Surveying and Nest Monitoring of the Gray Vireo (Vireo vicinior) in
the Sevilleta NWR
Vizzachero, Benjamin; Yu, Sze Wing; Granillo, Kathy
Abstract
The Gray Vireo is a small native songbird and species of concern of which little is known. Few
past studies have documented their preferred habitat, relative abundance, population size, nesting
success, or the impacts of brood parasitism by the Brown-headed Cowbird (Molothrus ater). We
conducted a census of the breeding Gray Vireo population within two large sample sites in the
Los Piños Mountains of the Sevilleta National Wildlife Refuge. Audio playback surveys were
performed across a grid of points encompassing these sites. We observed detected Gray Vireos
to locate their nests and returned regularly to monitor the success of these nests. This will be the
third season of data collection for this project. Study sites were selected to determine if distance
from rangeland affects rates of brood parasitism, though it seems differences in landscape are
more influential on cowbird abundance. By quantifying the abundance and nesting success of the
Gray Vireo, we can determine their population status and potential need for conservation action.
Documenting the vegetation around nest sites will create a snapshot of the ideal Gray Vireo
nesting habitat, which could be incorporated into future habitat management plans across their
range. Overall, we found 19 Gray Vireo nests, of which 7 successfully fledged Gray Vireos, 4
were parasitized by cowbirds, and 8 failed for other reasons.
Introduction
The Gray Vireo (Vireo vicinior) is a small migratory songbird that breeds in arid Pinyon-Juniper
woodlands and savannahs across the American Southwest and winters in Northwestern Mexico
(Barlow et al. 1999). With a naturally limited and sparse distribution, their listing as an
endangered species in New Mexico and a Bird of Conservation Concern at the federal level
comes as no surprise (NMDGF 2006, USFWS 2008). Although they have garnered scientific and
conservation attention since their listing, the body of knowledge on Gray Vireos is small
compared to most other North American birds. Breeding Bird Survey Data, our only relevant
long-term data set, inconclusively indicates increases in population since 1966 (Sauer et al.
2011). However, this likely reflects an insufficient recovery from drastic range reductions caused
by the cattle ranching boom of the early 20th century. In addition, studies have detected small or
nonexistent populations in historically documented or otherwise deemed suitable Gray Vireo
habitat (Hargrove & Unitt 2014, DeLong & Williams 2006).
Like many imperiled North American Songbirds, the primary conservation threats to Gray
Vireos are habitat modification and nest parasitism by the Brown-headed Cowbird (Molothrus
ater) (Barrow et al. 1999). Habitat modification includes development for both construction and
fossil fuel mining as well the (relatively) less destructive effects of cattle ranching, wildfire, and
other habitat management efforts (Hawks Aloft 2007, Wickersham & Wickersham 2006). Gray
Vireos typically exhibit low overall nest success, documented as low as 18% (of 95 nests).
Cowbird parasitism is often a cause of nest failure, with rates as high as 71% (of 19 nests) in
some sites. Gray Vireos seem to possess the ability to recognize a cowbird egg, as the majority
of documented parasitized nests are abandoned (Hargrove and Unitt 2014, Hawks Aloft 2006,
Hawks Aloft 2007).
In 2006, Delong and Williams published a thorough compilation of the work of themselves and
others, summarizing the breeding abundance of the Gray Vireo across New Mexico. Their
summary was exhaustive, but the researchers lacked access to the ideal Gray Vireo habitat within
the Sevilleta National Wildlife Refuge (SNWR), about 20 miles north of Socorro, New Mexico.
This refuge was founded by the US Fish and Wildlife Service in 1973, following a thirty year
period of intensive cattle ranching. This study took place during the summer of 2014 and aimed
to census the Gray Vireo population contained within three sample regions of SNWR, located in
or adjacent to the Los Piños Mountains on the East side of the refuge. These sample regions were
selected by previous researches in order to determine if cowbird parasitism is more frequent in
regions proximal to cattle ranches.
In order to complete the census, we located and monitored nests or family groups of any vireos
detected and evaluated the rates of nest success and parasitism by the Brown-headed Cowbird. A
vegetation analysis was also performed to compare characteristics of nest sites chosen by vireos
against the sample regions at large, including attributes of the vegetation community and the
specific nest tree selected. Hargrove and Unitt have also indicated in their 2014 publication that
Gray Vireos tend to nest on south-facing slopes, with the nest in the south side of the tree, and
measurements were taken to follow up on this pattern. We also followed up on the observation
that nest trees tend to be more sparsely foliated or contain more dead branches by determining a
method to quantify the sparseness of a juniper tree. The overarching goal of our study is to
provide data on the Gray Vireo’s population in SNWR in order to add to a growing body of
knowledge used to assess the species’ conservation status and aid potential management
decisions of the refuge and beyond.
