Loop migration by a Western Yellow-billed Cuckoo wintering in the Gran Chaco

Abstract

aBSTracT: a lack of information on the full life cycle of long-distance migrants, including nonbreeding periods, may hinder the recovery of threatened populations. in 2010, on the middle rio Grande, Sechrist et al. (2012) recaptured a yellow-billed cuckoo (Coccyzus americanus) fitted with a light-level geolocator, revealing for the first time wintering grounds and migration routes of an individual of this species. To further this knowledge, in 2011 we placed light-level geolocators on eight Western yellow-billed cuckoos at breeding sites on the lower colorado river in arizona and california. We recaptured one female in July 2012 at her previous capture site and analyzed the stored light data. During fall migration the bird flew ~9500–9900 km, passing through the caribbean region. it wintered from mid-november to late april in the Gran chaco of central South america, around the junction of paraguay, Bolivia, and argentina. The more direct spring route back to the breeding grounds passed through peru and central america. Following recapture, we discovered the bird was nesting while wearing the geolocator, and she later fledged young from two nests. Before and after migration, the bird appeared to pause in southern arizona or Sonora, paralleling the first tracked Western yellow-billed cuckoo, suggesting this monsoonal region may be important to the western population during these stages of the life cycle. The bird's migration timing and loop route, though reversed in direction, were also strikingly similar to those of the first bird tracked, and their overlapping wintering grounds suggests the possibility of a distinct winter range for the western population. Given the continuing expansion of agriculture into natural areas throughout this large region of South america, conservation of these forested areas is essential.

Full text

244 Western Birds 46:244–255, 2015
Loop Migration by a Western yeLLoW-biLLed
CuCkoo Wintering in the gran ChaCo
SHANNON E. McNEIL, DIANE TRACY, and CAROLINE D. CAPPELLO, Southern
Sierra Research Station, 7872 Fay Ranch Road, Weldon, California 93283; sem.
ssrs@gmail.com, dtracy.ssrs@gmail.com (current affiliation of Cappello: Department
of Biology, University of Washington, Box 351800, University of Washington, Seattle,
Washington 98195; ccappell@uw.edu)
ABSTRACT: A lack of information on the full life cycle of long-distance migrants,
including nonbreeding periods, may hinder the recovery of threatened populations.
In 2010, on the middle Rio Grande, Sechrist et al. (2012) recaptured a Yellow-billed
Cuckoo (Coccyzus americanus) fitted with a light-level geolocator, revealing for the
first time wintering grounds and migration routes of an individual of this species. To
further this knowledge, in 2011 we placed light-level geolocators on eight Western
Yellow-billed Cuckoos at breeding sites on the lower Colorado River in Arizona and
California. We recaptured one female in July 2012 at her previous capture site and
analyzed the stored light data. During fall migration the bird flew ~9500–9900 km,
passing through the Caribbean region. It wintered from mid-November to late April in
the Gran Chaco of central South America, around the junction of Paraguay, Bolivia,
and Argentina. The more direct spring route back to the breeding grounds passed
through Peru and Central America. Following recapture, we discovered the bird was
nesting while wearing the geolocator, and she later fledged young from two nests.
Before and after migration, the bird appeared to pause in southern Arizona or Sonora,
paralleling the first tracked Western Yellow-billed Cuckoo, suggesting this monsoonal
region may be important to the western population during these stages of the life
cycle. The bird’s migration timing and loop route, though reversed in direction, were
also strikingly similar to those of the first bird tracked, and their overlapping wintering
grounds suggests the possibility of a distinct winter range for the western population.
Given the continuing expansion of agriculture into natural areas throughout this large
region of South America, conservation of these forested areas is essential.
In the western U.S., the decline of the breeding population and range of
the Yellow-billed Cuckoo (Coccyzus americanus), recently listed as threat-
ened under the Endangered Species Act as a “distinct population segment”
(U. S. Fish and Wildlife Service 2014), is attributed mainly to loss or degrada-
tion of breeding habitat following large-scale modification of rivers (Gaines
and Laymon 1984, Hughes 1999). Knowledge of the conditions the cuckoo
experiences during the rest of its life cycle is limited, however, prohibiting a
full assessment of its year-round conservation needs. Habitat quality in the
winter range can affect an individual’s fitness, timing of its spring migration,
and reproductive success (Marra et al. 1998, Tonra et al. 2011). The threats
to western cuckoos once they leave their breeding grounds are unclear but
may be driven by the loss of forest to increased human population and by
the expansion and intensification of agriculture and cattle grazing, all exac-
erbated by climate change (Ramirez-Villegas et al. 2012).
Recent research spurred by the development of the light-level geoloca-
tor has revealed both stopover and wintering sites of many long-distance
migrants (Bridge et al. 2013, McKinnon et al. 2013), and the evolution of
this technology enables the tracking of smaller (<100 g) migratory birds
to their stopover and wintering grounds. Although the devices’ precision

