Avian Behavior, Ecology, and Evolution

INTRODUCTION

Few studies have addressed natal dispersal in small birds even though it is believed to play a major role in population growth and persistence (Wiens et al. 2006, Cox et al. 2014). Dispersal may be especially important for species that occupy a highly specialized niche and that rely on this process to find and colonize new habitat patches (Shitikov et al. 2012). Irruptions are defined as important influxes of individuals that are localized in time and space (Newton 2006). Such movements are usually linked to the exploration of breeding areas of high food availability but can also result from important exoduses due to a lack of food (Bock and Lepthien 1976, Koenig and Knops 2001). Although they are often considered resident species, boreal woodpeckers, such as Black-backed Woodpeckers (Picoides arcticus; BBWO) and American Three-toed Woodpeckers (P. dorsalis; ATTW), irrupt south of their respective ranges in certain winters but not necessarily synchronously (Tremblay et al. 2020a, 2020b). These irruptions are thought to be associated with an important increase in boreal woodpecker’s productivity originating from wildfires or large-scale insect outbreaks (West and Speirs 1959, Yunick 1985). In Black-backed Woodpeckers, colonization of recent burns is predominantly done by second year (SY) birds and thus related to natal dispersion (Huot and Ibarzabal 2006, Tremblay et al. 2015, Siegel et al. 2016). Whether irruptions of boreal woodpeckers indeed result from post-fledging movements remains to be determined however, as no study has yet documented the age and sex of individuals involved in such events.

For species such as boreal woodpeckers, ATTW and BBWO, who have juveniles that are not readily distinguishable from adults in the field, capturing individuals and aging them with moult patterns can contribute to a better understanding of the movements of the different age classes (Pyle 1997, Froehlich 2003, Johnson et al. 2011). Although mist netting remains the most prevalent capture method for forest birds, its capture efficiency varies greatly among species (Jenni et al. 1996, Remsen and Good 1996, Dunn and Ralph 2004) and habitats (Dunn and Ralph 2004). Standardized mist-netting operations using standard 30 mm mist-net to capture passerines in migration, are not efficient at capturing larger woodpeckers because of their body size (Wang and Finch 2002). In addition, because of their behavior and preferred habitat, visual migration counts are not always possible, meaning that bird counts acquired through capture could prove a useful tool for population management (Dunn and Ralph 2004).

Few techniques have been developed to improve the capture rates of woodpeckers undergoing dispersal or migration. Most methods for capturing nesting woodpeckers are inappropriate for documenting woodpecker’s dispersal and migration movements. Capturing boreal woodpeckers is particularly challenging outside of the breeding season because capture rates using mist nets for these birds are typically low, and boreal woodpeckers seldom visit feeders. This has led some authors to capture individuals using netguns as a last resort (Lehman et al. 2011). The use of audio lures to increase detection rate in audio/visual inventory is frequently used on different woodpecker species (Kosinski et al. 2004, Michalczuk and Michalczuk 2006, Wynia et al. 2019, and others). Yet, this technique is seldom used for large-scale capture operations. In addition, its efficiency at increasing capture rate has not been quantified.

In this study, we investigated the efficiency of a new capture technique specifically targeting this group of species and the age-class structure of dispersing boreal woodpeckers. We captured boreal woodpeckers using either enhanced capture methods (EMC) including playback of their calls broadcasted from within a mist net enclosure where dead snags were planted or with standard passive mist-netting. We then examined the age-class structure of two woodpecker species (Black-backed and American Three-toed Woodpeckers) caught using EMC, evaluated the capture rates of both methods, and finally compared the capture rates using EMC with standardized visual counts to assess the capacity of the former to estimate dispersal intensity. Because EMC may attract specific population classes (Whalen and Watts 1999, Lecoq and Catry 2003), we assessed whether capture rates using EMC differed from a 1:1 sex ratio (F:M) or from the expected productivity of 1:1.5 (adult:juvenile; Nappi and Drapeau 2009, Tremblay et al. 2015). We hypothesized that capture sex ratio using EMC would be unbiased because in both species, both sexes show aggressive behaviors against intruders or counterparts (Lawrence 1967). We hypothesized that capture rates would be biased toward juveniles because irruptions are thought to be driven by natal dispersal and post-fledging movements (Huot and Ibarzabal 2006, Tremblay et al. 2015, Siegel et al. 2016).

