Avian Conservation and Management

INTRODUCTION

The Western Cordilleran Flycatcher (Empidonax difficilis occidentalis), an insectivorous Neotropical migrant passerine, breeds throughout the montane regions of western North America (Lowther et al. 2020). Classified as a crevice-nester and considered nest-site limited (Collias and Collias 1984), this flycatcher typically places nests on recessed rock ledges, nooks in stream banks, behind loose bark on trees, or within the roots of wind-thrown trees. The birds also frequently nest on buildings, especially horizontal crossbeams (Boyce et al. 2020, Darrah and van Riper 2021). This broad range of nesting substrates has previously been documented for Western Cordilleran Flycatchers in Colorado and Arizona (Corman and Wise-Gervais 2005, Colorado Bird Atlas Partnership 2016).

The addition of 4-sided nest boxes has been shown to increase breeding bird numbers and enhance productivity of numerous hole-nesting birds around the world (Kluijver 1951, Perrins 1979, Brawn and Balda 1988, Fargallo et al. 2001, Sutherland et al. 2004, Mänd et al. 2009, Hepp and Bellrose 2013, Norris et al. 2018, Hannay et al. 2022). In North America, nest boxes for the Wood Duck (Aix sponsa) have been erected over much of the continent during the last 50 years, and breeding populations have responded by increasing at an overall continental rate of approximately 6% per year (Hepp and Bellrose 2013). Backhouse (1986) summarized recovery of the Eastern Bluebird (Sialia sialis) from the “brink of extinction” to locally common, following placement of nesting boxes throughout Eastern North America. Norris et al. (2018) found that nest boxes increased reproductive output for Tree Swallows (Tachycineta bicolor) in central British Columbia, Canada, with pairs nesting in boxes producing over twice the number of young as birds in natural nesting holes. East and Perrins (1988) found in European broadleaf temperate woodland forests that nesting success for species was lower in natural nesting holes than in nest boxes, because of a higher percentage of total nest failures.

There have been comparatively few studies to determine if crevice-nesting birds would respond to the addition of artificial nesting substrates. In Finland and Lithuania, Kuitunen and Aleknonis (1992) found increased breeding success of the crevice-nesting Common Treecreeper (Certhia familiaris) with the addition of nesting boxes. Weeks (1979, 2020) found that the crevice-nesting Eastern Phoebe (Sayornis phoebe) realized increased nest productivity when breeding on human structures, compared to natural locations. Kestrels (Falco spp.), normally a hole-nesting bird in Europe and North America, have responded to recent efforts to provide them with open 3-sided nesting platforms (Fargallo et al. 2001, Lambrechts et al. 2010).

During a study of Western Cordilleran Flycatcher breeding in southwestern Colorado, we found that nesting birds were absent from portions of riparian habitat along the Dolores River. We hypothesized that this was due to a lack of suitable nesting sites. Beginning in 2010, we tested this hypothesis by adding several types of artificial nesting structures. Although the Western Cordilleran Flycatcher is widespread and not presently a species of concern, we felt that this was an opportunity to test if artificial platforms would help breeding and if so, this could possibly be used as a management tool for crevice-nesting species of concern. After identifying a structure that flycatchers would utilize for nesting, we supplemented the habitat with 2-sided nesting platforms, and from 2012 to 2017 documented how those platforms influenced breeding, adult numbers, reproductive success, and site-level productivity. Then from 2018 to 2022 we tested our platform findings on a breeding Western Cordilleran Flycatcher population at Mt Lemmon in southern Arizona.

We hypothesized that, following the introduction of artificial nesting platforms, Western Cordilleran Flycatcher numbers would increase and that they would utilize artificial platforms where natural nests sites were limited. We also hypothesized that nest predation by predatory birds and mammals would probably increase because of platform visibility, but in terms of site-level productivity losses to predation might be compensated for by the increased availability of suitable nesting sites. If our nest platform supplementation efforts were successful, we felt that this would enable researchers to have easier access to nests for studying breeding birds (e.g., Katzner et al. 2005), and that this also might encourage similar efforts to increase numbers of crevice-nesting birds of concern at other locations.