Materials and Methods
Figure 1 – Map of Sample Regions
Within the Los Piños Mountains, three large sample sites were outlined in the field and overlaid
with a 300x300m grid of waypoints using GIS software. The sites, Piños Canyon, Bootleg
Canyon, and Sepultura Flats, included 52, 16, and 59 waypoints, respectively. Starting on May
29th, 2014, audio playback surveys were performed at an arbitrarily selected cluster of these
points each day, alternating between sample sites, and continuing until all points within each site
were surveyed. 17 points in Piños Canyon were abandoned due to time constraints and a
complete absence of Gray Vireos in the vicinity.
Audio surveys consisted of 2-5 minutes of listening immediately after arriving at a point,
followed by 30-60 seconds of playback at maximum volume on the speakers of an iPhone 4S,
using the recording of the Gray Vireo territorial song from The Sibley eGuide to Birds. After an
additional 2-5 minutes of listening, the playback was repeated similarly, followed by a final 2-5
minutes of listening.
If a Gray Vireo was heard at any point during the survey or while travelling in or near the sample
site and it was not thought to be coming from a previously marked nest or territory based on the
direction of the heard song, that vireo would be immediately tracked down. Once the singing
vireo or its mate was spotted, it was followed until its nest or fledgling(s) was located. If the bird
disappeared and was not seen or heard for at least 10 minutes, 2-3 rounds of playback were
performed (one “round” consisting of 30-60 seconds of playback and 2-5 minutes of listening). If
the bird remained unresponsive, or the search was cut short due to time constraints, a GPS
waypoint was taken and the point would be returned to within the next week to continue the nest
search. If a fledgling being attended to by the adults was spotted, a GPS waypoint was taken and
this was considered a “family” group. If a nest was spotted, a GPS waypoint was taken and
flagging was used nearby to indicate the location.
All nests were then revisited every 3-5 days (with occasionally longer lapses). Upon initial
detection and each revisit, the contents of the nest would be checked and recorded. Most nests
were above eye level and a mirror mounted on the end of an extendable pole was used to view
their contents. Birds found sitting on the nest were typically flushed by the proximity of the
researchers or the mirror pole, but in some cases they were more reluctant to leave. In these cases
they would be lightly prodded with the mirror pole, and playback of the song recording was used
nearby. Birds on nests at eye level could be lifted off the nest. In some cases, birds could not be
flushed and no data was recorded. The eggs and fledglings of Brown-headed Cowbirds raised by
Gray Vireos were also recorded.
After all of the grid points within both sample regions were surveyed, vegetation sampling
began. Vegetation surveys were all centered around Juniper trees, including the 19 nest trees and
the closest trees to each of 34 randomly selected survey points. This includes 9 randomly
selected points from Piños Canyon, 4 from Bootleg Canyon, and 21 from Sepultura flats. We
measured the height and canopy diameter of each tree. Canopy diameter was taken once at the
widest part of the tree and again at a 90º angle to this, and these two numbers were averaged. A
1m square board of 10cm checkerboard tiles was constructed to estimate sparseness. The board
would be held at a set height on one side of the tree, while an observer would stand on the other
side of the tree and estimate the percent of the board visible through the tree. Measurements were
taken twice per tree along the same 2 axes as canopy diameter was measured, and results were
averaged for each tree. For nest trees only, we took the nest’s vertical height above the ground
and the orientation of the nest from the center of the tree. For all points on sloped ground, we
took a compass bearing to measure the direction immediately downslope.
In order to assess the local vegetation community, we evaluated 4 1m ground cover quadrats,
with each placed 10 meters from the tree in each of 4 cardinal directions, measured with a tape
measure. We recorded estimates of ground cover in these categories: bare ground, grass, forb,
shrub, and succulent. Dead but rooted vegetation was counted as if alive, while dead litter and
debris were considered bare ground. “Succulent” includes both cacti (cholla and prickly pear)
and various types of yucca. Bare ground was tallied as the difference between the sum of all
other vegetation types from 100. Percent cover in each category was averaged for the 4 quadrats
at each point. Lastly, we identified to family and counted all succulents and woody shrubs over
50 cm tall within a 10 m radius of the survey tree.
