survey. With the ground and aerial counts made within 2 

weeks of one another, it is doubtful that significant changes 

had taken place. Then Nˆ /sa is a reasonably good estimator 

of the aerial Visibility Rate. The aerial count was multiplied 

by the aerial Visibility Rate to obtain population estimates for 

areas with only aerial counts. Separate Visibility Rates were 

initially estimated for nests in cacti, nests on cliffs, and ground 

nests on small islands. Several nests on other structures 

(power poles) were included in air: ground comparison study 

area near Kino in 2006. For this study the nests on cliffs, cacti 

and other structures were combined because of their similar 

Visibility Rates (Table 1). Visibility Rates (combined nesting 

substrates) in 1977, in 1992-1993 and in 2006 were similar 

(1.63, 1.71 and 1.75), which suggests that the Visibility Rate 

had minimal influence on the percent change over time in the 

osprey population estimates. 

Scammon’s Lagoon and vicinity included large numbers 

of nesting ospreys in a relatively small area. Therefore, we 

relied upon detailed ground counts of that area by Castellanos 

and Ortega-Rubio (1995) and Castellanos et al. (1999) from 
4 Report Title

Table 1. Number of occupied osprey nests (nesting pairs) seen from the air and ground. 

Year nest substrate Air Ground Both Total est. Visibility

 (sa

) (sg) (m) (N) Rate (N/sa) 

1977

 Cliffs 88 121 74 143.89 1.64a

 Cactus 7 9 6 10.50 1.50

 Cliffs and Cactus (combined) 95 130 80 154.38 1.63 

 Scammon’s Lagoon (ground nests) 26 23 22 27.18 1.05 

1992, 1993

 Cliffs (LA Bay)b 32 43 25 55.04 1.72

 Cactus (Kino) 16 27 16 27.00 1.69

 Cliffs and Cactus (combined) 48 70 41 81.95 1.71 

2006

 Cliffs (LA Bay) 25 37 22 40.22 1.61

 Cactus and Other (Kino and vicinity) 23 33 18 42.17 1.83

 All Combined 48 70 40 84.00 1.75 

a Values for three locations sampled in 1977 were 1.54, 1.75 and 1.77 

b

 Combined information for both 1992 and 1993 

1993 for our earlier survey and counts by Castellanos (unpublished data) from 2004 for this survey. This lagoon accounted 

for about 9% of the osprey population throughout our whole 

study area. For our study area, nests occupied at the time of 

the survey, but believed to have been missed by both air and 

ground surveys, were included in estimates presented here. 

However, occupied nests abandoned before the survey, or initiated after the survey, are not included in our initial population 

estimates (but see nesting chronology below). No Visibility 

Rates were available for nests in mangroves; therefore, the 

combined value for cacti, cliffs and other nests was used. We 

believe nests in mangroves were more difficult to locate from 

the air, thus, nesting pairs in mangroves in coastal Sinaloa and 

perhaps Magdalena Bay (although none were located) may 

be underestimated. In the text, we refer to observed occupied 

nests (nesting pairs) when raw counts are used and estimated 

occupied nests (nesting pairs) when Visibility Rate adjusted 

counts are used. For simplicity in the text, we will refer to 

either observed nesting pairs or estimated nesting pairs. 

The variance estimate was detailed in our earlier report 

(Henny and Anderson 2004), and will not be described here. 

Basically, let there be two similar areas with populations of 

sizes N, and M. It is assumed that the aerial visibility of nests 

is the same in both areas. In one area both air and ground 

counts are made. A critical assumption is that the air and 

ground counts are statistically independent. In the second area 

only aerial counts are made. 

Using the following notation: 

N population size in area covered by ground and air 

M population size in area covered by air only

 s nests seen by air in air-ground area a

 s

g

 nests seen by ground in air-ground area 

m nests seen by both air and ground methods 

C aerial count of nests outside of air-ground area, a 

 binomial random variable 

T = N+M, population total 

Estimation formulae used in the study were: 

Nˆ = sasg / m Mˆ = Cs g / m Tˆ = Nˆ + Mˆ 

Generally, in more northern latitudes where this type of 

survey approach was first conducted (Henny et al., 1974), the 

nesting cycle was more synchronized; however, this tight synchrony does not occur in Mexico. Jehl’s (1977:243) statement 

citing Kenyon (1947) regarding ospreys in Scammon’s Lagoon 

is typical, “nests there contained all stages from fresh eggs to 

flying young.” Henny and Anderson (1979) noted that more 

research on nesting season chronology was needed throughout 

the study area to evaluate the percentage of the total breeding 

population that was nesting at a certain time. To better address 

the issue of survey timing and nest occupancy in the region, 

Henny and Anderson (2004) used detailed studies of ospreys 

in San Ignacio Lagoon (26o 54’ N; 113o 09’ W) by Danemann 

(1994). He provided information on the number of nests 

occupied on two small islands (Ballenas) between mid-January and early June 1989 (see Table 2 in Henny and Anderson 

[2004]). Our surveys in late-March approached the peak of 

occupancy, although only 83.9% of the total nests occupied 

for the season were occupied at that time (used multiplication 

factor of 1.19 times estimated occupied nests at time of the 

survey). Because of this lack of nesting synchrony, there was 

no survey time when all nests for the year were occupied. A 

final adjustment of population estimates for 1977, 1992/1993, 
Heading 1 5 

and 2006 is made here based upon Danemann’s studies at San 

Ignacio Lagoon, and the double-adjusted estimates (for birds 

missed that were nesting at the time of the surveys and for 

those nesting earlier or later) will only be presented in the last 

Table. Future studies may show variability in nesting chronology among regions, but only one detailed chronology dataset 

currently exists. The same adjustment for nesting chronology 

was used for all regions and all survey time periods (1977, 

1992/1993 and 2006). 

