Introduction

Billions of birds are killed annually worldwide by window collisions (Klem, 2015). In the United States, the annual estimated death toll is 365 to 988 million (^{Loss, Will, Loss, & Marra, 2014}) and 16 to 42 million in Canada (^{Machtans, Wedeles, & Bayne, 2013}).Although many of these deaths are caused by collisions with high rise urban buildings (Klem, Farmer, Delacretaz, ^{Gelb, &S aenger, 2009}; ^{Hager & Craig, 2014}; ^{Cusa, Jackson, & Mesure, 2015}), the majority of deaths occur on single and two-story residences in suburban and rural areas due to their proportionately larger numbers (^{Machtans et al., 2013}; ^{Loss et al., 2014}) and surrounding habitat features, such as the presence of trees (^{Gelb & Delacretaz, 2009}; ^{Klem et al., 2009}; ^{Borden & Lockhart, 2010}). ^{Loss et al. (2014}) and ^{Machtans & Thogmartin (2014}) recommended assessing the vulnerability of specific species to window collisions in order to determine effects on their respective populations and to establish species-oriented conservation management priorities.

While many studies document bird collisions in the United States and Canada, relatively few studies have taken place in the Neotropics; these include studies from Colombia (^{Agudelo-Álvarez, 2006}; ^{Agudelo-Álvarez, Moreno-Velasques, & Ocampo-Peñuela, 2010}; ^{Ocampo-Peñuela et al., 2016}), Costa Rica (^{Graham, 1997}; ^{Menacho-Odio, 2015}; ^{Oviedo & Menacho-Odio, 2015}), Mexico (^{Cupul-Magaña, 2003}; ^{Gómez-Moreno, Herrera-Herrera, & Niño-Maldonado, 2018}), and Brazil (^{Briske, Campos-Silva, & Piratelli, 2017}; ^{Santos, Ferreira, & Ferreira, 2017}). Studies conducted in Important Bird Areas (IBA) are even more infrequent (^{Santos et al., 2017}).

Monteverde-Arenal is one of 21 Important Bird Areas (IBA) designated for Costa Rica (^{Sánchez, Criado, Sánchez, & Sandoval, 2009}). Its creation was based on three global criteria: the presence of threatened species, species with restricted range, and restricted biome (^{Bird Life International, 2017}). Monteverde is a well-known tourist destination (^{Davis, 2009}) where urban development has favored the use of panels of glass in the infrastructure which is often within or adjacent to forest areas (^{Menacho-Odio, 2018}). Although ^{Young & McDonald (2014}) stated that hundreds of birds die every year due to window collisions in Monteverde, no studies describe the species affected.

In this study, we documented species and quantified species-specific characteristics of window casualties in the highly diverse rural Monteverde area, Costa Rica (^{Young & McDonald, 2014}). We include characteristics such as the life zones where they were found, migratory habits, forest dependence, trophic guild, weight, endemism, and conservation status of each species. Our findings provide a baseline for future studies to understand and quantify the full extent of this problem in Monteverde as well as elsewhere in Costa Rica and the Neotropics.

Material and methods

Study area: We conducted this study from August 2014 to December 2017 in Monteverde, located on the Pacific slope of the Tilarán mountain range (10°18’ N, 84°49W; elevation 700-1 400 m a.s.l.), Puntarenas, Costa Rica. The category “Monteverde” refers to the region that includes the community area of Monteverde, the Monteverde Cloud Forest Preserve, the Children´s Eternal Rainforest, and areas on both sides of the Continental Divide down to about 700 m elevation (^{Nadkarni & Wheelwright, 2014}) (Fig. 1).

Fig. 1 Life zones and urban development of Monteverde, Costa Rica 2017 (^{Fogden, 2014}; SNIT, 2017). 

Monteverde encompasses seven life zones of the twelve found in Costa Rica (^{Holdridge, 1967}). Our study area includes three life zones (^{Fogden, 2014}): premontane moist forest, premontane wet forest, and lower montane wet forest (Fig. 1). The moisture gradient on the Pacific slope causes a more abrupt variation than on the Caribbean slope, resulting in high species diversity both for plants and birds in each life zone (^{Kappelle, 2016}).

