Post-fledging habitat use in a declining songbird

Introduction

Over the last several decades, many species of birds have declined as forested habitats have been altered or destroyed by humans. As a consequence, many species are listed as threatened. Therefore, it is imperative that we closely manage and preserve the remaining habitats for these species (Martin & Finch, 1995; Robinson et al., 1995; Holmes, 2007; Sauer et al., 2012). Many songbird studies focus on breeding habitat with a focus on adult birds, but leave out a large part of a songbird’s annual cycle, the fledgling period (Robinson et al., 1995; Campbell, Witham & Hunter Jr, 2007; Bakermans, Rodewald & Vitz, 2012). To best protect and manage habitats for a particular species, it is vital to understand a species’ entire life cycle. Based on a handful of studies, many species shift in their use of habitats between the breeding and post-breeding/fledgling periods (e.g., Streby et al., 2011; Porneluzi et al., 2014; Burke, Thompson III & Faaborg, 2017).

The Cerulean Warbler (Setophaga cerulea) is considered one of the fastest declining Neotropical wood-warblers in North America (Sauer et al., 2012). It is a small, migratory songbird that breeds in forests of the central and eastern United States (USFWS, 2006). According to the Breeding Bird Survey (BBS), its population has declined more than 75% from 1966 to 2006 and it is now considered a species of conservation concern by the U.S. Fish and Wildlife Service (Buehler et al., 2008). Cerulean Warblers are also listed as species of international concern and are listed as endangered in Canada (COSEWIC, 2010). Birdlife International in partnership with the International Union for Conservation of Nature (IUCN) classifies Cerulean Warblers as ‘vulnerable’ (BirdLife International, 2019). In Indiana, Cerulean Warblers are endangered (Indiana General Assembly, 2007). Species trend information, based on BBS data collected from 1966 to 2012, indicate that Cerulean Warblers are declining at about 3% per year. Declines in Cerulean Warbler populations have been attributed to habitat losses on their breeding and wintering grounds (Weakland & Wood, 2005; Buehler et al., 2008).

Research on the breeding ecology of Cerulean Warblers has been conducted in some parts of its distribution. However, this species has been a challenge for researchers because it nests and forages high up in the canopy. To date, very little research has been conducted on juvenile survivorship, fledgling movements, or pre-migratory activity. Until recently, it was difficult to follow individuals (Hamel, Dawson & Keyser, 2004). However, with new technology such as radio-transmitters and geolocators that are small enough for use on small Passerines, researchers can now explore this aspect of its ecology. According to Hamel (2000a, 2000b), unknown aspects of Cerulean Warbler biology are the behaviors and habitat preferences exhibited by this species during the fledgling period on its breeding grounds before migration.

The objective for this study was to better understand fledgling movements. Specifically, we were interested in identifying habitats used by fledglings. We chose to analyze microhabitat variables from locations where fledgling Cerulean Warblers were found and compared these data to microhabitat variables from random non-use locations. For the first time, we discuss fledgling Cerulean Warbler habitat characteristics, and offer forest management recommendations.

Materials & Methods

Results

From 2015–2017, ten fledgling Cerulean Warblers from different nests were tracked via radio-telemetry. Seven radio-transmitters were attached to fledgling Cerulean Warblers. Two radio-transmitters were placed on adult males, and one was placed on an adult female as a proxy for tracking adults to locations of fledglings. Adults were used as a proxy because fledgling Cerulean Warblers can be near impossible to capture in some instances. Also, the dates of tracking fledgling were during a period of time right after fledging when young are dependent on adults for food and not independent to start moving on their own. When adult birds were tracked instead of juveniles, fledgling locations were recorded where the juveniles were found being fed by the adults. The distances moved by each bird, and the number of days tracked varied among individuals. The average distance traveled by fledgling Cerulean Warblers from their nest during tracking was 355.2 m (range 12–1,396 m), and the average number of days individuals were tracked was 10 (range 1–22 days; Table 1). Of the 10 fledglings that were tracked during this study, one may have either lost its radio-transmitter, or died of unknown causes. In this instance, the radio-transmitter was tracked to the same tree for three days, but no radio-transmitter was recovered.