Results 

Using seven designated regions, we summarize our 

results as follows: 

Northwest Baja California, Lower California 

(L.C.) 

The region extends from the U.S.-Mexico border south to 

and including Scammon’s Lagoon, and west to Punta Eugenia, 

including Natividad, Cedros, and San Benito Islands (Figure 

1). No nesting pairs were observed between the border and 

Desembarcadero de Santa Catarina in either 1977 or 1992 

with the first nesting pair observed near Santa Catarina at 29° 

35’ N; 115° 22’ W. From this point south to Morro Santo 

Domingo, an estimated 20 pairs were nesting in cliffs in 1977 

with an estimated 31 pairs nesting in cliffs in 1992. However, 

by 2006 the nesting range extended north of Santa Catarina 

to 29° 52’ N; 115° 4’ W where a nest was built on a platform 

near a house, while a few other nests were on cliffs (Table 2). 

The estimated nesting population between Santa Catarina and 

Morro Santo Domingo was essentially unchanged at 32 pairs 

in 2006. 

Scammon’s Lagoon has been part of the El Vizcaino 

Biosphere Reserve since 1988 and has a long history of osprey 

studies (see summary 1946-1993 in Henny and Anderson 

[2004]). Basically, the population increased from 27 pairs 

nesting on the ground on small islands in 1946, to 50-86 

nesting pairs between 1977 and 1982 when artificial structures (channel markers, power poles, platforms, etc.) became 

important nesting substrates, and then further increased to 126 

in 1993 and 120 in 2004 (including 30 in the town of Guerrero 

Negro) when artificial nesting structures became even more 

important. 

No nesting pairs were observed to the west along 

“Scavenger’s Beach” between Scammon’s Lagoon and Punta 

Eugenia in 1977, but an estimated 10 pairs were present in 

1992. Henny and Anderson (2004) referred to this as an 

apparent “overflow” from the population increase at Scammon’s Lagoon. However, in 2006 only 2 pairs were estimated 

in the area. Due to fog in 2006, Natividad, Cedros and San 

Benito Islands (where an estimated 68 and 60 nesting pairs 

were present during the two earlier surveys) could not be 

surveyed. The estimated osprey population in Northwest Baja, 

L.C. (excluding the islands mentioned above) increased from 

1977 (70 pairs) to 1992 (167 pairs), but showed little change 

between 1992 and 2006 (160 pairs) (Table 2). 

Southwest Baja California, L.C. 

This region extends south from Punta Eugenia along the 

Pacific Coast to Cabo San Lucas. In 1977, only 35 pairs of 

osprey were estimated nesting in the region with the majority 

(27 pairs) nesting on the ground on two small islands (Ballenas) in San Ignacio Lagoon (Table 2). However, Reitherman 

and Storrer (1981) reported 129 occupied nests on the two 

islands only 4 years later in 1981. Danemann (1994) visited 

the islands regularly between January and June 1989 and 

counted 143 occupied nests (total count). During the 1992 

survey of the lagoon, we counted ospreys at 85 nests, and no 

ospreys at 50 nests, but our counting technique for the dense 

colony was inadequate, especially when many birds were 

flying. Therefore, although the nesting colony remained large 

in 1992, we opted to use the 1989 complete count of Danemann in the report. Excluding San Ignacio Lagoon, only 8 

additional pairs were estimated nesting in the region in 1977 

and 55 additional pairs in 1992, with most nesting south of the 

lagoon (Table 2). 

In contrast to the two earlier surveys, the San Ignacio 

Lagoon nesting colony of ospreys was no longer present in 

2006. Roberto Carmona (personal communication) made a 

ground count on the islands in 2006 and reported only three 

occupied nests. Therefore, we did not fly low over the islands 

and only saw one flying osprey from our higher-altitude flyover. Where did the San Ignacio Lagoon birds go, and when 

did they depart the lagoon? Basically, we believe they began 

nesting on power poles and other artificial structures in nearby 

towns in the region. In 1992, an estimated 8 pairs were nesting on artificial structures in the region (Henny and Anderson 

2004); but by 2006, the number increased to an estimated 175 

pairs (Table 2). The towns with nesting pairs (from highest 

counts to lowest counts) and distance (north or south) away 

from San Ignacio Lagoon included: Puerto San Carlos (south 

263 km), Punta Abreojos (north 46 km), Puerto Adolfo Lopez 

Mateos (s. 217 km), La Bocana (n. 56 km), Las Barrancas 

(s. 139 km). Other towns in the region with fewer numbers 

included: El Datil (s. 48 km), San Juanico (s. 98 km), military 

base on Isla Santa Margarita (s. 301 km), Bahia Tortugas (n. 

192 km) and Punta Prieta (n. 90 km). The total estimated 

population in the region in 1992 was 198 pairs, and remained 

unchanged at 199 pairs in 2006, but with a major redistribution 

to towns both north and south of the lagoon. 