In our study area 267 bird species had been recorded by ^{Fogden (2014}), including 156 residents, 41 altitudinal migrants, 67 latitudinal migrant species, and three latitudinal migrants with reproductive populations. This list does not include vagrant or uncertain species. Two species are endemic to the continental area of Costa Rica and 18 are restricted to the highlands of Costa Rica and western Panama (^{Obando-Calderón et al., 2017}).

The populations of most bird species in Monteverde have not been estimated. ^{Stiles & Skutch (2003}) and ^{Garrigues (2014}) provide a general idea of bird abundance. Likewise, ^{Fogden (2014}) described the abundance of the species in each Monteverde life zone. The only exception is the Three-Wattled Bellbird (Procnias tricarunculatus) for which annual counts during a six year period estimate 116-248 individuals in Monteverde (^{Sandoval, 2014}).

Data collection: We documented window-strike species by using museum records and citizen observations, and by monitoring designated buildings (^{Klem, 1989}). Specifically, in 2014 and 2017, we obtained from the National Museum of Costa Rica and the Museum of Zoology of the University of Costa Rica a list of specimens with tags specifying that they had collided with windows in Monteverde. We also obtained records from residents of Monteverde who submitted specimens of window-killed birds.

Carcass surveys: Buildings designated for carcass surveys were those of organizations dedicated to education, conservation, and research, as well as private homes of residents, because they were willing to collect and share data. These included the Monteverde Institute, Monteverde Friends School, and a private home located in premontane wet forest. In lower montane wet forest we visited the Monteverde Cloud Forest Reserve, the Monteverde Biological Station, and one private building. The private residences were chosen because the residents of one of them reported high window-strike mortality and the other one because it had large panes of glass. All the buildings are one or two stories, 2.5 m to 20 m height, and surrounded by secondary forest and gardens. Another criterion was that the buildings were close enough together for the main researcher (RMMO) to visit during a day in order to do a carcass search. In all cases, we obtained permission from property owners (Fig. 1). From March 2015 to February 2016, RMMO conducted surveys of carcasses eight days per month, with the exception of four days in April and seven days in September and February, for a total of 90 sampling days per buildings. She conducted surveys around 27 buildings including the Monteverde Institute (n = 5 buildings), Monteverde Biological Station (n = 7), Monteverde Cloud Forest Reserve (n = 8), Friends School (n = 5), and two houses belonging to area residents. The protocol consisted in looking for carcasses around each building on a transect of two meters width from the building´s wall (^{Hager & Cosentino, 2014}). She collected all the carcasses, taking pictures of the bird and the window where she judged it had hit.

From October 2014 to March 2016, we asked inhabitants of Monteverde and the staff of the collaborating organizations to collect bodies of birds killed by window collisions. We made this request through electronic media, such as the newsletters of the Monteverde Conservation League, the Monteverde Institute, and the Friends School, and through signs posted in the area, and personal visits to community meetings. The carcasses were stored in freezers at the Monteverde Institute, the Monteverde Biological Station, and the Monteverde Cloud Forest Reserve laboratory. Following this study, we delivered most of the bodies to the Natural History Department of the National Museum of Costa Rica. These specimens are registered with the following catalogue numbers: MNCR-O28910, MNCR-O28912, MNCR-O28913, MNCR-O28915, MNCR-O28916, MNCR-O28918, MNCR-O28920, MNCR-O28921, MNCR-O28923, and MNCR-O28927 (G. Alvarado, pers. comm., February 20, 2018).

Photo reports: Through social media, we also received from Monteverde residents photo reports of birds, dead or alive, that collided with windows. Photographs from bird guides were used to identify the species (^{Kummer, Bayne, & Machtans, 2016}). From March 2015 to December 2017, we received and took photographs of birds that had collided with windows at the University of Georgia Costa Rica campus in San Luis Valley, located in premontane moist forest (Fig. 1).

Naturalists: From August 2014 to December 2017, we received specimens of birds that were collected and frozen by local biologists and residents. Each specimen was identified, a photo was taken, and the date and place where it was found were registered.

Expert interviews: From August to November 2014, we interviewed five biologists who have lived in Monteverde for more than ten years. We asked them to identify the species they were aware of striking windows most frequently.