Results
Table 1 – Survey Results
% of Survey Points Detected
Site
Gray Vireo
Brown-Headed
Cowbird
Piños Canyon
5.3%
0%
Bootleg
12.5%
0%
Sepultura Flats
32.1%
1.9%
Total
19.6%
0.9%
Nests
Families
5
2
12
19
1
1
5
7
“Table 1 – Survey Results” shows the percentages of survey points for each site at which we
detected (including birds heard and/or seen) Gray Vireos and Brown-Headed Cowbirds. It also
gives totals for active nests and families found at each site, excluding families presumed to have
been previously counted by us at their nest site.
Table 2 – Nest Monitoring Results
Region
Nest
Fate
1
Fledged
2
Predated
Piños
3
Fledged
Canyon
4
Fledged
5
Fledged
Bootleg
Canyon
1
2
Fledged
Predated
Sepultura
Flats
1
2
3
4
5
6
Abandoned
Failed
Abandoned
Predated
Fledged
Predated
Secondary Fate
Cowbird
N
N
N
N
N
N
N
Predated
Predated
Y
N
Y
N
N
N
7
8
9
10
11
12
Partially Predated
Incomplete
Parasitized
Predated
Abandoned
Fledged
Abandoned
N
N
Y
N
Y
N
Predated
The previous chart details the fate of each nest we detected. 8 nest fledged young, 7 of them
fledging Gray Vireos and one (Sepultura Flats Nest 9) fledging only a cowbird chick. 6 Nests
experienced premature disappearance of eggs or young, presumed to be predation (Piños Canyon
Nest 2, Bootleg Canyon Nest 2, and Sepultura Flats Nests 4, 6, 7, and 10). One nest (Sepultura
Nest 7) had all but one egg disappear, and after this the egg did not hatch and the parent was not
seen incubating the nests. 4 Nests were parasitized by cowbirds (Sepultura Flats nests 1, 3, 9, and
11), and these nests were all abandoned (parents were not seen incubating) and later predated
(remaining eggs suddenly disappeared) with exception of nest 9, aforementioned. Sepultura Flats
Nest 8 was found while it was actively being constructed, but this nest was never completed.
Table 3 – Tree Attributes
Site
Group
Nest Height (cm)
Piños
Bootleg
Sepultura
Total
Nest
288
Random
Nest
249
Random
Nest
216
Random
Nest
238
Tree Height (cm)
Canopy (cm)
Sparseness (%)
439
613
6.2
326
385
2.8
376
352
13
393
560
11.4
353
535
3.8
283
393
3.4
378*
536**
5.4
411**
4.3
Random
308*
*Two-tailed t-test indicates significantly different results, p=0.0058
** Two-tailed t-test indicates significantly different results, p=0.022
The above chart displays average nest heights for each site, which averaged at 238cm and ranged
from 45-350cm with no significant differences between sample sites. It also displays average
height, canopy, and sparseness for nest trees and randomly selected trees across the three sample
sites and overall. Variation between sites is significant but sample sizes were too small to
analyze statistics of anything but the totals. Nest trees were found to be significantly larger in
both height and canopy diameter, though the difference in sparseness is insignificant.
Table 4 – Ground Cover Quadrats
Site
Group
Bareground (%) Grass (%) Forb (%) Shrub (%) Succulent (%)
Nests
86.6
7.4
0.0
2.7
3.8
Piños
Random
87.3
8.5
1.0
2.2
0.9
Bootleg Nests
89.4
7.9
0.0
1.1
1.0
Random
Nests
Sepultura
Random
Nests
Total
Random
85.4
87.3
91.3
87.3
89.6
7.6
11.7
7.3
10.2
7.7
0.3
0.4
0.4
0.2
0.6
3.6
0.5
0.8
1.1
1.5
3.0
0.1
0.1
1.2
0.7
The above chart displays average percent ground cover around nest trees and random trees at 3
sites and overall. Samples sizes are too small to meaningfully analyze the difference at each site,
and overall there were no significant differences in any category between nest trees and random
trees.