Based on the results of the Spearman’s correlation analysis, we reduced the number of variables from 14 to 10. Based on ΔAICc values ≤ 2.0, nine models were selected (Table 2). Variables included in the nine models were grapevine presence, percent of white oak, vegetation density, ground cover, basal area, canopy cover, aspect, and the percent of nest tree species nearby. Fledgling presence was strongly correlated with presence of grapevines (1.11; Table 3; Fig. 1) and vegetation density (0.85; Fig. 2), and negatively correlated with presence of white oak abundance (−0.52; Fig. 3). Grapevine, white oak abundance, and vegetation density were found in all ten models. Fledgling presence was positively correlated with ground cover (0.31; Fig. 4) and ground cover was important in 7 models. In contrast, fledgling presence was negatively correlated with basal area (−0.12; Fig. 5). Fledgling presence was positively correlated with canopy cover (0.03), but negatively correlated with aspect (−0.02) and found in only two models. Fledgling presence was negatively correlated with the abundance of mature trees (tree species used by Cerulean Warblers for nesting; −0.01) and it was only found in one model.

Discussion

This study is the first to examine where Cerulean Warbler fledglings disperse once they leave the nest. Based on our research findings, vegetation density is a key factor that determines where adult Cerulean Warblers take their young after leaving the nest. Whether it is on the ground, in the shrub layer, or in the canopy, these birds are favoring areas of greater vegetation density. Of 97 fledgling locations surveyed, 51.5% of the vegetation plots had grapevine. However, only 18.6% of random vegetation surveys had grapevine. Grapevine provides excellent cover, and it is an important habitat component in fledgling habitats, likely because it offers protection from predators. Fledglings, especially when less than one week out of the nest, do not typically move very far, and are not strong fliers. Therefore, staying hidden in thick vegetation is beneficial to their survival. Areas that have high vegetation densities in our study sites include clearcuts, patch-cuts, and riparian areas. Adult Cerulean Warblers have been caught in clearcuts in our study sites during the fledgling period (P Ruhl, pers. comm., 2015), and we have observed adult and fledgling Cerulean Warblers in patch-cuts at our sites. As suggested by other studies, fledgling songbirds may utilize these areas after leaving the nest to avoid predation (Vitz & Rodewald, 2006; Vitz, Rodewald & VegaRivera, 2007; Stoleson, 2013).

Based on previous research, nesting habitats differ from post-fledging habitats in some Passerine species (Akresh et al., 2009; Porneluzi et al., 2014). For example, forest-interior Passerines, such as the Black-throated Blue Warbler (Setophaga caerulescens) and Ovenbird (Seiurus aurocapilla) used clearcuts within a forest matrix during the fledging period (Porneluzi et al., 2014; Stoleson, 2013; Streby et al., 2011). In other species, such as the Golden-winged Warbler, fledglings moved from early successional habitats to forested habitats during the post-fledging period (Streby et al., 2015b; Streby, Peterson & Andersen, 2016). In Passerines, parents will often lead fledglings to riparian forests after their nestlings fledge (Akresh et al., 2009). Proposed explanations for these habitat changes from the nesting and fledgling periods include, higher biomass of insects or fruit, and greater protection from predators (Vitz, Rodewald & VegaRivera, 2007; Akresh et al., 2009; Stoleson, 2013). Similar to our research on Cerulean Warblers, Anders, Faaborg & Thompson III (1998) found that fledgling Wood Thrushes (Hylocichla mustelina) dispersed to sites where forests consisted of a dense understory with thick ground cover. Vitz & Rodewald (2006) also found that smaller clearcuts resulted in a greater abundance of forest-interior songbirds than larger clearcuts.

At our sites, Cerulean Warblers often forage in white oak during the nest building, egg-laying, and nestling stages (CD Delancey, pers. obs., 2015–2017; MacNeil, 2010; Auer et al., 2016). Fewer mature trees of preferred nest tree species were present in areas with fledglings. White oak, the main nest tree species used by Cerulean Warblers, was more common in random sites than in areas where fledglings were found. Cerulean Warblers may place territories around areas with clumped white oak distributions. However, once the nestlings fledge, the birds move beyond the territory boundaries (12–1,396 m [from their nest]) where white oak was found to be less common.