Community survey: During 18-22 February 2016, with the collaboration of two volunteers, we interviewed 58 Monteverde residents aged 15 and older. One survey was conducted per household. The Monteverde community area was divided into seven conglomerates with an average of 20 houses, and RMMO and two volunteers visited each conglomerate during half of a day, aiming to visit as many houses as possible. The survey questionnaire asked: Has a bird ever died when it hit a window in this house? The people who answered affirmatively were asked: What species have hit the house? A list of species was obtained and the frequency of strike records per species was collected. Additionally, residents were shown eight pictures with names: Green Hermit (Phaethornis guy), Swainson’s Thrush (Catharus ustulatus), Three-wattled Bellbird, Resplendent Quetzal (Pharomachrus mocinno), Long-tailed Manakin (Chiroxiphia linearis), White-tipped Dove (Leptotila verreauxi), Lesson’s Motmot (Momotus lessonii), and Northern Emerald-Toucanet (Aulacorhynchus prasinus). We asked each person: Which of these birds died colliding with windows? We categorized the answers as “yes”, “no”, “I don´t know” or “no answer” for each picture.

Data analysis: We identified each specimen and designated the casualties according to categories: 1) Migratory status (^{Fogden, 2014}): residents, altitudinal migrants, latitudinal migrants, and breeding resident with latitudinal migration populations; 2) Forest dependence (^{Stiles & Skutch, 2003}; ^{Biamonte, Sandoval, Chacón, & Barrantes, 2011}): 1= species that live in mature forests, 2=species that live in habitats with 50 % or less forest covers, and 3=species that live in open areas. Species between a-b and c-d where classified as 1.5 and 2.5 using data from ^{Stiles & Skutch (2003}); 3) Feeding guilds (Stiles & Skutch, 2003; ^{González-Salazar, Martínez-Meyer, & López-Santiago,2014}): frugivore, insectivore, granivore, nectarivore, carnivore, aquatic invertebrates eaters, omnivores, and scavengers; 4) Weight (^{Stiles & Skutch, 2003}); 5) Abundance (Fogden, 2014): Common, Fairly common, Uncommon, Rare, Status Uncertain, Vagrant, and not reported in the zone;6) Endemism (^{Garrigues et al., 2017}): Endemics to continental Costa Rica, Endemics to highlands of Costa Rica and western Panama, Endemics to Caribbean, Honduras to Panama, and Endemics to Eastern Honduras to western Panama; 7) Conservation and population trend status (^{IUCN, 2017}; ^{SINAC, 2017}).

Carcasses collected were compared by age, sex, and collision frequency per month. To determine the species that collided most frequently, we used records from interviews with biologists, specimens collected, and the area surveys.

Results

Number of species: We recorded 103 species, classified in 25 families and eight orders, that collided with windows in Monteverde (Table 1). 69 % (71 species) belonged to the order Passeriformes. We recorded 98 species (36.7 %) of the 267 species documented and five species not registered in the three life zones by ^{Fogden (2014}). The families with the most species represented were Parulidae (14), Trochilidae (13), Turdidae (10), and Tyrannidae (9).

Migratory status: Most collisions were by resident species, including 57 no migrants and 21 altitudinal migrants (Table 2).Twenty one (51 %) of 41altitudinal migrants documented by ^{Fogden (2014}) collided with windows.

Species and carcasses collected by method: Carcasses gathered by the Monteverde community resulted in 130 bodies of 46 species. Naturalists collected 81 bodies of 33 species. Photo reports generated data for 67 individuals of 41 species. Museums reported 22 individuals of 14 species, and the carcass survey allowed collecting 22 bodies of 14 species.

Forest dependence: We found different levels of forest dependence among the species that collided with windows. Most species (35 %) occur in areas with at least 50 % of forest cover and just five species were classified as highly dependent on mature forest and secondary forest (1 and 1.5) (Table 3). In lower wet montane forest we found thirteen of 31 (42 %) species that collided as highly dependent on mature and secondary forest (1 and 1.5),13 (54 %) were also found to collide in the premontane wet forest; and only five (21 %) in the premontane moist forest (Table 1).

Feeding guilds: Collisions were mostly by insectivore species (57spp) (Table 4). Only 36 % of the insectivore species documented by ^{Fogden (2014}) were found to collide. On the other hand, 64 % of nectarivore and 63 % of frugivore were documented to have collided with glass panes. Insectivores not documented in our study as window-kills include swallows, woodpeckers, and 28 species (76 %) of the Tyrannidae family (^{Fogden, 2014}). Alternatively, the Olive-striped Flycatcher hit frequently in lower montane wet forest. No carnivores, scavengers, omnivores, nor birds that feed on aquatic macroinvertebrates were recorded as window casualties (Table 4).