245
remains rough, particularly in latitude (in forest, error averaging close to
200 km; Fudickar et al. 2012), geolocators can reliably be used to track
the movements of long-distance migrants (Ryder et al. 2011). The track-
ing of sufficient numbers of Yellow-billed Cuckoos could reveal the species’
migratory connectivity rangewide (Webster et al. 2002), including whether
the western and eastern populations use separate migration corridors or
wintering grounds, as found between distinct breeding populations of other
species (Delmore et al. 2012). If the two populations are allopatric during
the nonbreeding season, the decline of the western population may also be
associated with greater rates of forest loss on its wintering grounds.
The first geolocator-carrying Yellow-billed Cuckoo to be tracked over
one year was a female captured on the middle Rio Grande, New Mexico
(Sechrist et al. 2012). The data revealed a post-breeding dispersal phase
in northwest Mexico, fall migration through Mexico and Central America,
wintering in central South America in the Gran Chaco region of Bolivia,
Brazil, Paraguay, and Argentina, spring migration back to the breeding
grounds via the Caribbean and Yucatan Peninsula, and a pre-breeding
stop-over again in northwest Mexico. With the year-round movements of
just one individual known, our objectives were to gain further understanding
of western cuckoos’ migration and wintering periods, including identifying
the main areas used for wintering. This information may help to clarify the
birds’ risks year round.
METHODS
Between mid-June and mid-August 2011, we captured, banded, and
weighed 29 adult cuckoos at three riparian-forest-restoration sites man-
aged under the Lower Colorado River Multi-Species Conservation Program
(2004): Palo Verde Ecological Reserve, California (33.7° N, 114.5° W), Ci-
bola Valley Conservation Area, Arizona (33.41° N, 114.66° W), and Cibola
National Wildlife Refuge, Arizona (33.36° N, 114.69° W). We modified a
targeted mist-net technique (Sogge et al. 2001), raising the top of the net to
the height of the canopy (up to 12 m) to increase the likelihood of capture.
We attached two to four stacked mist nets (each 2.6 m high, 9–18 m long,
mesh 60 mm) between two canopy poles (Bat Conservation and Manage-
ment, Inc.) placed in a vegetation gap and broadcast cuckoo vocalizations
from portable speakers hidden on each side of the net. From each bird
captured, we drew up to 40 µL of blood for molecular sexing (McNeil et
al. 2012). We fitted eight that we knew or assumed were breeding on the
basis of nest observations, proximity to nests, or residency with Mk20 ASLT
geolocators (British Antarctic Survey), with light stalks 15 mm long angled
at 30°. Following Rappole and Tipton (1991), we attached the geoloca-
tors to lower-back leg-loop harnesses made of 1 mm elastic cord weighing
1.1 g total (0.9 g geolocator plus 0.2 g cord attachment; 1.5–1.9% of the
birds’ total mass). We released the birds where captured, and the following
breeding season we tried to recapture them at the same or adjacent sites to
retrieve the geolocators.
We used BASTrak software to download and decompress geolocator data,
and TransEdit to analyze the data (Fox 2010). We used a light threshold level
LOOP MIGRATION AND CHACO WINTERING BY A YELLOW-BILLED CUCKOO