Northeast Baja California, L.C. 

An estimated 117 pairs of ospreys nested in this region 

along the Gulf from the mouth of the Colorado Rivers south 

to Santa Rosalia in 1977, 106 pairs in 1992, and 126 pairs in 

2006 (Table 3). The terrain from the Colorado River south to 
6 Report 

Title

Table 2. Distribution and abundance of occupied osprey nests at the “time of the aerial survey” on the Pacific side of Baja California in 1977, 1992 and 2006.

1977 1992 2006

Location Total Total Max. Total

 Estimatea Estimateb Cliff Cactus Ground Other Observed Estimate

Northwest Baja, L.C.

U.S. Border to Puerto Santa Catarina 0 0 5.25 0 0 1.75 4 7.0

Puerto Santa Catarina to Morro Santa Domingo 19.5 30.8 26.25 0 0 5.25 18 31.5

Scammon’s Lagoon and vicinityc 50.1 126.0 0 0 18.00 102.00 120 120.0

Punta Mallarrimo to Punta Eugenia 0 10.2 0 0 0 1.75 1 1.8

Natividad Island 22.8 8.6 NSd

Cedros Island 19.6 18.8 NS

San Benitos Islands 26.0 32.5 NS

Subtotal 138.0 (69.6)e 226.9 (167.0)e 31.50 0 18.00 110.75 143 160.3

Southwest Baja, L.C.

Punta Eugenia to Punta Abreojos 1.6 5.1 14.00 0 0 54.25 39 68.3

San Ignacio Lagoon 27.3 143.0 0 0 3.00f 0 3 3.0

El Datil to Cabo San Lucas 6.5 49.6 7.00 0 0 120.75 73 127.8

Subtotal 34.5 197.7 21.00 0 3.00 175.00 115 199.0

Grand Total 173.4 (104.1)e 424.6 (364.7)e 52.50 0 21.00 285.75 255 359.3

a From Henny and Anderson (1979) 

b From Henny and Anderson (2004) 

c Ground count by Aradit Castellanos Vera 5-7 Feb 2004 

d NS = not surveyed in 2006 due to fog 

e Value in ( ) estimate without Natividad, Cedros and San Benito Islands (comparable to 2006 total estimate) 

f Ground count by Roberto Carmona, 25 Jan 2006
Heading 1 

7

Table 3. Distribution and abundance of occupied osprey nests at the “time of the aerial survey” on the Gulf side of Baja California and Midriff Islands in 1977, 

1992 and 2006.

1977 1992 2006

Location Total Total Max. Total

 Estimatea Estimateb Cliff Cactus Ground Other Observed Estimate

Northeast Baja, L.C.

Colorado River to Punta Remedios 37.4 23.9 21.00 3.50 0 5.25 17 29.8

Punta Remedios to Punta de las Animas 

 Los Angeles Bay Islands 35.0 29.3c 40.22 0 0 0 39 40.2d

 Other Locations 22.1 17.1 21.00 0 0 12 21.0

Punta de las Animas to Santa Rosalia 22.8 35.9 24.50 8.75 0 1.75 20 35.0

Subtotal 117.3 106.2 106.72 12.25 0 7.00 88 126.0

Southeast Baja, L.C.

Santa Rosalia to Loreto 65.6 49.5 42.00 0 0 14.00 32 56.0

Loreto to Timbabichi 42.4 49.6 36.75 0 0 1.75 22 38.5

Timbabichi to Cabo San Lucas 29.3 30.8 22.75 0 0 8.75 18 31.5

Subtotal 137.3 129.9 101.50 0 0 24.50 72 126.0

Grand Total 254.6 236.1 208.22 12.25 0 31.50 160 252.0

Midriff Islands

Guardian Angel 40.8 44.5 105.00 0 0 0 60 105.0

San Lorenzo, San Lorenzo Norte, Partida, 

 Salispuedes, Raza 52.2 53.0 36.75 0 0 0 21 36.8

Tiburon 71.8 164.2e 40.25 66.50 0 1.75f 62 108.5

San Estabon, Turner, Chollag 22.8 46.2e 39.00 0 0 0 29 39.0

Grand Total 187.6 307.9 221.00 66.50 0 1.75 172 289.3

a From Henny and Anderson (1979) 

b From Henny and Anderson (2004).

c Total estimate for 1993 was 25.2. 

d Used adjustment factor determined for LA Bay Islands in 2006

e Aerial survey conducted in 1993 

f Only tree nest in Midriff Islands (near a well) 

g Includes ground count by Tad Pfister in 2006 (4 occupied nests)
8 Report Title

Puertecitos is flat and contains few cacti suitable for nesting 

sites (such as the giant cardon, Pachycereus pringlei). The 

osprey breeding range has extended north from the tripod 

nesting structure placed on top of a building (30o

 45’ N; 114o 

42’ W) first observed in 1992 and still occupied in 2006. The 

most northern nest is now (2006) on a power pole at San 

Felipe (31o 03’ N; 114o 49’ W), with two additional nests 

observed north of the tripod nesting structure (another power 

pole at 30o 56’ N; 114o 43’ W, and a cactus at 30o 46’ N; 114o 

42’ W). Bancroft (1927, 1932) stated that historically the San 

Luis Archipelago (29o 57’ N to 30o 06’ N) was the location 

of the most northern nesting of the species in the region with 

about 60-75 nesting pairs. The estimated number of nesting 

pairs at the Archipelago in 1977 declined to 16 pairs, than 

further declined to 7 pairs in 1992 and remained unchanged at 

7 in 2006. 