Canopy cover was slightly higher at fledgling sites, which is beneficial for young birds that are more vulnerable to predation (Vitz & Rodewald, 2006; Vitz, Rodewald & VegaRivera, 2007; Stoleson, 2013). During the fledgling period, it would be advantageous to move into areas with a higher abundance of insects and areas of dense vegetative growth, which are both characteristic of riparian areas (Akresh et al., 2009). In some instances, fledglings were found in riparian areas. For example, our control units do not have patch-cuts or clearcuts but are characterized by riparian areas. These areas often possess many shrubs and a dense canopy across all strata. Akresh et al. (2009) suggested that songbirds, especially juveniles, preferred riparian areas during the post-fledging period due to higher vegetation densities and an abundance of food. In Ontario, insect abundance was found to be higher in riparian areas compared to upland sites from 16 June-28 July, which correlates to the post-fledging period for many songbirds (Mosley, Holmes & Nol, 2006).

There was a slight negative relationship with presence of fledglings and southwest aspect. Aspect was only found in two models when fledgling locations were compared to random locations and therefore, may not be as important as other variables that showed stronger associations with fledgling presence. There was less basal area in areas where fledglings were present, which would allow for a denser shrub layer. A study in West Virginia examined microhabitat characteristics of vegetation, soil, and climate with respect to aspect and found that afternoon temperatures on west and southwest-facing slopes were about 4.86 °C warmer than on north and east-facing slopes (Desta et al., 2004). Before nestlings fledge, they need to thermoregulate themselves; therefore, moving to warmer locations will help them regulate their body temperatures without expending additional energy. Also, the relative humidity on these western and southwestern slopes was found to be about 25% less than that on north and eastern slopes. Basal area also averaged 7.84 m²/ha higher on north and eastern slopes (Desta et al., 2004).

In three instances, fledglings moved southward and downstream from their nest locations (Fig. S1). Riparian areas are often corridors for migration with areas of dense cover to protect fledglings from predators; riparian areas also offer ample food to help fledglings grow fast. The Cerulean Warbler is a single-brooded species that leaves its breeding sites in southern Indiana by the beginning of August (C.D. Delancey, pers. obs.); it is possible that once the young fledge, family groups move southward, following streams. In some instances, fledglings moved up slope, and in a northward direction (Figs. S1, S2). One of these individuals chose to spend some time in a pine (Pinus spp.) stand, which provided abundant dense cover (Fig. S1). Some birds chose to stay relatively close to their nests (Figs. S1, S3 and S4). In these instances, surrounding areas near the nest tree had higher vegetation densities than riparian areas, including harvested areas nearby or areas where grapevines had spread into the canopy. In the case of one fledgling that was tracked in a control unit, the area surrounding the nest site received a light harvest a few years ago, resulting in growth of shrubs and saplings in the understory (Fig. S3). Harvest was allowed in this area because it was in the buffer zone of the HEE study sites.

Many observations of nestlings at the time of fledging demonstrated that nestlings could leave the nest and still stay high up in the canopy, making it impossible to capture. We noticed that nestlings that flapped their wings while on the nest were less likely to be caught and stayed high up in the canopy. Only fledglings that came within ∼7 m of the forest floor could be captured for this study. These nestlings may have been the weaker individuals in the nest, or maybe nestlings that left the nest prematurely. Once on the ground, fledglings worked their way back into the canopy within a couple of days.

We were unable to compare fledgling movements among the different study sites due to our sample size of 10 fledglings. Additionally, the data on fledglings is skewed toward uneven-aged study sites because one of the sites (unit 8) has the greatest relative abundance of Cerulean Warblers compared to the other sites. Seven of the ten fledgling Cerulean Warblers tracked were located within unit 8. One fledgling was tracked in unit 6 (an uneven-aged unit), and two fledglings were tracked in control units (units 4 and 5). Both Cerulean Warbler fledglings tracked in control units were in disturbed areas along forest roads or in an area within the buffer that was recently harvested. The areas where these two fledgling Cerulean Warblers were tracked resembled an uneven-aged forest stand.

Conclusion

This is the first study that examined habitat use by fledgling Cerulean Warblers. We found that fledgling habitat differed from other habitats that Cerulean Warblers utilize during the breeding season. Clearcuts or smaller patch-cuts near breeding sites can also benefit Cerulean Warblers in the post-fledging period as areas with plentiful food and protection from predators. Identifying the different vegetation types that Cerulean Warblers use throughout the breeding season can best inform natural resource personnel on how to manage forests to meet the habitat requirements of this declining songbird. However, continued research across the breeding distribution of Cerulean Warblers can best determine if our results are regional preferences, or if our results are similar throughout their range.

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