METHODS

Study areas

Our study took place from 2008 to 2018 in Dolores County, southwestern Colorado, and then from 2019 to 2022 on Mt Lemmon, in Pima County Arizona USA. In Colorado, study areas were established along the east fork of the Dolores River from 2280 to 2590 m elevation, with a core study site that encompassed 28 ha centered on 37.576234°N, -108.22559°W at 2423 m elevation. We also had a paired 30 ha study site without buildings at Ryman Creek, 20 km up the Dolores River at 2667 m elevation (37.615230°N, -108.045953°W). The Dolores River runs through a glaciated valley bottom and is bounded on either side with riparian cottonwood (Populus fremontii) gallery forests. This natural montane riparian habitat is interspersed with patches of open areas created by pasture and residential buildings. The riparian habitat mosaic also includes alder (Alnus spp.) and willow (Salix spp.) thickets along sections of stream bank, and mixed species stands of spruce (Picea spp.), Douglas fir (Pseudotsuga menziesii), and aspen (Populus spp.) on the more elevated portions of the flood plain. The steep valley walls on either side are dominated by mixtures of mature Ponderosa pine (Pinus ponderosa), aspen, and fir (Abies spp.) forest.

The Arizona study site encompassed 32 ha, centered on 32.2645°N, -110.4554°W at 2423 m elevation between the town of Summer Haven and the Mt Lemmon summit. In Arizona potential nesting locations are more abundant than in Colorado, and there are greater numbers of breeding Western Cordilleran Flycatchers (personal observation). The habitat is composed of mixed fir and spruce stands at moister locations, while ponderosa pine dominates on the drier south facing slopes. Aspen is present, but less abundant than in Colorado, while mountain maple (Acer grandidentatum) is more common. Sabino Creek, which is spring fed, runs through the study site and provides moisture to the riparian vegetation. Residential structures are mixed with U.S. Forest Service lands along the riparian corridor.

The bird

The Western Cordilleran Flycatcher is a small (~11 g), migratory aerial insectivore in the suboscine family Tyrannidae, whose breeding is confined to the higher elevation mesic zones of the intermountain west, from southern Canada through the United States and into central Mexico (Greeney et al. 2018, Lowther et al. 2020). Breeding aspects of this species are known from studies of brood provisioning rates and fledgling behavior in Colorado (Darrah and van Riper 2021), nesting studies in northern Arizona (Martin et al. 2015, Boyce et al. 2020), while other life history aspects have largely been extrapolated from the better-studied Western Pacific Slope Flycatcher Empidonax difficilis (Davis et al. 1963, Sakai 1988). In Arizona Western Cordilleran Flycatchers arrive from Mexico in May, but in Colorado during early June. At both locations birds defend Type A territories (Nice 1941, Brown 1969); the average clutch size is four eggs; young fledge between 15 and 17 days; and fall migration begins in August at both locations (personal observation).

In Colorado and Arizona, the Western Cordilleran Flycatcher is limited to one complete nesting cycle each breeding season (Kingery 1998, Darrah and van Riper 2021), as are most Empidonax flycatchers in North America (Mumford 1964, Walkinshaw 1966, Sogge et al. 2003). An abbreviated nesting season greatly influences annual productivity and population dynamics of small passerine birds (e.g., Kus et al. 2017, Sutton and Freeman 2023), and if a nest fails during late incubation or the nestling stage, it is not possible for the Western Cordilleran Flycatcher to successfully re-nest. Rearing a second successful brood is prohibited by the late initiation of nesting, nesting cycle length (43 days from nest building to fledging), and the species’ August departure for the wintering grounds in Mexico.

Field methods

Each year from 2008 to 2022 we began by listening for males that were singing and exhibiting territorial defense, and then searched each territory until the nest was located. We also checked previously utilized nesting locations, as Western Cordilleran Flycatchers often use the same site for nesting in consecutive years. Nests were also discovered by following adults in the process of nest building or provisioning young. At all locations we checked nests daily, took physical measurements, and at some nests, recorded behavior via a home video system (Diwill 4-port H.264 Digital Wireless). We established 12 permanent mist net sites within each core study area, and at those locations captured and banded birds throughout the breeding season (after Ralph et al. 1998). We also captured unbanded parents at each nest using target mist-netting, but those data were not incorporated into our birds/net hour computations. All birds were banded with a single U.S. Geological Survey numbered aluminum band, and each adult flycatcher received a unique combination of 2-colored plastic leg bands on the opposite leg. We sexed adults by the presence of a brood patch or cloacal protuberance (Pyle et al. 1987, Ralph and Hollinger 2003), which was later corroborated with behavioral evidence (incubation; song). Wing chord was measured to the nearest mm using a wing rule, length of tarsometatarsus to the nearest 0.1 mm with calipers, and mass to the nearest 0.1 g using a digital balance.