Bahia de los Angeles and the nesting population on the 

small islands have been a focal point for osprey studies for 

many years. The estimated population on the islands was 

35 pairs in 1977, 29 pairs in 1992 and 40 pairs in 2006, with 

the pattern over time paralleling that observed for the whole 

region, except perhaps for the southernmost part of the regions 

(Punta de las Animas to Santa Rosalia) which is a more 

remote area. Isla Tortuga was not surveyed in 1977 and 2006 

(fog), but 2 occupied nests were located in 1992. The southern 

area showed an increase from an estimated 23 pairs in 1977 to 

36 pairs in 1992 and then stabilized at 35 pairs in 2006. 

Southeast Baja California, L.C. 

The coastal region south of Santa Rosalia to Cabo San 

Lucas contained an estimated 137 pairs in 1977, 130 pairs in 

1992 and 126 pairs in 2006 (Table 3). The islands along the 

coast listed in order of importance (nesting pairs) included 

Carmen, San Marcos, Cerralvo, Santa Cruz, Coronado and 

Santa Catalena; and those with only one nest located include 

Monserrat, San Jose, Partida, Espirita Santo and three small 

near-shore islands. These islands accounted for an estimated 

70 of the 126 nesting pairs in the region in 2006. Isla Santa 

Ynez, where five occupied nests were observed on the ground 

and one on a fishing shelter in 1977, only one occupied nest 

was observed on the ground and one on a tower in 1992, but 

none were observed in 2006. A new southern record for nesting osprey in Baja California was located on a rock pinnacle 

about 53 km northeast of San Jose del Cabo at 23o 26’ N; 109o 

25’ W. The previous southern record for the Gulf side was at 

the southern end of Isla Cerralvo in 1992 (24o 09’ N; 109o 52’

W), and for the Pacific Side in a cactus in 1992 at 23o 49’ N; 

110o 43’ W. 

Midriff Islands 

These islands are located in the Gulf of California in the 

vicinity of 29o N Latitude (Table 3). An estimated 188 pairs 

nested on the islands in 1977, 308 pairs in 1992/1993 and 

289 pairs in 2006. The two largest islands, Guardian Angel 

and Tiburon on opposite sides of the Gulf accounted for an 

increasing percentage of the total during the surveys; an estimated 113 pairs (60% of the total) in 1977, 209 pairs (67.8%) 

in 1992/1993 and 214 pairs (73.8%) in 2006. But, perhaps 

more intriguing is the relative importance of the two islands 

during the interval of surveys: Guardian Angel accounted for 

an estimated 21.7% of the population in the region in 1977, 

14.5% in 1992/1993, and 36.3% in 2006. Conversely, Tiburon 

Island accounted for an estimated 38.3% of the population in 1977, 53.3% in 1992/1993 and 37.5% in 2006. The 

other islands on the Baja California side (San Lorenzo, San 

Lorenzo Norte , Partida, Salispuedes, Raza) showed a consistent decline as a percentage of the total in the region (1977, 

27.8%; 1992/1993, 17.2%; 2006, 12.7%), while on the Sonora 

side (San Estabon, Turner, Cholla) remained fairly consistent 

(1977, 12.2%; 1992/1993, 15.0%; 2006, 13.5%). Tershy and 

Breese (1997) report the osprey as a rare visitor on San Pedro 

Mártir; the island was not surveyed in 1977, 1992/1993 or 

2006. 

Coastal Sonora 

An estimated 124 pairs nested in the region in 1977, 214 

pairs in 1993 and 158 pairs in 2006 (Table 4). The highest 

population estimate for Sonora occurred in 1993, but was 

followed by a general decline in 2006, which appeared more 

precipitous south of Punta Sargento. Cartron (2000) reported 

some years with extremely poor reproduction in his study 

area in coastal Sonora. The extreme northern portion of the 

region is flat with no cacti; then cardon appear sporadically. 

Further south along the coast, there are a few sandy cliffs and, 

eventually, some rocky cliffs. Some pairs nested in the cliffs, 

but most pairs in the region were in cardon cacti. The most 

northern occupied nest located in 2006 along the mainland 

of Mexico was on a power pole at 31o 18’ N; 113o 13’ W. 

Another nest was on a platform at 31o 16’ N; 113o 27’ W with 

two other nests on fake (wooden or plastic) cactus at 31o 14’

N; 113o 13’ W. These were all located in the vicinity of Puerto 

Peñasco which is an area of rapid and extensive development. These nests were all north of the previous northern 

nesting record in the region in 1993 which was south and east 

of Puerto Peñasco at 31o 12’ N; 113o 04’ W and located on a 

metal power pole. Mellink and Palacios (1993) reported an 

osprey nest in the same general location in 1982, and reported 

several nests along a railroad between Lopez Collada and 

Sahuaro in 1991 and 1992. All of the specific nests mentioned 

above were on artificial structures which strongly supports the 

concept that ospreys colonized this area with few natural nest 

sites available (land flat and no suitable cacti) because of the 

artificial nest sites. The power line towers were constructed 

shortly after our first survey in 1978 and 1979 (Mellink and 

Palacios 1993). No osprey in Sonora were nesting on artificial 

structures in 1977, but increased to 3.2% in 1993 and 16.7% 

in 2006. 
Heading 1 

9

Table 4. Distribution and abundance of occupied osprey nests at the “time of the aerial survey” in coastal Sonora and Sinaloa in 1977, 1993 and 2006.