METHODS

Study site

The village of Tadoussac, located on the north shore of the St. Lawrence River at its confluence with the Saguenay River (Fig. 1), lies along an important southward migration corridor for raptors (Berthiaume et al. 2009) as well as for passerines and transient boreal woodpeckers (Savard and Ibarzabal 2001). A seasonal banding station, the Observatoire d’oiseaux de Tadoussac (OOT; 48°09′N, 69°42′W; Fig. 1) was set up in 1994 to monitor and study fall migrating birds. The immediate area around the capture site (250 m radius) is covered by a very low density of jack pines (Pinus banksiana), spruces (Picea spp.) and balsam fir (Abies balsamea) over sandy naked soil. At a broader scale, the site is characterized by mix of deciduous and coniferous forest stands. This site, and its surroundings (> 10 km) is not proper for boreal woodpeckers because the site does not contain mature trees or recently burned areas, therefore the number of individuals that are not transient is null or limited.

Visual counts

We counted dispersing woodpeckers daily between 2000 and 2003 and over hourly periods from 2004 until 2006 following Explos-Nature protocols (Côté and Dumas 2022). We used two sites for visual counts, one located on the marine terraces (Dionne and Occhietti 1996) bordering the St. Lawrence River and the other located 800 m inland from the first. Counts were performed simultaneously at the two sites by the same two observers who alternated between the sites every day for the entire season. Counts started each day at 08:00 and stopped at around 14:00, usually from mid-August until mid to late November. All birds actively moving south that were detected and properly identified, either by sight or by ear, were recorded. The observation point closer to the shore was located at 130 m from the mist net locations (slightly more inland) so that the observers could not see either the nets or the birds therein (Fig. 1). These daily counts were used to estimate dispersal intensity.

Mist netting

Mist netting was performed concurrently with visual counts (i.e., 2000–2006) using two methods: (1) passive mist netting involving one to eight nets set at different locations, depending on financial resource capacity, and (2) EMC where birds were actively attracted with playbacks broadcasted from within one to three net enclosures planted with dead snags depending on years and weather. Although the location of the passive nets did change at a local scale between years, the EMC locations remained the same but were discontinued after 2006.

Passive mist netting

Passive capture effort (hours of activity × length of nets in meters) included a varying number of mist nets set from mid-August/late September until late September/late October of each year, with up to 71 days of mist netting per season. Although some changes in net locations occurred between years, nets were always positioned in known woodpecker and passerine trajectories to optimize the capture of transient individuals. Passive nets were at least 150 m away from active enclosures to limit the impact of the active systems on their capture rates. No passive captures were made in 2003 and 2004. Primarily aimed at capturing passerines, nets were 9 to 12 m long with a mesh size of 30 mm. Nets ranged from 20 to 30 cm from the ground to a maximum height of 2.6 to 2.7 m. Passive netting began 30 min before sunrise and continued for 7 hours, weather permitting.