At nests we documented hatching success (percentage of eggs laid that hatched), nestling success (percentage of hatchlings fledged), and fledging success (the percentage of eggs laid that fledged). Nest measurements of height, bowl width, depth, and rim thickness were taken with a micrometer, while we used a tape measure for height from ground and substrate measurements. Nest weights were taken on an Ohaus digital scale accurate to 0.01 grams.

Development and use of a 2-sided artificial nesting structure

During 2010 and 2011 breeding seasons, we tested four types of artificial nesting structures: (1) a flat board; (2) a 2-sided platform with roof; (3) a 3-sided platform with roof; and (4) a completely enclosed 4-sided nest box with 4.12 cm (1 5/8 in) diameter entry hole. Five replicates of each structure type were placed throughout the Colorado study area. During the 2012 breeding season, prior to flycatcher arrival, we placed out 30, 2-sided platforms (Fig. 1). Fifteen platforms were distributed on five cabins, with the remaining 15 placed in groups of three on trees in a natural forest setting. In Arizona, starting in 2019 residents of Mt Lemmon began placing 2-sided nesting platforms on cabins and by 2022 platforms had been placed at 30 residences and at 15 locations in natural forest settings.

Statistical analyses

To determine whether survival from nest to fledgling differed between study sites or among the types of nesting substrates, we fit generalized linear mixed effects model in which the fate of individual eggs was modeled using a binomial distribution and eggs within a nest were given a common random effect. We fit four models: a null model, a model in which survival varied by study site, a model in which survival varied by nest structure, and a model in which effects of study site and nest structure were additive. Models were fit using the lmer package in R and compared using AIC and we based inferences on the best model (which was greater than 10 AIC units better than all models without uninformative parameters sensu Arnold 2010). For inferences regarding parameters, we fit a Bayesian version of the best model using the rstanarm package in R (R Core Team 2017).

Utilizing daily nest checks, we determined incubation stage length (number of days elapsing between laying of the last egg to hatching of all eggs), and nestling stage length (the time from hatching of the last egg to fledging of all young). Annual productivity was calculated as the total number of young fledged per year within study areas. To test if use of platforms affected nesting behavior or success we compared within and between study locations, clutch size, incubation behavior and length, nestling stage length, hatching success, nestling success, fledgling success, and total young fledged. We tested among flycatcher pairs using natural nest sites, nests on human-made structures, and those on artificial platforms with ANOVA utilizing SPSS, a 2017 IBM Corp statistical package.

We used a 2-sided t-test and chi-square tests to compare nests on the three types of substrates producing ≥ 1 fledging and the probability of depredation by predatory birds and mammals, during incubation and nestling stages. The proportion of flycatchers captured each year was corrected with a time-series analysis utilizing a damping factor of 0.3. Trends in the time series capture rates were analyzed with a Mann-Kendall trend test.

RESULTS

Acceptability of artificial nesting structures

During our 2010–2011 exploratory testing of four different platform types, Western Cordilleran Flycatchers did not choose to nest on flat boards, 3-sided platforms, or completely enclosed nest boxes with a round entry hole, but did use a 2-sided platform with roof (Appendix 1). In 2012 when we placed 30 2-sided nesting platforms within eight territories, two flycatcher pairs bred on platforms, and from 2013 through 2016 from 3 to 4 pairs used platforms (Table 1). In Arizona, where natural nesting locations were more abundant, Western Cordilleran Flycatchers utilized two nesting platforms in 2019 (11% of all nests), increasing use to eight in 2022 (53% of all nests).

Nest size and structural integrity

Nest maximum basal width was significantly larger on platform vs naturally placed nests (T-test = -2.208; df = 16; P < 0.05), but not between nests on platforms and human-made structures (T-test = 1.595; df = 22; P = 0.06). There was no significant difference between nest cup depth (ANOVA; df = 2; F = 1.74; P = 0.20) or cup width (ANOVA; df = 2; F = 3.67; P = 0.07) when comparing among the three nesting substrates. Because of structural issues of placement locations, nests at natural sites failed more frequently than did those on platforms. Nests placed behind tree bark sometimes failed when the bark sloughed off the tree. At nine locations the nesting crevice was shallow, and nests fell to the ground. We did not observe any eggs or young fall from platform nests.