1977 1993 2006c

Location Total Total Max. Total

 Estimatea Estimateb Cliff Cactus Mangroved Other Observed Estimate

Coastal Sonora

Colorado River to Punta Sargento 78.4 106.0 12.25 66.50 0 15.75 61 94.5

Punta Sargento to Sinaloa Border 45.6 107.7 21.00 31.50 0 10.50 39 63.0

Subtotal 124.0 213.7 33.25 98.00 0 26.25 100 157.5

Coastal Sinaloa

Sonora border to Topolobampo 6.5 13.7 0 15.75 0 7.00 13 22.8

Topolobampo to Punta Baradito 61.9 165.9 0 243.25 15.75 3.50 150 262.5

Punta Baradito to Mazatlan 1.6 0 0 0 0 0 0 0

Subtotal 70.0 179.6 0 259.00 15.75 10.50 163 285.3

Grand Total 194.0 393.3 33.25 357.00 15.75 36.75 263 442.8

a From Henny and Anderson (1979)

b From Henny and Anderson (2004)

c Survey extended south from Mazatlan to San Blas, Nayarit in 2006, but no nesting ospreys located

d Or other trees/bushes
10 Report Title

Coastal Sinaloa 

In previous surveys, the region extended from the Sonora 

border south to Mazatlan, but in 2006 the survey extended 

south from Mazatlan to San Blas, Nayarit, but no nesting 

ospreys were observed south of Mazatlan (Table 4). An 

estimated 70 pairs nested in the region in 1977, 180 pairs 

in 1993, and 285 pairs in 2006. From the Sonora border to 

Topolobampo, an estimated 7 pairs nested in 1977, 14 pairs 

in 1993 and 23 pairs in 2006. The majority of the nests were 

located between Topolobampo and Punta Baradito where an 

estimated 62 pairs nested in 1977, 166 pairs in 1993 and 263 

pairs in 2006. This area was further subdivided into two areas 

with the split at 25o 10’ N. From Topolobampo south to 25o 

10’ N, which included the barrier islands of San Ignacio and 

Macapule, we estimated 31 pairs nesting in 1977, an estimated 

118 pairs in 1993, and 213 pairs in 2006. South of 25o 10’

N, which included Santa Maria Bay and the barrier island of 

Altamura, we estimated 31 pairs in 1977, 48 pairs in 1993 

and 49 pairs in 2006. Carmona and Danemann (1994) studied 

Santa Maria Bay in 1988 and reported about 40 pairs of nesting osprey which supports our earlier observations. Most 

ospreys nesting in Sinaloa were nesting on barrier islands and 

peninsulas associated with several large bays. Mangroves and 

other brushy trees were abundant and some ospreys nested 

in them. The species of cacti changed again to a type that 

branches about 1.5 m above the ground (Pachycereus pectenaboriginum), and most nested again in cactus. The most rapid 

increase from 1993 to 2006 occurred at the barrier islands of 

San Ignacio and Macapule. The most southern nesting osprey 

along the mainland was south of Punta Bardito in 1977 at 

Ensenada del Pabellón (24o 38’ N) in a mangrove, although an 

occupied nest was reported on an island in Mazatlan Harbor 

(~23o 13’ N) in March 1978 (see addendum to Henny and 

Anderson [1979]). No nests were located south of Punta Baradito in 1993 or 2006 (Figure1). 

Nesting Sites 

Historically, ospreys at more northern latitudes nested in 

dead trees or trees with dead tops, but ospreys in more recent 

years (i.e., beginning in the 1970s) in the western United 

States nested on power poles and transmission towers and 

other structures in response to a shortage of suitable trees and 

the more recent appearance of many potential nesting structures near rivers, bays and estuaries (Henny and Kaiser 1996). 

In the eastern United States where more artificial structures 

have been available for a longer time, ospreys began using 

these structures for nesting much earlier (see Bent 1937). This 

study area in Mexico, where trees were rare, with the exception of mangroves at more southern latitudes, other options 

including cliffs, rock pinnacles, cacti and the ground on small 

islands provided the historic nesting substrates. By 1977, 

ospreys were using artificial nesting structures, at Scammon’s 

Lagoon primarily; however, towers, pilings, channel markers, 

debris washed ashore, boats (sunk and aground) and power 

poles were occasionally used at various locations in the study 

area (Table 5). The 1992/1993 survey recorded not only an 

increase in the overall nesting population, but also an increase 

in the percentage of the population (from 4.3% to 6.2%) nesting on artificial structures. Some artificial structures used in 

flat terrain with no cacti; i.e., no suitable nesting substrates, 

resulted in relatively short northward range expansions for 

the species. By 2006, artificial structures became much more 

important accounting for 26.4% of all nests, including 79.5% 

of nests on the Pacific Ocean side of Baja California, but only 

6.9% of nests elsewhere. Artificial structures became more 

important on the Pacific Coast side of Baja California because 

of two factors: (1) the ground nesting birds on the small 

islands in San Ignacio Lagoon departed and began nesting on 

power poles and towers at nearby towns, and (2) development of the salt industry at Scammon’s Lagoon beginning in 

1953 (Castellanos et al. 1999) resulted in power poles, channel 

markers and other associated structures being built in the area 

(including growth of the town of Guerrero Negro). Ospreys 

already nested on some artificial structures by 1977 which 

were becoming numerically more important over time (see 

summary of studies 1946-1993 in Henny and Anderson 2004). 