Enhanced capture methods (EMC)

The net enclosure consisted of four mist nets forming a closed squared area. Nets (Avinet 110/2) were 9 m long by 2.8 m high with a mesh size of 60 mm. Nets ranged from 20 to 30 cm from the ground to a maximum height of 2.6 to 2.7 m. Trees within the enclosure were kept at or below the nets’ height so that birds flying on and off these trees were more likely to be captured. Two to three snags were planted vertically in the ground within the enclosure as perching areas to encourage woodpeckers to enter the enclosure. These planted snags were kept at or below the height of nets to lower birds’ flight altitude. In the center of the enclosure, we installed a sound broadcasting system comprising two speakers set off the ground and opposite each other (Ibarzabal and Desmeules 2006). The broadcasting unit could produce sounds having an amplitude of 100 dB, although we always kept the amplitude lower (at approximately 75 dB) to avoid sound saturation. We continuously broadcasted a single track consisting of a mix of calls and drumming of Black-backed and American Three-toed Woodpeckers for a period of up to seven hours; playbacks generally began 30 min after sunrise, depending on field conditions. Both the call and drumming soundtracks were from the 1997 Stokes field guide to bird songs (Stokes et al. 1997). EMC effort varied from year to year but usually spanned from mid-September until early November.

Statistical analyses

The capture rate (captures/net meters-hour) was modeled using negative binomial models for each species. A set of two candidate models was designed for model selection based on AICc (Burnham and Anderson 2002) to quantify the effect of EMC on capture rate. The first model only included the effect of dispersal intensity (observed transient individuals divided by the visual count effort in time) on capture rate (H₀), while the second model included both the dispersal intensity and the enhanced capture effort (H₁). Both candidate models contained an offset of the natural logarithm of capture effort. We compared both candidate models based on their AICc. The objective behind the previous model selection is to provide an order of magnitude of the efficiency of the EMC on boreal woodpeckers’ capture rate. Despite smaller mesh size, passive netting is still efficient at capturing transient boreal woodpeckers.

Capture biases per species for sex and age ratios were investigated using t-tests. Using simple linear models, we found no significant effect of capture effort either on sex or age ratio therefore, no capture effort was included in these analyses. The null hypothesis for the t-tests was a theoretical 1:1 ratio of female:male, which is accepted as the normal ratio for the wild population (Clutton-Brock 1986), and a ratio of 1:1.5 adult:juvenile. Our use of the 1:1.5 ratio is conservative as the highest observed productivity ranges from 2 to 3 juveniles per couple (Nappi and Drapeau 2009, Tremblay et al. 2015). All analyses were conducted using R 3.6.3 (R Core Team 2020); models were made using glm.nb from the MASS package (Venables and Ripley 2002).

RESULTS

Standardized visual counts from 2000 to 2006 recorded averages of 205.4 ± 30.4 ind. yr⁻¹ for the Black-backed Woodpecker and 114.3 ± 25.6 ind. yr⁻¹ for the American Three-toed Woodpecker. Distribution of capture efforts in relation to visual counts indicated a relatively good coverage of the dispersing window (Fig. 2). Black-backed Woodpecker captures averaged 0.7 ± 0.5 ind. yr⁻¹ without EMC and 69.3 ± 9.3 ind. yr⁻¹ with EMC. American Three-toed Woodpecker captures averaged 0.6 ± 0.4 ind. yr⁻¹ without EMC and 93.6 ± 21.7 ind. yr⁻¹ with EMC. For the Black-backed Woodpecker, capture rate using EMC is positively correlated to visual counts across years (0.71, p = 0.071; Fig. 3). For the American Three-toed Woodpecker, the correlation between capture rate using EMC and visual counts was strong (0.97, p < 0.001; Fig. 3).

The proportion of adults in both species was low (Table 1). Black-backed Woodpecker caught using EMC were biased in favor of juveniles. The average captured age ratio was 1 adult for 13.9 juveniles (ratio of 0.07 ± 0.03, p < 0.001; Table 1). The same pattern was found for the American Three-toed Woodpecker, with a ratio of 1 adult for 16.7 juveniles (ratio of 0.06 ± 0.04, p < 0.001; Table 1). The observed sex ratio using EMC capture did not differ significantly from the 1:1 ratio for both species and all age classes, where the average captured Black-backed Woodpecker sex ratio was 1.26 females for 1 male (p = 0.06, df = 6), and was 0.75 female for 1 male (p = 0.1, df = 6) for American Three-toed Woodpecker (Table 1).