Weight: Most collision species weighed less than 80 grams (84 spp.; 81 %). The smallest species were Canivet’s Emerald (Chlorostilbon canivetii) 2.6 g and the Ruby-throated Hummingbird 2.8 g. The heaviest were Gray-headed Chachalaca (Ortalis cinereiceps) 500 g.; Keel-billed Toucan (Ramphastos sulfuratus) 500 g.; Montezuma Oropendola (Psarocolius montezuma) 520 g.; and the Black Guan (Chamaepetes unicolor) 950 g. Most of the birds are small or medium sized. Neither Accipitridae family species documented in the area (n =15, Mdn = 480 g) were found to collide with windows, nor Cathartidae family species (n = 2, Mdn = 1600 g).

Abundance per life zone: More species collided with windows in premontane wet forest than in premontane moist forest or in lower montane wet forest (Table 5). Nine species were found in the three life zones, 21 only in premontane moist forest, 32 only in premontane wet forest, and 16 only in lower montane wet forest (Table 1). Most victims in all life zones were classified as Common and Fairly Common, but among the affected were also species previously classified as Uncommon, Rare, Vagrant, and others not reported by ^{Fogden (2014}) in the life zones (see Table 5).

Endemism, population trend, and species vulnerability status: We found 12 endemic species colliding with windows, four of them with declining populations (^{Garrigues et al., 2017}). Thirty-six species were recorded having declining populations while 34 are considered as stable populations; 18 increasing, and the population trend for 15 species is unknown (^{IUCN, 2017}). According to the ^{IUCN Red List (2017}), most collision species are classified in the category of Least Concern (97 spp.; 94 %) and five as Near Threatened, and one Vulnerable (Table 1). Six species are on the List of Endangered Species of the National System of Conservation Areas of Costa Rica and five on the list of reduced populations or threatened species for Costa Rica (^{SINAC, 2017}) (Table 1). The populations of six endemic species are considered to be decreasing or unknown and are on the list of endangered species of Costa Rica: Black-thighed Grosbeak (Pheucticus tibialis), Three-wattled Bellbird, Black Guan, Black-faced Solitaire, Golden-browed Chlorophonia, and Orange-bellied Trogon (Table 1).

Species that collide most frequently with windows:

A) Expert interviews. Five resident biologists interviewed cited Swainson’s Thrush as the migratory species that collides with windows in higher numbers than other species in the area, mainly during April-May and September-October. Three of the five interviewees stated that, among the resident species, the Northern Emerald-Toucanet collides most often, the other two judged Clay-colored Thrush (Turdus grayi), pigeons, and hummingbirds, Long-tailed Manakin, and Keel-billed Toucan.

B) Community survey. A total of 58 residents answered the survey, one per house in premontane wet forest and lower montane wet forest. Forty-three surveyed residents (74 %) could recall a bird hitting their home windows at least once. Of these, the Northern Emerald-Toucanet was described (n= 16) as the most common casualty, followed by hummingbirds (n = 15), and pigeons (n = 9). After showing pictures of eight species, 35 (60 %) householders recognized and reported that Swainson’s Thrushes collided with windows, and 34 (59 %) reported hummingbirds and Northern Emerald-Toucanets.

C) Museum records. Data from the National Museum of Costa Rica (NMCR) consisted of 14 window-killed specimens of seven species from Monteverde; of that total, eight specimens (57 %) were Swainson’s Thrushes (Table 1). Eight window-killed specimens of seven species were in the collection of the Zoology Museum of the University of Costa Rica: Silver-throated Tanager (Tangara icterocephala), Mountain Elaena (Elaenia frantzii), Scaled Antpitta (Grallaria guatimalensis), Purple-throated Mountain-gem Lampornis calolaemus, Olive-striped Flycatcher (Mionectes olivaceus), Gray-throated Leaftosser (Sclerurus albigularis), and two Northern Emerald-Toucanet specimens, including a specimen collected by Gary Stiles in 24 December 1978.

D) Naturalists. Eighty-one frozen specimens (31 species) collected prior to this research and stored in freezers were shared by naturalists (biologists and people interested in nature) of Monteverde. Date of collection was not always available, but the oldest specimens with information dated from 2008. Twenty- eight specimens were collected in lower montane wet forest, 45 in premontane wet forest and eight did not have information about the place of collection. The most abundant species was Swainson´s Thrush (n=16), followed by Black-faced Solitaire (n=10), Northern Emerald Toucanet (n= 7), and Long tailed Manakin (n=7).