Differences in breeding behaviors between artificial platforms and other nest sites

When comparing artificial platform nests to natural nests, and those built on human-made structures in Colorado and Arizona there was no significant difference in numbers of eggs among clutches in Colorado (ANOVA, F = 0.1725; df = 2; P = 0.84) or Arizona (ANOVA, F = 2.575; df = 2; P = 0.07; Table 2), and incubation attentive periods were virtually identical between platform and natural nesting locations (T-test = -1.301; df = 13; P = 0.2158). We also found no significant difference in the incubation periods among natural sites, human-made locations, and artificial platforms in Colorado (ANOVA, df = 2; F = 0.056; P < 0.94) or Arizona (ANOVA, F = 2.596; df = 2; P = 0.08). There was, however, a significant difference in nestling periods among the three nesting substrates as young remained in artificial platform nests significantly longer in Colorado (ANOVA, df = 2; F = 7.609; P < 0.01) and Arizona (ANOVA, F = 4.584; df = 2; P = 0.01; Table 2).

In Colorado platforms fledged an average of 1.65 young/nest, while 1.11 young fledged/nest on human-made structures, and natural substrates averaged 1.41 fledged young/nest (Fig. 2A). There was no significant difference in the number of young fledged/nest among the three nesting substrates (N = 92; ANOVA F = 0.38; df = 4; P < 0.82). On Mt Lemmon, AZ, platforms fledged an average of 3.26 young/nest, while 2.44 young fledged/nest on human-made structures and natural substrates averaged 1.49 fledged young/nest (Fig. 2B), and there was a significant difference in the number of young fledged/nest among the three types of nesting substrates (N = 69; ANOVA F = 8.98; df = 2; P < 0.01).

Changes in adult Western Cordilleran Flycatcher numbers following artificial platform nest introduction

In Colorado and Arizona standard mist net capture/recapture data provided us with an abundance index of birds/net-hour (after Ralph et al. 1998). In 2007 no Western Cordilleran Flycatchers were captured at our core study site. From 2008 to 2011 the proportion of captured Western Cordilleran Flycatchers was consistently less than 10% of all birds (Fig. 3a). Following platform introduction in 2012, over the next 10 years flycatcher numbers rose to greater than 30–40% of total captured birds, and the increasing trend was significant (Mann-Kendall test; tau = 0.818; P < 0.01). In Arizona, there were initially many breeding Western Cordilleran Flycatchers, and the addition of nesting platforms did not increase numbers of adult flycatchers (Mann-Kendall test; tau = 0.429; P = 0.23; Fig. 3b).

Artificial platform nest contributions to productivity

After introduction in 2012 of artificial nesting platforms in Colorado, the total annual number of Western Cordilleran Flycatcher young produced more than doubled in our study area (Table 1). The following year platform nests again doubled the number of fledged young, but in 2014 and 2015 platform nest productivity decreased. In 2016 platform nests again fledged two-times the number of young compared to natural nests. In Arizona, artificial platform nests added at least four young each year from 2019 to 2021, and 20 additional young in 2022, which was over 80% of all young fledged that year (Table 1).

Our top nest survival model included “substrate type” and was 27.2 AIC units better than both the null model and the study site model indicating very strong support for differences in nesting substrate productivity. Parameter estimates from the top model suggested substantial differences among substrate types when individual random effects were set equal to zero (Fig. 4), with nests on platforms experiencing higher survival (mean: 0.85, 95% credible intervals: 0.46–0.99) than nests in human-made structures (mean: 0.39, 95% credible intervals: 0.12–0.74) or nests placed in natural situations (mean: 0.03, 95% credible intervals: 0.01–0.11). Nonetheless, there was substantial unmodeled heterogeneity among nests (i.e., standard deviation of random effect distribution was 6.7) pointing to the high frequency of shared fates among eggs in the same nest (i.e., often all the individuals in a clutch survived to fledglings or no individuals in a clutch survived).

Comparison of predation rates at natural vs artificial platform nests

In Colorado, during the four years prior to introduction of nesting platforms, loss of Western Cordilleran Flycatcher eggs averaged 14.5% annually (n = 115 nests), while nestling predation was 3% of all hatched young. Principal nest predators, as identified from direct and video-recorded nest observations, included the Black-billed Magpie (Pica hudsonia) and Steller’s Jay (Cyanocitta stelleri). We also observed one American Robin (Turdus migratorius) involved with egg removal and the American red squirrel (Tamiascirus hudsonicus) with young predation. We had hypothesized that predation rates would be higher on nesting platforms, but this was not the case. From 2012 to 2016, 26.3% of eggs at platform nests were depredated, while 17.9% of natural nests and 5.88% of nests on human-made structures were eaten by predators. Even though there was higher egg predation on experimental platforms, there was no significant difference among the three substrates (X² = 1.13, df = 2, N = 159, P = 0.57). Nestling depredation averaged 10.7% on artificial platforms, 25.0% on human-made structures, and 15.6% at natural locations, and there was no significant difference among the three (X² = 1.94, df = 2, N = 69, P = 0.38).