The addition of an EMC factor to the baseline model containing only dispersal intensity improved the model, increasing the log-likelihood by 279.43 (ΔAICc = 554.15) for the Black-backed Woodpecker and by 14.73 (ΔAICc = 24.73) for the American Three-toed Woodpecker (Table 2). Indeed, the number of captures was positively affected by EMC; EMC had an additive effect on the average number of captures of 4.5 ± 0.5 (p < 0.001) for the Black-backed Woodpecker and 4.6 ± 0.6 (p < 0.001) for the American Three-toed Woodpecker.

DISCUSSION

Our main objectives were to investigate the efficiency of EMC calls in capturing woodpeckers and to determine the population structure of Black-backed and American Three-toed Woodpeckers during fall movements at the OOT, located at the southern limit of Québec’s boreal forest. Although our experimental design does not permit to quantify the effect of the different mesh size used, the use of EMC within an enclosure greatly increased capture rate by an additive effect of > 4 for both species. The proportion of individuals caught (BBWO = 1:13.9 and ATTWO = 1:16.7) differed from the expected productivity of both species (1:1.5; adult:juveniles; Nappi and Drapeau 2009, Tremblay et al. 2015), supporting our hypothesis that most individuals caught during the fall are juveniles likely dispersing (i.e., post-fledging movements).

There are two main explanations for the observed age ratio bias toward juveniles. First, juveniles, through a higher attraction or a lack of experience, are caught more often in nets; this has been observed for passerines (Brotons 2000, Oñate-Casado et al. 2021). Because of the proximity of the visual count site (~100 m, in their flight trajectory), we believe that the high juvenile ratio in American Three-toed Woodpecker captures is likely not linked to EMC bias, as EMC captures represent an average of 82% of the individuals observed through the dispersing counts. We believe it is the same situation for Black-backed Woodpecker, but with an average of 33% of observed individuals caught, we cannot rule out a potential bias of EMC on the high juvenile ratio. However, no field cue led us to believe we may have experienced such bias. Another possible explanation is that most individuals caught and observed at the study site are juveniles undergoing natal dispersal, with adults less likely to disperse. Indeed, it is possible that these surpluses of dispersing individuals originate from high productivity breeding seasons linked to natural disturbances such as forest fire or spruce budworm outbreaks; both disturbances create suitable habitat for these species (Tremblay et al. 2020a, 2020b). Variations in standardized capture rates from one year to another may reflect yearly variations in productivity and newly created habitats by natural disturbances. Black-backed Woodpecker abundance and productivity are closely linked to pulsed resource interactions resulting from recently burned stands (Tremblay et al. 2015). Indeed, recent burns provide a high-quality habitat available for only a short period (Saab et al. 2007, Hutto and Patterson 2016, Tingley et al. 2018); however, these ephemeral habitats are likely responsible for a large output of juveniles in the population (Nappi and Drapeau 2009). Accordingly, younger individuals are predominant in recently burned habitats, suggesting colonization via natal dispersal (Huot and Ibarzabal 2006, Tremblay et al. 2015, Siegel et al. 2016). Future analysis could use data collected at bird observatories such as the OOT, i.e., population structure and abundance, and link the data to the frequency, intensity, size, and distribution of natural disturbances in boreal forests. Unfortunately, little is known with regard to the American Three-toed Woodpecker demographic response to natural disturbances, especially in the eastern part of its range (Tremblay et al. 2020b), making it difficult to interpret the variations in productivity observed at our study site. Observed fluctuations in age ratios and abundance may yield interesting insights into the species’ breeding ground productivity and annual demography. Indeed, the species could also benefit from natural forest disturbances, but in lesser magnitude than the Black-backed Woodpecker (Tremblay et al. 2020b). It should be acknowledged that population structure studies using EMC capture data should be taken with caution because biases are well known in different species (Schaub et al. 1999) and results could change according to location and time. Accordingly, we used a conservative test, and our results still show strong and significant differences in age ratio.