Carcass surveys: From October 2014 to August 2016 a total of 152 carcasses consisting of 50 species classified in 15 families were collected by at least 63 volunteers (n = 130 carcasses) and RMMO (n = 22) in Monteverde between 4 October 2014 and 29 August 2016 in premontane wet forest and lower montane wet forest. One hundred and six bodies (70 %) were collected in premontane wet forest and 45 (30 %) in lower montane wet forest. The most common species were Swainson’s Thrush (n = 34), Northern Emerald-Toucanet (n =14), Black-faced Solitaire (Myadestes melanops) (n=14), and the Olive-striped Flycatcher(n = 10). The families with the highest strike rates were Turdidae (n = 57), Trochilidae (n = 23), Tyrannidae (n = 16), and Ramphastidae (n = 14). We were not able to identify two carcasses to the species level.

The most common species collected in premontane wet forest was the Swainson’s Thrush (n = 26) and the Northern Emerald-Toucanet (n = 14), whereas the most common species collected in lower montane wet forest was the Black-faced Solitaire (n = 14), followed by Swainson’s Thrush (n = 6), and Olive-striped Flycatcher (n = 6). Ten (71 %) of 14 Black-faced Solitaire were obtained from 24 June to July 25, 2015 in lower montane wet forest.

In relation to age and sex, 75 (49 %) of window casualties were mature, 60 (39.5 %) immature, and 17 (11 %) age unknown. For specific species, Northern Emerald-Toucanet specimens consisted of five (36 %) immatures and nine (64 %) matures; the number of casualties were equal for Black-faced Solitaire with six (50 %) immatures, six (50 %) matures; Swainson’s Thrush consisted of 19 (56 %) matures, 11 (32 %) immatures; and four unknown (12 %).

For all individuals of all species combined, 59 were males, 33 females, and 60 unknown. For Swainson’s Thrush we identified seven females, nine males, and 18 unknown. For Northern Emerald-Toucanets, both sexes were equally represented with six males, six females, and two unknown. For the Black-faced Solitaire we counted seven males, two females, and five unknown.

In relation to migratory status, 62 (41 %) were altitudinal migrants, 51 latitudinal migrants (34 %), and 37 residents (25 %). We found an increase in the number of long distance migrants colliding in April 2015 (n = 17) and October 2015 (n = 10) (Fig. 2). Fifteen (88 %) of the long-distance migrants in April were Swainson’s Thrush, the other two were Wood Thrush and a Ruby-throated Hummingbird. In October, eight of the migratory species were Swainson’s Thrush and the other two were Black-and-white Warbler and Yellow-throated Vireo (Vireo flavifrons).

Fig. 2 Number of window-killed birds collected by month and migratory status in Monteverde, Costa Rica (from October 2014 to April 2016, n = 145). 

The number of resident species colliding with windows increased in June and July. The number of immature residents collected increased only slightly in June (nine immature, seven mature), but more so in July (11 immature, five mature) and August (seven immature, 0 mature) (Fig. 3.)

Fig. 3 Number of window-killed resident birds collected by month and age in Monteverde, Costa Rica (from January 2015 to December 2015). 

Discussion

A substantial number of bird species known to occur in Monteverde were documented as victims of bird-window collisions. Even though the number of species recorded in this study is much higher than the 57 species registered in Reserva Particular do Patrimônio Natural Santuário do Caraça, Minas Gerais, an Important Bird Area in Brazil (^{Santos et al., 2017}) and 25 species in a rural area in Colombia (^{Ocampo-Peñuela, Peñuela-Recio, & Ocampo-Durán, 2015}), it likely underestimates the true number. Some of the factors for the presumed underestimation of our results are: a) in museums, it is not mandatory for the specimen tag to indicate the cause of death of birds. b) With the use of the citizen science approach, participants may not have been aware of or interested in this study. Participation could have been affected by motivation (^{Cooper, Dickinson, Phillips, & Bonney, 2007}), time available to search for and to report dead birds, cost of taking the carcasses to the different storage freezers, and the long time frame of the study. Even when participants were highly motivated, they could have failed to hear a bird striking a window or could have been absent during the collision event (^{Bracey, Etterson, Niemi, & Green, 2016}). Despite this, citizen science was crucial for gathering data and also probably increased awareness and knowledge about this subject (^{Chandler et al, 2017}; ^{Cooper et al., 2007}). c) A further factor that could have affected bird collisions detectability was the removal of the bird bodies by scavengers, such as coatis (Nasua narica) and domestic cats (Felis domesticus ). Also, carcasses of birds that fell into vegetation, or injured birds that moved before dying, would also go undetected (^{Bracey et al., 2016}).