246
of 2 to define sunrise and sunset, visually assessed each of these transitions,
and rated the quality of the transitions on a scale of 0 to 9. Transitions lacking
smooth curves (evidence the bird was in deep shade) received low scores.
We then rejected transitions scoring less than 8. We also discounted clearly
erroneous locations, such as those >1000 km apart within 12 hours or falling
far off shore. We used a sun-elevation angle of –4.09°, which best calibrated
to the capture location for the week after deployment (8–15 August 2011),
thus assuming a similar degree of shading throughout the year. We used
BirdTracker software (Fox 2010) to estimate latitude and longitude (datum
WGS 84) at noon and midnight each day. Because Yellow-billed Cuckoos
migrate at night (Crawford and Stevenson 1984), we compensated for lon-
gitudinal movement when estimating latitude (Fox 2010). For the periods
within 15 days of the fall and spring equinoxes (23 September 2011 and
20 March 2012, respectively) when day length was similar everywhere, we
inferred coordinates in longitude only. We estimated mean positional error
by measuring the distance between calculated and known locations for the
week after deployment and the week before recapture, when the bird was
at the breeding site. We imported the locations into ArcMap 9.3 (ESRI) for
visual assessment, and defined a buffer around pre- and post-breeding points
and around winter points, the width equal to our mean positional error, to
represent the areas of staging for migration and wintering, respectively.
RESULTS
Of four males and four females fitted with geolocators in 2011, we recap-
tured one female on 17 July 2012 at the Palo Verde Ecological Reserve, at
the same net location where initially captured on 7 August 2011. We failed
to refind the other seven or any of the 21 other cuckoos banded but not
fitted with geolocators. We removed the leg harness and geolocator and ex-
amined the bird thoroughly; she appeared healthy with no obvious ill effects
from the harness. She weighed 64 g on recapture, 4 g heavier than when
captured in 2011. We then fitted her with a tail-mounted radio transmitter
(McNeil et al. 2013) and found her to be a week into nesting, about 230 m
from the capture location. We radio-tracked her until 2 September, when
we lost her signal and assumed she left the site. She nested three times in
2012, the first and last attempts successfully.
The geolocator generated 687 location points over 344 days between
7 August 2011 and 16 July 2012. We omitted 409 low-quality points
(59.5%), including 114 (16.6%) near the equinoxes, when geolocation data
are unreliable, and another 15 that were clearly erroneous (2.2%). This left
263 points (38.3%) with which we assessed the migration routes and winter
range (Figures 1, 2). After the data from the first week of deployment (8–13
August) were calibrated to the capture location, the mean positional error
during that week was 87.2 km (range 46.8–146.4 km, SD = 28.2 km, n =
11 points). Longitudinal error averaged 56.3 km (range 19.1–89.1 km,
SD = 19.0 km, n = 11), and latitudinal error averaged 57.2 km (range
2.9–142.9 km, SD = 41.5 km, n = 11). The geolocator stopped recording
data on removal so we were unable to calibrate it after retrieval. Because
the bird was a week into nesting when recaptured, so presumably at or near
LOOP MIGRATION AND CHACO WINTERING BY A YELLOW-BILLED CUCKOO

247
the nest the entire week, we compared the data from the week prior to
recapture, 10–15 July 2012, to the location of her active nest. The mean
distance between the estimated locations and the nest was 221.2 km (range
22.1–387.7 km, SD = 110 km, n = 10). We used this distance (221 km)
LOOP MIGRATION AND CHACO WINTERING BY A YELLOW-BILLED CUCKOO
Figure 1. Estimated locations of a Western Yellow-billed Cuckoo from the lower
Colorado River to/from central South America, 7 August 2011 to 17 July 2012. No
latitude data are available for September 2011 because of the fall equinox; possible
routes are shown as dashed arrows.