Capturing woodpeckers can be challenging; this heightens the difficulty in obtaining sufficient data and metrics to fully understand woodpecker biology, ecology, and population dynamics. Boreal species are a particular challenge to survey and capture (Lehman et al. 2011), in part because of the limited large-scale movements of adults (Tremblay et al. 2020a, 2020b). The net enclosure system proved useful and efficient for capturing large numbers of dispersing boreal woodpeckers and yielded data on population size not obtained through passive mist netting. Indeed, from 2000 to 2006, 78% of all American Three-toed Woodpeckers and 36% of Black-backed Woodpeckers in North America were banded at the OOT (Bird Banding Office, unpublished data). Black-backed Woodpeckers capture representation at the OOT is underestimated as an additional 216 captures were made from 2003 to 2006 through research projects during the breeding season (Huot and Ibarzabal 2006, Tremblay et al. 2009). In addition, our study shows that the number of captured individuals using EMC is strongly correlated with visual counts. Hence, long-term capture with EMC could prove useful in providing accurate species population trends for boreal woodpeckers, two species known to be challenging for monitoring efficiently with other techniques, in sites that are not fit for visual counts.

Previous studies on owls and passerines using playbacks to lure the birds into mist nets show sex biases to be common (Schaub et al. 1999, Whalen and Watts 1999). These and other studies consistently document higher capture rates for males than females. Our captures were not significantly skewed toward females nor males for both species. The lack of a strong bias could be explained by the fact that woodpeckers are a group where both sexes are known to be aggressive against intruders or counterparts (Lawrence 1967).

Our data quality on passive netting fluctuated in part because of differences in effort between years. In addition, certain locations for passive nets were more successful than others, and we therefore relocated the nets to the same area the following year, whereas some nets were moved between years; however, the net enclosures used the same type of nets and the same locations. A more constant effort in passive capture both in location and in capture effort would provide a better comparison between both methods. Nevertheless, we believe that using the effort in terms of net length × hours partially corrects for any bias from all net locations not being the same. Another possible bias in our study was differences in net mesh size. Because we also wished to capture passerines with the passive netting, we used a mesh size of 30 mm, whereas the mesh used in the enclosure was 60 mm, which is better adapted for capturing woodpeckers. Indeed, smaller mesh sizes are appropriate for passerines, although sturdier and larger woodpeckers could bounce off the nets (Jenni et al. 1996). Unfortunately, our design does not allow us to fully distinguish between the specific effects of mesh size and EMC. Despite different mesh size, our passive nets proved efficient at capturing boreal woodpeckers where 1.0% of the captures of Black-backed woodpeckers and 0.6% of American Three-toed woodpeckers originated from them. These observations suggest passive nets were not entirely ineffective at catching larger birds, despite their smaller mesh size. Because boreal woodpeckers fly mostly above the canopy while dispersing, they are less likely to be caught in passive nets set below the canopy. Therefore, we believe that even if the EMC effects could be overestimated because we could not control for mesh size, EMC likely still had a significant positive effect on the capture rate. The potential of this method to be replicated at other bird banding locations is very high, and we recommend using such a method as a means of increasing our knowledge of fall movements by woodpeckers and their population dynamics on a year-to-year basis.

In conclusion, our observations of the skewed age-class structure toward juveniles of the Black-backed Woodpecker and the American Three-toed Woodpecker highlight the importance of juveniles in irruptive events of boreal woodpeckers, and possibly for other specialist species. Moreover, bird numbers captured each year at the net enclosure were positively correlated to the numbers detected during visual observations, therefore supporting our hypothesis that EMC capture, if highly standardized, can be used as an effective population monitoring tool.

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