We found that most species affected by window collisions in the study area were residents. ^{Santos et al. (2017}) and ^{Ocampo-Peñuela et al. (2015}), also found mostly resident species in their studies in rural conditions in the Neotropics. It is possible that the high density of year-round resident birds of Monteverde, and the deception caused by reflection of vegetation in windows, causes the high mortality of resident individuals (^{Gelb & Delacretaz, 2009}; ^{Klem et al., 2009}; ^{Borden &Lockhart, 2010}). Since both juveniles and adults were found among the victims, we think birds are unable to learn to avoid the danger, as ^{Klem (1989}) also concluded.

We documented window-kills in endemic Neotropical families, including guans (Cracidae), toucans (Ramphastidae), motmots (Momotidae), and manakins (Pipridae). ^{Ocampo-Peñuela et al. (2016}) and ^{Santos et al. (2017}) also found species of these families in their studies in rural areas in the Neotropics. The Northern Emerald-Toucanet was cited as one of the species most often reported as victim of window collisions.

As ^{Loss et al. (2014}) described for the United States, warblers, hummingbirds, and thrushes also were found to be vulnerable in Monteverde. The families Parulidae and Trochilidae had the highest number of species affected. ^{Menacho-Odio (2015}) found that hummingbirds are frequently reported as window casualties in Costa Rica. ^{Klem (2014}) mentioned that any attractant to the vicinity of the windows increases fatalities. It is likely that the presence of flowering plants and feeders in home and institutional gardens attract hummingbirds, thus increasing their risk of hitting glass in the buildings. In this case, incentives, like the Costa Rican government´s Certification for Tourism Sustainability, that promote planting vegetation to attract resident and migratory birds around hotels should be re-evaluated. Simultaneously, methods preventing window collisions should be promoted (^{Instituto Costarricense de Turismo, 2013}). Hummingbird behaviors such as high-speed flight, male territoriality and associated aggression, may also increase the risk of collisions (^{Graham, 1997}). Artificial feeders are common in Monteverde and in many other touristic areas of Costa Rica. In Costa Rica it is illegal to feed wildlife; however, this regulation is not enforced so it is important to communicate to residents and managers of lodges that feeders should be located less than one meter from any glass surface (^{Klem, 2014}). Most of the Parulidae species found are migrants; among species extremely vulnerable to building collisions (^{Loss et al., 2014}), all of which we found window killed in Monteverde, include Golden-winged Warbler (Vermivora chrysoptera), Canada Warbler (Cardellina canadensis), and Kentucky Warbler (Geothlypis formosa). In their study, ^{Loss et al. (2014}) pointed out that, for these three species, building collisions may contribute to their population decline. In our study, we found that buildings in rural areas may also be affecting bird populations during migration.

Swainson’s Thrush and Black-faced Solitaire, two species found to collide with windows in high numbers in Monteverde, belong to the Turdidae family. In Colombia, ^{Ocampo-Peñuela et al. (2016}) found that the Black-billed Thrush (Turdus ignobilis) was one of the two species with highest collision frequency. ^{Klem (2014}) described how birds such as hummingbirds, ovenbirds, and thrushes regularly fly through restricted passageways in dense vegetation, making them more susceptible to hit windows frequently. Although neither the expert interviews nor community survey indicated that the Black-faced Solitaire was killed in large numbers, we found a relatively high number of individuals of this species colliding with buildings in the lower montane wet forest. Since most people we interviewed do not live in the lower montane wet forest, we suspect that many window related deaths of the Black-faced Solitaire were overlooked. For this reason, we can expect that more urbanization in lower montane wet forest will increase the mortality of the Black-faced Solitaire, a species considered Vulnerable in Costa Rica due to the popular practice of capturing and keeping them as pets in cages (^{Stiles & Skutch, 2003}; ^{Arévalo, 2010}; ^{Oviedo & Menacho-Odio, 2013}).