248
LOOP MIGRATION AND CHACO WINTERING BY A YELLOW-BILLED CUCKOO
Figure 2. Comparison of migration routes, schedules, and wintering grounds of two
Western Yellow-billed Cuckoos from the lower Colorado River (a) and the middle Rio
Grande (b; data from Figure 2 in Sechrist et al. 2012). Points along the routes also
coded as 1, capture/breeding location; 2, post-breeding, Aug–Sep, and pre-breeding,
Jun; 3, fall migration, early Oct; 4, fall migration, mid-Oct; 5, winter range, Dec–Mar;
6, spring migration, late Apr; 7, spring migration. mid-May; 8, spring migration early
Jun; 9, spring migration, mid-Jun.

249
as the width of the buffer to display positional error around the points. The
mean latitudinal error from the nest was 211.2 km south (range 16.3–353.9
km, SD = 107.1 km, n = 10), or approximately 2° south, large compared
to the longitudinal error (mean 57.2 km, range 10.7–158.3 km, SD = 43.6
km, n = 10). Visual assessment of the locations suggested that through the
year, several other points outside the grasp of the equinoxes were shifted
south by a similar amount, including a cluster of points placed in the Pacific
Ocean south of Central America, when the bird was probably in Panama,
Nicaragua, or Honduras during spring migration.
We inferred the following from the estimated locations:
Post-breeding Dispersal and Fall Migration
• The bird left the breeding site around 17 August, moving east toward
central southern Arizona or northwest Mexico, where she remained until
13 September.
• On 14 September, she flew east to 104°–107° W longitude, staying
until 27 September.
• After spending six weeks in the southwest U.S. or northern Mexico post-
breeding, the bird swiftly moved east, apparently flying ~2000 km from
27 September to 1 October (mean 500 km/day). By 1 October she was
at 85.5°–87.5° W longitude.
• On 3 October, she was in line with Florida, Cuba and Central America.
From 6–19 October she was east of Florida’s longitude.
• From 20 to 25 October, she apparently moved through eastern Co-
lombia, then spent late October to mid-November in Amazonian Brazil.
Winter
• The bird spent mid-November through March, and probably April, in the
Gran Chaco of central South America, in the region where the borders
of Paraguay, Bolivia, and Argentina intersect.
• Latitude data from 12 to 29 March were unusable, the dates being too
near the equinox, but longitude was static. By late March she had possibly
moved south toward coastal Argentina. It is unclear if this shift was error
due to weather or a prolonged effect of the equinox.
Spring Migration
• She began moving north by 28 April and was in Central America from
late May to early June.
• Data from June were largely unusable (possibly because of weather or
the geolocator being too shaded), but she likely moved northwest through
southern Mexico to arrive in central Mexico 18–23 June. By 28 June,
she was back in southern Arizona or Sonora.
• By 9 July she had returned to the breeding site, and began nesting
around 12 July.
Fall migration from the southwest U.S. to South America took three weeks
(mean 225–250 km/day), with another month taken to arrive on the wintering
grounds (mean 117 km/day). Spring migration lasted two months, one month
LOOP MIGRATION AND CHACO WINTERING BY A YELLOW-BILLED CUCKOO