Table 5 – 10m Shrub Survey
Most Abundant Shrubs
Site
Sample
1
2
Cactaceae
Rosaceae
Nests
Piños
Cactaceae
Random Rosaceae
Cactaceae
Rosaceae
Total
Bootleg
Cactaceae
Nests
Random Rosaceae
Rosaceae
Total
Juniperus
Yucca
Cactaceae
3
Juniperus
Yucca
Yucca
Quercus
Quercus
Quercus
Juniperus
Parthenium Amaranthaceae
Nests
Sepultura Random Amaranthaceae Larrea
Juniperus
Amaranthaceae Juniperus
Larrea
Total
The above chart displays results from the 10 m shrub census. The three most abundant families
are given for all the nest trees, all the random points, and overall for each sample site. Family
name was replaced with genus when that genus contains all plants within its family that were
detected. Cactaceae includes primarily Tree Cholla (Cylindropuntia imbricate) as well as larger
Prickly Pear individuals (Opuntia spp.). Juniperus refers exclusively to One-seed Juniper
(Juniperus monosperma), preferred nest tree of vireos. Quercus refers to multiple species of
shrubby desert oaks, predominantly Gray Oak (Quercus grisea) and Shrub Live Oak (Quercus
turbinella). Rosaceae, a large family of flowering shrubs, includes Apache Plume (Fallugia
paradoxa) and Mountain Mahogany (Cerrocarpus sp.). Yucca refers to several unidentified
species in this genus, family Asparagaceae. Parthenium incanum, New Mexico Rubber Plant, is
the only member of the Asteraceae family we detected. Amaranthaceae, the Amaranth Family,
includes winterfat (Krascheninnikovia spp.) and Saltbush (Atriplex sp.). Creosote Bush (Larrea
tridentata) of the Zygophyllaceae family was abundant in some plots.
Figure 2 (left) – Orientation of nests from center of the tree
Figure 3 (right) – Direction of slope from center of the nest tree
North
2
North
4
2
1
0
5
East
West
2
1
East
West
0
3
3
1
2
4
0
South
0
South
The above figure on the left shows the number of nests and their orientation from the centers of
the nest trees. 4 nests were in the approximate center of the tree and not counted. The figure on
the right displays the number of nests trees and the orientation of the slope from the centers of
the trees.
Figure 4 – Weeks of first egg observed
Week of First Egg Observed
Number of Nests
6
5
4
3
2
1
0
Week
The graph above is a summary of when eggs were first observed in nests. This figure does not
include nests that were found after eggs and young were present, where we could not accurately
estimate when the first eggs were layed.
Discussion
Sample Regions
The three sample regions studied seem to present different distributions of Gray Vireo Habitat,
with associated differences in nest success. Piños Canyon had 4 of its 5 nests concentrated in the
dry creek beds at the bottom of the canyon, and only one nest along the more sparsely vegetated
slopes leading up the mountains. This observation is consistent with the documented tendency of
Gray Vireos to reside in relatively lower elevations (Schlossberg 2006). We attribute the low
detection rate of Gray Vireos (5.3% of points) to the number of points that were high on the
slopes, as well as the largest portion of the sample site that wrapped around the outskirts
Whiteface Mountain, sloping down towards an extensive grassland to the west known as
McKensie Flats. No Gray Vireos were detected in this portion of the sample region.
Figure 5 – Piños Canyon (left), Bootleg Canyon (right)
Bootleg Canyon, though smaller, showed a similar trend to Piños Canyon. Areas facing outward
towards McKensie flats were more sparsely vegetated and devoid of vireos. 2 nests were found
deeper into the canyon, in a relatively wide and flat area of juniper savanna that gradually slopes
up towards high ridge tops.
Sepultura Flats told a very different story. 12 nests were found, a much larger population
reaching greater densities. However, only 2 nests fledged Gray Vireos, with rates of predation
and cowbird parasitism being drastically higher.
Cowbirds
In both canyons, no cowbirds were detected by surveys and no instances of cowbird parasitism
were observed. Sepultura flats had a higher rate of parasitism and some recorded cowbird
observations, though there were more sightings while not performing surveys, which were
unrecorded. Although Piños and Bootleg were closer to a large cattle ranching operation
immediately north of the refuge, it seemed that the terrain influenced cowbird presence more
than proximity to cattle. Female cowbirds are known to disperse up to 10 km from feeding
ground to lay eggs, and cowbirds tend to remain confined to flat, open, grassy areas that loosely
resemble the shortgrass prairie in which they evolved. (Lowther 1993). However, we recommend
that in future years all cowbird observations in all sample regions be recorded (instead of just
those observed while performing the Gray Vireo survey) to confirm this notion.
Predation
Predation rates were higher in Sepultura Flats than in both Canyons. This could be the result of
greater population density of Gray Vireos (or possibly songbirds at large) supporting a greater
community of predators in Sepultura Flats. A study in Southern California indicates the Western
Scrub-Jay as a prominent nest predator of the Gray Vireo, and this bird was observed several
times at Sepultura Flats (Hargrove & Unitt 2014). Other likely predators observed within sample
regions include the Northern Mockingbird, Pinyon Jay, Loggerhead Shrike, Scott’s Oriole,
Bewick’s Wren, and a variety of non-venomous snakes (Barlow et al. 1999). The Gray Fox and
Bobcat are also vireo predators known to live within the refuge, but were not observed (Hargrove
& Unitt 2014). We recommend keeping more exact counts of possible predator sightings in both
sample regions. One must keep in mind that any instance of predation was assumed to have
occurred due to sudden disappearance of eggs or young.