When compared to Colorado, in Arizona Western Cordilleran Flycatchers experienced higher egg and nestling predation rates. Of 257 flycatcher eggs monitored in Arizona, 34.25% were lost to predation. Nestling predation averaged 21% in Arizona when compared to 9.9% in Colorado. Magpies are absent in Arizona, but mist net capture rates of Steller’s Jay and Common Raven (Corvus corax), two major egg predators, were higher in Arizona. Nest-video playback substantiated that jays and ravens contributed to the higher Arizona egg and nestling predation rates.

DISCUSSION

Nest platform types and influence on adult numbers

We found three platform structures that were not utilized by Western Cordilleran Flycatchers (completely flat, 3-sided, and closed 4-sided with entry hole), and only the 2-sided structure with roof was used for nesting (Appendix 1). By installing 2-sided nesting platforms, we found that at locations where flycatcher breeding numbers were low, adult numbers could be increased. But in an area that contains more suitable natural nesting sites and higher adult populations, like Mt Lemmon, AZ, the addition of nesting platforms did not increase breeding bird numbers. The increase in adult Western Cordilleran Flycatcher numbers that we documented in Colorado is like what has been found when nest boxes have been added to hole-nesting bird species (Mänd et al. 2009, Hannay et al. 2022).

Nest platform influence on breeding behaviors

There were no significant differences in clutch size, incubation period, or four adult breeding behaviors (nest building, attentiveness, feeding, and brooding rates) between platform nests and nests at other locations. But the nestling period at platform nests was approximately 1 day longer than on other substrates, and this difference was statistically significant. Platform nests had higher fledging rates than did nests on other substrates. We have demonstrated that, with the addition of nesting platforms, the number of fledged young can be more than doubled per unit area of habitat. A contribution to this success was likely the more secure nest-basal area that platforms provided for nesting. Nests that were placed on narrow ledges or small crevices sometime failed because of structural constraints.

Nest platform influences on productivity

Western Cordilleran Flycatchers are nest-site limited in many areas of the intermountain west (Martin et al. 2015), and our results demonstrate that artificial nesting structures can enhance productivity. This would be especially true when platforms were placed at locations where there are not initially large numbers of breeding Western Cordilleran Flycatchers. Following introduction of nesting platforms in Colorado, production of fledged young in our core study site doubled, while in Arizona artificial platforms added annually from 16% to 80% additional fledged young.

Predation at artificial nest platforms

Our initial hypothesis was that there would be higher predation rates on platform nests, as predators might develop a “search image” for the structure. This was, however, not the case and in fact we found predation lower on platform nests when compared to nests at natural locations and on human-made substrates. This may be because the platform roof hides nests from aerial predators, while natural nest locations are closer to the ground and eggs are more visible, as Chiavacci et al. (2015) demonstrated in grassland birds. It may be that over time predators will develop a search image for the platforms and focus on them as a potential food resource, but during the 15 years of our study this did not occur.

Conservation

As demonstrated in this study and found in other areas of the world (Kuitunen and Aleknonis 1992, Weeks 1979, 2020, Warakai et al. 2013), crevice nesting birds will readily utilize artificial platforms. This should also hold true for other regions where crevice-nesting birds occur. For example, Warakai et al. (2013) found in New Guinea that crevice nesting birds will readily utilize artificial platforms. In Hawaii, utilizing nesting platforms might prove beneficial for enhancing populations of crevice-nesting birds like the endangered Akepa (Loxops coccineus). Freed et al. (1987) found that Akepa will not accept nest boxes, but this endangered species might possibly benefit from the addition of nesting platforms. A slightly larger platform could also be used for the declining Hawaiian Thrush (Myadestes obscurus) as, like the Western Cordilleran Flycatcher, this endemic thrush utilizes bark crevices for nesting (van Riper and Scott 1979, Wakelee and Fancy 2020). Even for non-endangered species, like the Western Pacific Slope Flycatcher and Black Phoebe (Sayornis nigricans), placement of nesting platforms could provide a buffer by adding additional young each year to those populations. Our study clearly demonstrates that artificial platforms can be utilized to increase numbers and to enhance productivity of the crevice-nesting Western Cordilleran Flycatcher. We hope that the addition of artificial nesting platforms as a management tool will prove useful for scientists who wish to take advantage of more readily accessible bird nesting locations and for the conservation of crevice-nesting bird species of concern.

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