250
from the wintering grounds to Central America (mean 150 km/day), another
to the southwest U.S. (mean 123 km/day). The distance from the breeding
site to the core wintering grounds was 9500–9900 km (depending on the
fall route), and the distance back to the breeding site was 9100–9200 km.
In comparison to that tracked by Sechrist et al. (2012), our cuckoo took a
similar but reversed loop route (Figure 2). That is, she apparently migrated
through the Caribbean in the fall and through Central America in the spring,
whereas the cuckoo captured on the middle Rio Grande took a fall route
through Central America and returned in the spring through the Carib-
bean. The timing of the two cuckoos’ migration was also strikingly similar;
both apparently left their breeding grounds around late August, arrived in
northern Colombia in mid-October, and arrived on the wintering grounds
mid-November. Both began spring migration in late April, and after staging
in other areas in northwest Mexico or southern Arizona, they both returned
to their respective breeding grounds around the end of June.
DISCUSSION
Understanding the cuckoo’s migration strategies and where it stages and
winters enables the expansion of management beyond the present limited
scope of its breeding grounds. On a more basic ecological level, this infor-
mation may help answer questions about flexibility in migration strategy and
divergence of eastern and western Yellow-billed Cuckoo populations. Addi-
tionally, as geolocator technology for smaller birds is still evolving, our results
support the application of this technology to smaller migratory land birds.
We did not find the geolocators to affect the cuckoos’ rate of recapture
or ability to breed; the fraction 1/8 recaptured is comparable to the 9.9%
rate of recapture of 183 cuckoos banded along the lower Colorado River
from 2009 to 2012 (McNeil et al. 2013). In a review of geolocator studies,
Bridge et al. (2013) also found rates of return of birds with and without
geolocators to be comparable. Though we failed to refind any of the 21
other adults captured but not fitted with geolocators in 2011, we recaptured
three birds banded in the study area in earlier years (McNeil et al. 2013),
suggesting some site fidelity with a relatively low probability of recapture.
That all geolocator-fitted cuckoos thus far recaptured have been female
(three including another female recaptured on the Pecos River whose geo-
locator failed soon after deployment, Sechrist and Best 2012) is somewhat
unexpected because we typically capture and recapture fewer females than
males (16% and 2.8% fewer, respectively; McNeil et al. 2013). Females
average around 10% heavier than males (in this small sample of eight birds,
the females averaged 14% heavier), though all our attachments weighed no
more than 1.9% of the body mass of any bird, below the recommended limit
of 3% (Gustafson et al. 1997). As birds may lose mass during migration, we
recommend long-term attachments be as light as possible.
Our exclusion of over 60% of the data is consistent with other studies
that have discarded as much as 62% of data from birds inhabiting forested
environments (e.g., Ryder et al. 2011), the result of shade confounding
the light-level readings from which locations are calculated (Fudickar et al.
2012). As we found, location error averaging over 200 km (Fudickar et al.
LOOP MIGRATION AND CHACO WINTERING BY A YELLOW-BILLED CUCKOO