At Scammon’s Lagoon sometime prior to the 2004 nesting 

season, a series of platforms/towers were also placed on the 

three small islands in the lagoon where ospreys traditionally 

nested on the ground. Thus, in 2004, the complete ground 

count used for this survey, included 35 pairs nesting on platforms/towers on small islands and 18 on the ground. However, when we flew over the small islands in 2006, all island 

Table 5. Types of nest sites selected by ospreys in the study area, 1977, 1992/1993 and 2006.

Nest site substrate 

E

 1977 

stimated number of occupied nests 

1992/1993 2006 

Cliff 479 (59.1%) 542 (39.8%) 515 (38.3%) 

Cacti 213 (26.3%) 506 (37.2%) 436 (32.5%) 

Ground 59 (7.3%) 213 (15.6%) 21 (1.6%) 

Artificial Structures 35 (4.3%) 85 (6.2%) 354 (26.4%) 

Mangrove/Other Trees 24 (3.0%) 16 (1.2%) 17 (1.3%) 

Total 810 (100.0%) 1,362 (100.0%) 1,343 (100.1%) 

Note: No adjustment of aerial survey population estimates was made here for nesting chronology, i.e., data reflect 

“time of aerial survey.” 
Heading 1 11 

nests were back on the ground because the platforms/towers 

had fallen down. Thus, the artificial structures count for the 

latest survey (based on 2004 data at Scammon’s Lagoon) was 

biased high probably by ~ 35 pairs and the ground nests biased 

low by a similar count. Nevertheless, artificial structures now 

play an important role, especially along the Pacific Ocean side 

of Baja California. 

Numerically, the most important nesting substrate 

throughout the years was cliffs adjacent to the sea (1977, 

59%; 1992/1993, 40%; 2006, 38%) both on the mainland and 

on islands. Sometimes the cliffs consist of large pinnacles 

or stacks upon which the ospreys build their nests. However, 

small sandy cliffs contain nesting ospreys in a few areas. 

Large cacti remained in second place (1977, 26%; 1992/1993, 

37%; 2006, 33%), especially in flat terrain, and were numerically most important in Sonora, Sinaloa and Tiburon Island. 

Mangroves and other trees were generally available only in the 

southern portion of the study area. Ground nesting ospreys 

during the studies were restricted to very small islands at three 

locations: Scammon’s Lagoon, San Ignacio Lagoon and Santa 

Ynez Island. Ospreys were no longer nesting at Santa Ynez 

Island in 2006, and the population at San Ignacio Lagoon 

declined from 143 occupied nests in 1989 to 3 in 2006. The 

Scammon’s Lagoon ground nesting population on the small 

islands was 18 pairs in 2004 with 35 nesting on newly placed 

platforms on the islands; however, by 2006 all were again 

nesting on the ground. 

Discussion and Conclusions 

As in the past, much of our discussion relates to populations in the seven regions with 95% C.I. provided as estimates of precision (Table 6). The maximum number of nests 

observed for each area is shown in the earlier Tables as well as 

the estimated size of the population which provides additional 

insight into data quality. Furthermore, the change in population size between the three survey periods (1977, 1992/1993 

and 2006) was not a function of changes in Visibility Rates, in 

fact, the combined Visibility Rates were very similar for the 

three surveys (1.63, 1.71 and 1.75), attesting to the consistency 

of data acquisition with the same two observers. We found 

that the general distribution of ospreys in the survey area had 

not changed (with the exception of some short range expansions) between 1977 and 2006, although local and regional 

changes in abundance were apparent and variable. 

The region of Northwest Baja California has the best 

documented historical changes in osprey population numbers 

over time in Mexico. These changes were summarized in 

detail in our earlier reports, but briefly, the four most northern 

islands or groups of islands (Los Coronados, Todos Santos, 

San Martín and San Gerónimo) had no nesting ospreys during 

our three surveys. Los Coronados historically had no known 

nesting pairs, but ospreys were common on the other three 

islands. The last pair was observed at San Martin in 1971. 

These extirpations were concommittant with extirpations on 

islands off southern California (Kiff 1980). The population is 

now; however, slowly re-extending its range from the south, 

and by 2006 the range extended northward from 29o 35’ N; 

115o 22’ W to 29o 52’ N; 115o 41’ W (~ 42 km in 14 years, or 

~ 3 km/yr). Perhaps, reintroductions from Scammon’s Lagoon 

into this historic range all the way north to California’s Channel Islands could expedite the repopulation where osprey 

have been gone for decades. Scammon’s Lagoon is certainly 

the focal point for osprey in this region and the large population, with most now nesting on artificial structures, stabilized 

between 1992 and 2006 with much higher numbers than in the 

Table 6. A summary of estimated number of occupied osprey nests ± 95% C.I. in the study area, 1977, 1992/1993, and 2006. 