Most species that collided with windows in our study were insectivores, the trophic guild with the most species in Monteverde (^{Young & McDonald, 2014}). However, the number of insectivores killed was small, based on the total number of species in this trophic guild. Insectivores not documented as window-kills include swallows, woodpeckers, and many species of the Tyrannidae family (^{Fogden, 2014}). Alternatively, the Olive-striped Flycatcher hit frequently in lower montane wet forest.

Among the omnivore species, the Brown Jay (Psilorhinus morio) is considered common in Monteverde, but there was no evidence of collisions for this specie. ^{Kahle, Flannery, & Dumbacher (2016}) mentioned that flock species may be underrepresented because it may easier for them to detect windows and signal to others.

Although we did not find carnivorous and omnivorous species as windows casualties, they are recorded as victims in other studies (^{Menacho-Odio, 2015}; ^{Kahle et al., 2016}). Based on this, we believe that with an increased sampling effort, birds of these trophic guilds would be likely recorded striking windows in Monteverde.

The number of window collision casualties considered Endangered, Near Threatened, Vulnerable, and Endemic species demonstrates cause for conservation concern. Thus the increasing number of buildings and the preference for large panes of glass in residences and tourist structures (^{Menacho-Odio, 2018}), should be monitored and regulated to reduce avian mortality. It is also necessary to focus attention on species such as the Orange-bellied Trogon, Black Guan, Three-wattled Bellbird, the Golden-browed Chlorophonia and other Neotropical endemic species whose populations are decreasing.

Twenty-one altitudinal migrant species were recorded as casualties in this study. Window collision risk is predicted to increase as birds move to lower more urbanized areas during their migratory passage. The Three-wattled Bellbird, the Black-faced Solitaire, the Resplendent Quetzal, and the Golden-browed Chlorophonia are known window casualties that are altitudinal migrants. Our results document window strikes by two juvenile Three-wattled Bellbirds; one (survived) in Los Llanos and another (died) in Monteverde (premontane wet forest), during June and July 2015 (Fig. 1). An average of 90 Three-wattled Bellbirds were counted in the last six years in Monteverde (^{Sandoval, 2014}), making every individual killed in a window a significant loss to the local population (Debra Hamilton, pers. comm., 2015.).

The expected increase in the number of buildings in Monteverde is anticipated to intensify the risk of bird-window collisions, as occurred in the premontane wet forest in our study area. Currently, Costa Rica Wildlife Conservation Law (^{SINAC, 2017}) does not include any regulation for preventing bird-window related deaths. Moreover, there are no regulations in Monteverde to prevent an increase in urbanization. Consequently, window-kills are expected to become an increasing threat to the local avifauna. Education and research for collision prevention will be required for conservation, economic, legal, and ethical planning to avoid the unwanted losses of the birds that more and visitors come to Monteverde to see (^{Klem, 2014}). For example, there is a need to promote the use of effective methods to prevent bird-window collisions and to discourage the use of non-effective large panes of glass on new buildings.

Future studies should address the estimated or, ideally, actual abundance of specific species in and around human dwellings, and the use of more rigorous protocols by volunteers, as suggested by ^{Loss, Will, & Marra (2015}), to obtain a more comprehensive understanding. It is essential to include the monitoring of hotels and other commercial buildings in future investigations, given risks these specific structures, with their large areas of sheet glass, pose to birds.

Bird-window collisions affect a high number of species of birds in Monteverde. Most window-killed species are residents, some endemics, threatened with declining populations. Seeking a solution to protect birds from windows should be an important goal for this IBA and the rest of Costa Rica.

We suggest that additional research should include a greater proportion of the population of this area in order to increase concern and knowledge about this important conservation issue for birds and people, especially among commercial interests such as hotel owners and others employed in the birding-related industry. Additionally, more effort should be given to areas where high diversities of birds coexist with buildings boasting large surface area in windows. This study reinforces the importance of public participation (citizen science) for achieving a comprehensive understanding of the problem.

Ethical statement: authors declare that they all agree with this publication and made significant contributions; that there is no conflict of interest of any kind; and that we followed all pertinent ethical and legal procedures and requirements. All financial sources are fully and clearly stated in the acknowledgements section. A signed document has been filed in the journal archives.