Other Causes of Failure
Outside of cowbird parasitism and predation, causes of failure were odd and aberrant. One nest
(Sepultura Nest 2) contained a clutch of eggs that for unknown reasons never hatched and were
incubated for over a month until the bottom of the nest fell out. Another nest was found while it
was being built, but was never completed. This was a pair that had already built a nest and laid a
clutch that was predated, and perhaps was wary of our return. We remained <10m from the nest
under construction at all points, but speculate that the abandonment was caused by the vireos
associating our presence with the failure of their earlier nest.
Of the nests that were successful, there were no unhatched eggs or vireos that died in the nest.
This suggests that predation and parasitism are limiting factors of Gray Vireo reproduction.
Adult Gray Vireos seemed to face no challenges in keeping their young fed and healthy. This is
perhaps true of the Gray Vireo overall, but might also be due to the Fall of 2013 being unusually
rainy and productive, making it a more productive spring overall.
Nest Placement
Average nest height was consistently near 2.5 meters for all sample regions, though it varied
considerably from <0.5m to 3.5m, with a slight left skew to this distribution. The orientation of
the nest within the tree presents few apparent trends, though sample sizes are too small for
statistical analyses. There is a distinct lack of nests on the east side of trees, which is perhaps an
effort to avoid the direct morning sun. However, this means less protection from the more
intense afternoon sunlight, and is most likely the result of random variation. Measurements of
slope orientation appear similarly random in distribution. The lack of nests on South-Facing
slope could perhaps indicate an attempt to avoid direct sunlight. However, our data in no way
support the previously published tendency of Gray Vireos to nest on South-facing slopes in the
southern part of the tree (Hargrove & Unitt 2014).
Phenology
Because nests were discovered at all stages of the nesting process, we lack a data set concerning
the timing of reproduction that is comparable across all 19 nests. However, we could estimate the
date of the first egg being layed for 14 nests. The dataset on hatching and fledgling dates is
sparse due to the high number of nests that were abandoned or predated. Literature shows that
Gray Vireos in New Mexico tend to hatch from mid-May through late July, with a mean lay date
from May 13th to May 22nd (DeLong & Williams 2006, Hargrove & Unitt 2014). Most of the
Gray Vireos surveyed in SNWR had a later lay date in mid-June, which was atypical. However,
our survey was performed across the sample regions throughout the breeding season, so the data
may be biased as a result of us spending more time in the less densely populated canyons earlier
in the season, while not finishing surveys of Sepultura Flats until the middle of July. Regardless,
Gray Vireos continued to reproduce until far later in the season than was expected.
Vegetation
Concerning tree height, canopy diameter, and sparseness, variation was considerable across
sample sites but sample sizes were too small to analyze trends in any case but the totals. Nest
trees were significantly larger in both diameter and height overall, and this is probably because
smaller juniper trees were more abundant and thus more likely to be the closest tree to the
randomly selected GPS coordinate.
Though nest trees were found to be sparser on average, this could not be statistically verified. We
are dubious that the method we devised of estimating the percent of a 1m square board visible
through the foliage accurately quantified sparseness. Trees that appeared sparser overall often
did not have a higher percent of the board visible, and instead the girth of the canopy and
placement of main trunks seemed more influential on the reading. Perhaps this trend could be
better detected by pairing nest trees with random trees of very similar sizes, and adapting a new
method of measuring sparseness, such as with a canopy densitometer.
The 10-meter radius census of all shrubs and succulents greater than 50 cm tall showed little
variation between groups but appropriately characterized trends in shrub community across sites,
All sites had a significant Juniper component, though Junipers are apparently most dense around
nest sites in Sepultura flats. Piños and Bootleg Canyon had many similarities characteristic of
mountainous terrain and higher-elevations. This includes large component of Apache Plume and
Mountain Mahogany, as well as abundant Cacti (mostly Chollas). Both canyons also had
significant amounts of yuccas and oaks, though there were far more abundant yuccas in Piños
and more oaks in Bootleg. In Sepultura flats, the Rosaceae shrubs appear largely replaced by
Saltbush and Winterfat of the Amaranth family, as well as New Mexico Rubber Plant and
Creosote Bush. These hardy shrubs are characteristic of flat areas at lower elevations.
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