Location 1977a 1992-1993a 2006a 

NW Baja, L.C. 138.0 (164.2) 226.9 ± 20.6b (246.1)c 160.3 ± 7.4bd (190.8)d 

SW Baja, L.C. 35.4 (42.1) 197.7 ± 11.1b (208.1)c 199.0 ± 36.4 (236.8) 

NE Baja, L.C. 117.3 (139.6) 106.2 ± 21.6 (126.4) 126.0 ± 23.1 (149.9) 

SE Baja, L.C. 137.3 (163.4) 129.9 ± 26.5 (154.6) 126.0 ± 23.1 (149.9) 

Midriff Is. 187.6 (223.2) 307.9 ± 62.8 (366.4) 289.3 ± 52.9 (344.3) 

Sonora 124.0 (140.7)c 213.7 ± 43.6 (254.3) 157.5 ± 28.8 (187.4) 

Sinaloa 70.0 (83.3) 179.6 ± 36.6 (213.7) 285.3 ± 52.2 (339.5) 

Total 810 (957) 1,362 ± 278 (1,570) 1,343 ± 246d (1,598)d 

a

 First estimate refers to those nesting at the time of the survey including those missed by aerial survey; estimate in ( ) refers to double-adjusted 

population estimate which includes those not nesting at the time of the survey due to asynchronized nesting season at southern latitudes 

 (initial estimate X 1.19, see Henny and Anderson 2004). 

b

 N ± 95% C.I. for regional population estimates (C.I., assumes variance 0 for total counts at Scammon’s and San Ignacio Lagoons) 

c

 Complete counts used for Scammon’s Lagoon (1993), San Ignacio Lagoon 1989 for 1992 survey, and behind Punta Sargento (1977), thus those 

portions not adjusted for asynchronized nesting. Ground count at Scammon’s Lagoon for the recent survey was conducted over short time 

 interval, therefore, counts adjusted for asynchronized nesting. 

d

 Estimated numbers not comparable to earlier years because of fog at several islands (see text) 
12 Report Title

1940s, or even the 1980s. Unfortunately, we have no recent 

information to report for the islands to the west (Natividad, 

Cedros, and San Benito); they were not surveyed in 2006 

because of fog. 

Perhaps the most dynamic region is Southwest Baja 

California where ground nesting osprey at San Ignacio Lagoon 

were at “apparently” low numbers in 1977 (27 pairs), but 

increased tremendously (129 pairs) by 1981 (Reithermann 

and Storrer 1981) and where 143 pairs were counted nesting 

in 1989 (Danemann 1994). When the survey was repeated in 

2006, only 3 pairs were likely nesting on the islands, but the 

numbers lost at San Ignacio Lagoon could be accounted for 

at nearby towns and associated power lines. In an attempt to 

understand why the re-distribution occurred, it is first useful to 

determine when the change occurred. Large counts of nesting 

ospreys at the towns of Puerto San Carlos and Puerto Adolpho 

Lopez Mateos were already observed in 2002 by Eduardo 

Palacios and colleagues (pers. comm.). Thus, we conclude 

that the osprey redistribution occurred sometime before 2002. 

These were also two of the three towns in the region with the 

largest nesting populations in 2006. DWA visited the two 

islands in San Ignacio Lagoon on 8 May 1998 and recorded 

“...there were many osprey on the islands nesting, but although 

I looked, I did not see a single flying osprey young. We did 

not specifically check osprey nests, but on both islands they 

were occupying nests in large, typically active numbers.” A 

count by Laura Rivera (personal communication) on April 11, 

2001 included 70 pairs and perhaps more were present if some 

early breeders were missed (see osprey nesting chronology 

at San Ignacio Lagoon by Danemann in Henny and Anderson [2004]). These observations indicate that osprey had not 

totally abandoned the islands through 2001, although some 

loss in numbers apparently occurred. Thus, the major redistribution away from San Ignacio Lagoon to the adjacent towns 

was perhaps gradual and completed sometime after the 2001 

nesting season. The cause remains uncertain and it may not be 

a single factor. Possibilities for the redistribution include: (1) 

reduction in fish availability in the lagoon, (2) nests in towns 

are now more acceptable than 30 years ago due to presence of 

wildlife personnel and education, e.g., many reports of ospreys 

shot from power poles in earlier years including at San Carlos 

(Henny and Anderson 1979), or (3) a combination of the two 

factors. Disturbance on the islands could be a factor, but not 

likely, because the area is patrolled by wildlife personnel. 

The Gulf Coast of Baja California (North and South) 

showed the least change over the span of the three surveys, 

but our initial concern was that it did not respond with a large 

population increase as recorded for all other regions between 

1977 (255 pairs) and 1992/1993 (236 pairs). Furthermore, the 

population estimate for 2006 was within the same relatively 

narrow range (252 pairs). Although the population appeared 

stable during the three surveys in the last 29 years, there is 

evidence that it was historically much larger at least in the San 

Luis Archipelago (see earlier discussion referring to Bancroft). 

This regional population primarily nests on cliffs (82.6%) with 

artificial structures (12.5%) of minor importance, although 

nests were occasionally located in towns. The present distribution of the scattered nesting is quite similar to earlier years. 