251
2012, Lisovski et al. 2012) is typical for latitude (see Hill 1994 for detailed
explanation), and latitude data from the periods around the fall and spring
equinoxes were largely unusable, often implying locations near the poles
(data not shown). Unfortunately, our bird spent the period most affected by
the fall equinox making the largest movements east. Because readings for
longitude are unaffected during the equinoxes, we were able to infer possible
routes for this portion of fall migration. As the first tracked cuckoo passed
between the West Indies and Yucatan Peninsula (Sechrist et al. 2012), this
route (though reversed in direction and season) seems the most plausible.
The apparent staging by both tracked cuckoos in southern Arizona or
northern Mexico pre- and post-breeding suggests this region is important to
the western population during these stages of its life cycle. The Yellow-billed
Cuckoo is often described as wandering or nomadic during periods surround-
ing the breeding season, exploiting outbreaks of large insects (Hamilton and
Hamilton 1965, Hughes 1999); birds of many species wander considerable
distances after breeding but before migration (Rappole and Ballard 1987).
Nomadic or exploratory behavior should aid the cuckoo in locating ephem-
eral patches of cottonwood–willow forest.
The North American monsoon typically develops over southwest Mexico
from late May to early June, arriving in northwest Mexico from mid- to late
June, and in the southwest U.S. by early July (Adams and Comrie 1997)—
roughly tracking the movement of our cuckoo through Mexico in June to
its arrival in the southwest U.S. by early July. The spike in rainfall in July
and August over the center of the monsoonal region in northwest Mexico
(Douglas et al. 1993) coincides with the peak of western cuckoo nesting.
If cuckoos track monsoonal flushes of new vegetation and insects (Wallace
et al. 2013), they may be pursuing a multi-stage strategy for breeding and
migration, as found in some other birds (e.g., Stach et al. 2013).
Sechrist et al. (2012) raised “migratory double-brooding” (breeding in two
regions in one year, separated by a migration) as a possible reason for cuck-
oos to visit Mexico late in the breeding season. First suggested by Rohwer
et al. (2009), this hypothesis is based on circumstantial evidence alone, and
it appears increasingly unlikely (Rohwer and Wood 2013). Sechrist et al.
(2012) also suggested molt migration as a possible cause for the stopover
in Mexico, while acknowledging that the stopover was too brief; also, the
Yellow-billed Cuckoo molts its flight feathers mainly in its winter range (Pyle
1997, Rohwer and Wood 2013). Tracking many more individuals, through
more than one annual cycle, is needed to test these hypotheses and assess
fidelity to staging areas.
The similarity in the timing of the two tracked cuckoos’ migration was
unsurprising. In long-distance migrants, it is often highly consistent within a
population (Stanley et al. 2012) and determined genetically (Berthold and
Helbig 1992), though it can be affected by factors such as energetic condi-
tion and nesting date (Stutchbury et al. 2011). Western cuckoos’ breeding
much later than the eastern population (Hughes 1999) also suggests the
western population may winter farther south and thus travel farther from the
winter to the breeding grounds (Rubolini et al. 2005), or it may begin spring
migration later than the eastern population. The lack of migration data on
eastern individuals currently prevents further comparison.
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252
The reversal of the two tracked cuckoos’ loop routes implies that western
cuckoos’ migration routes are flexible, as found in some other species (Al-
erstam et al. 2006, Vardanis et al. 2011, Stanley et al. 2012). Sechrist et
al. (2012) already dispelled speculation that only eastern cuckoos migrate
through the Caribbean (Hughes 1999), and both birds’ passing through the
Caribbean suggests migration of western cuckoos through this area may
even be common. Within a population, migration routes are generally more
flexible than timing (Delmore et al. 2012, Stanley et al. 2012), as we found.
Loop routes are common (Klaassen et al. 2010, Stanley et al. 2012), though
typically the movement is in a consistent direction (e.g., clockwise). Because
wind direction can be the greatest predictor of flight direction (Able 1973),
the direction of the wind at the start of each migration may have driven each
bird’s decision whether to pass through Central America or the Caribbean.
The threats to western cuckoos on their breeding grounds, primarily
habitat loss and degradation (Gaines and Laymon 1984), increasingly exac-
erbated by long-term drought and climate change (Ault et al. 2014), may be
even greater in their winter range. The Gran Chaco, containing the second
largest native forest in South America after the Amazon Basin, has, over
the last few decades, experienced large-scale conversion and fragmentation
of forest for expanding cattle and soybean production (Berbery et al. 2006).
Deforestation of the Chaco in Argentina, Paraguay, and Bolivia represents
the greatest loss of forest cover globally in the 21st century (Hansen et al.
2013). From 2005 to 2010, annual rates of deforestation in this region
(1.5–2.5%) surpassed Latin American and world averages (0.5% and 0.2%,
respectively, reviewed by Seghezzo et al. 2011). Gasparri and Grau (2009)
found deforestation of Chaco dry forest accelerating, with over 1.4 million
ha destroyed in the last 35 years. Mastrangelo and Gavin (2014) encouraged
alternatives to intensive agriculture, such as selective clearing, to lessen the
continuing reduction of habitat for birds in this region. A better understanding
of the winter range, including identifying and supporting actions to reduce
these threats, will promote conservation of western cuckoos through their
full life cycle.
ACKNOWLEDGMENTS
We thank the Bureau of Reclamation, Lower Colorado River Multi-Species Con-
servation Program (contracting officer representative Barbara Raulston) for funding
the long-term monitoring of the Western Yellow-billed Cuckoo that made this study
possible. We also thank our coworkers at the Southern Sierra Research Station and
2011–2012 field crews. Additional thanks to Melanie Culver and the Conservation
Genetics Lab, University of Arizona School of Natural Resources and the Environment,
for help with methods for sexing. Juddson Sechrist (Bureau of Reclamation) gave us
advice on fitting geolocators as well as helpful comments on the manuscript, and James
Fox (British Antarctic Survey) provided information on geolocator specifications. We
gratefully thank the anonymous peer reviewer for constructive comments and edits.
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Accepted 25 March 2015
Yellow-billed Cuckoo
Drawing by George C. West