However, in contrast to this general concept, the population at 

Espiritu Santo Island seems quite dynamic with only one pair 

observed during our 1977 survey, but the number increased 

sharply beginning in 1984 and peaked in 1986 with 22 occupied nests (Carmona et al. 1994). After 1986, the number of 

nesting pairs decreased and by 1992 (our second survey) only 

one pair was again nesting on the island although 16 unoccupied nests were observed. The Espiritu Santo count was back 

to one pair again in 1999 and 2006 (the last survey). Thus, 

short-term local changes can certainly occur during the long 

interval between the regional aerial surveys. 

The Midriff Islands in the Gulf of California showed an 

interesting dichotomy during the 1977 and 1992/1993 surveys, i.e., the islands near the Gulf coast of Baja (Guardian 

Angel and the series of islands associated with San Lorenzo) 

maintained nearly identical numbers (93 pairs versus 98 pairs), 

while those closer to Sonora (Tiburon, San Estaban, Turner 

and Cholla) showed major increases (95 pairs versus 210 

pairs). These findings paralleled those reported for the adjacent mainlands for the same time periods. However, in 2006, 

the nesting population on the islands nearer Baja California 

increased to an estimated 142 pairs, while the island population nearer Sonora decreased to 148 pairs. Thus, the size of 

the two populations was again similar in 2006 (142 and 148 

pairs), but at higher numbers than in 1977 (93 and 98 pairs). 

Factors responsible for the osprey increase in the early 1990s 

on the islands off the Sonora coast and eventual decrease by 

2006 may involve fish availability and associated lower reproductive success. After recording the population shift from San 

Ignacio Lagoon, it would be tempting to speculate that about 

50 pairs moved from islands off the Sonora coast to islands 

off the Baja California coast, but recall that a 13-14 year 

interval took place (1992/1993 and 2006) between the two 

surveys with limited information during the interim. Reduced 

reproductive success over a long period of time associated 

with food shortages (as reported along the coast of Sonora 

by Cartron) could account for the population reduction, and 

likewise increased reproductive success could also account for 

the population increase at the other islands. 

The estimated osprey population along mainland Mexico 

(Sonora and Sinaloa) doubled (194 pairs to 393 pairs) between 

1977 and 1993 with the increase more pronounced in Sinaloa 

than Sonora. By 2006, the mainland Mexico population 

continued to increase (443 pairs), but the increase was solely 

due to Sinaloa, as the population in Sonora declined (much 

like the population on the adjacent Midriff Islands). Most of 

the increase occurred on the relatively remote barrier islands 

of San Ignacio (different from San Ignacio Lagoon mentioned 

earlier) and Macapule (estimated 31 pairs in 1977, 118 pairs 

in 1993 and 213 pairs in 2006). Both of these islands and all 

others in the area are designated an “Area de Proteccion de 

Flora y Fauna (Islas e Islotes)” (Carabias-Lillo et al. 2000). 

Carmona and Danemann (1994) mentioned the possibility of 

agricultural pesticides flowing into the bays and estuaries from 
Heading 1 13 

streams that drain Sinaloa farmlands. The unique pattern of a 

continued population increase of this barrier island population 

that nests on natural substrates (cacti and mangrove, 98.7%) 

may be the result of reduced and/or termination of persistent 

pesticide use. This location with natural nesting sites seems 

ideal for ospreys. If DDT/DDE or other persistent pesticides was involved in an earlier (pre-1977) osprey population 

decline in Sinaloa (the location most likely influenced by agricultural pesticides) then a population recovery and increased 

productivity (Wiemeyer et al. 1988) would be expected. 

Unfortunately, no pesticide studies of ospreys and no osprey 

production studies were conducted in the region to evaluate 

possible effects of pesticides in earlier years. Our explanation 

for the observed increase is therefore problematic. 

Artificial structures used by ospreys in Mexico increased 

from 4.3% in 1977 and 6.2% in 1992/1993 to 26.4% in 2006. 

This increase primarily occurred on the Pacific Ocean side 

of Baja California, and included many nests on power poles. 

At many locations in the United States and elsewhere in the 

world, the percentage of ospreys nesting on artificial nest 

structures is extremely high, e.g., 85% along the Willamette 

River in western Oregon (Henny and Kaiser 1996). With 

many ospreys now nesting on power poles and transmission 

towers, the utility companies need to address, and in many 

places have already addressed the issue of power outages 

cause by nests as well as osprey electrocutions. Modification 

of some nests on power poles has already occurred at several 

locations in Mexico. The science of managing osprey nests on 

power poles is rapidly developing (APLIC 2006) and a recent 

report is available online: http://www.aplic.org/SuggestedPr 

actices2006(LR).pdf. 

Acknowledgments 

We thank Roberto Carmona and Tad Pfister who provided osprey ground counts for selected sites. Jim Bredy 

safely piloted the aircraft, especially in areas where fueling 

stops were limited and planning was critical. The survey 

was partially funded by the U.S. Geological Survey, the U.S. 

Fish and Wildlife Service, the University of California, the 

Instituto Nacional de Ecología, and Comisión Nacional de 

Áreas Naturales Protegidas. Christine Custer provided useful 

comments on an early draft of the manuscript. This scientific 

survey would not have been successfully completed without 

the approval of La Secretaria de Relaciones Exteriores (Permit 

No. DAN04059) and the assistance of many wildlife biologists 

in Mexico. 
14 Report Title

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