Yearling proportion correlates with habitat structure in a boreal forest landbird community

Introduction

Vital rates such as productivity and survivorship have increasingly been used to assess the status of landbird populations and inform conservation actions (DeSante, Nott & O’Grady, 2001; Anders & Marshall, 2005; Saracco, DeSante & Kaschube, 2008; Robinson, Julliard & Saracco, 2009; Rushing et al., 2015). While trends in landbird abundance are useful for highlighting species of conservation concern, the assessment of vital rates may help identify causes of declining trends; for example, whether they are driven by factors on or away from breeding grounds (Newton, 2004; Albert et al., 2016). Data on vital rates collected at constant-effort landbird capture stations can also be used to predict population viability (Ryu et al., 2016) and can be modeled as functions of habitat variables to inform conservation-management strategies (DeSante, 1995; Saracco et al., 2016, 2018), including reclamation and restoration programs (Foster et al., 2017). To date, modeled vital-rate terms have primarily included those of productivity, often indexed as the probability that a captured bird at a station had fledged that year and survivorship of breeding adults, as estimated from capture-mark-recapture models (Saracco, DeSante & Kaschube, 2008).

Population age structure within breeding populations of iteroparous species has received less attention but may reflect population dynamics and aspects of habitat quality (Rodenhouse, Sherry & Holmes, 1997). In landbirds, advances in distinguishing yearlings from older adults for many species (Jenni & Winkler, 1994; Pyle, 1997) provide new opportunities to estimate proportions of first-year breeding adult landbirds in populations. Yearling proportions have been found to correlate negatively with landbird population densities (Graves, 1997; Sillett & Holmes, 2005) and positively with marginal, lower-quality, or disturbed habitats, likely the result of competitive exclusion of yearlings by older birds from higher-quality breeding habitats (Graves, 1997; Holmes, Marra & Sherry, 1996; Hunt, 1996; Bayne & Hobson, 2001; Haché & Villard, 2010). Yearling proportion has also correlated positively with reproductive success within a population the previous year (Sillett, Holmes & Sherry, 2000), reflecting a direct demographic effect. Yearling proportions from capture stations have the potential to increase our understanding of landbird demography, habitat selection and annual recruitment rates; however, assessments of yearling proportion thus far have been limited to a few directed studies on individual species in which one-year-old landbirds, in most cases males only, are readily distinguished from older birds in the field.

Here we examine yearling proportions using data from 18,799 captures of 12,714 individual landbirds, of 29 species (Table 1), captured at 35 Monitoring Avian Productivity and Survivorship (MAPS) constant-effort bird-capture stations over a 6-year period in the oil sands region of northeastern Alberta (Fig. 1). The goals of this study were to (1) examine the accuracy and utility of yearling proportion as a measure of habitat structure in a boreal-forest landbird community, (2) assess the relationship between the probabilities of capturing a yearling and habitat based on multi-species hierarchical models and (3) assess locally measured and remote-sensed structural vegetation changes in relation to habitat reclamation maturity (e.g., number of years since reclamation commenced) as it may affect species-specific and habitat-specific variation in yearling proportions.

Materials and Methods

Results

Extent of understory cover as measured in situ showed little relationship with station age since reclamation (Fig. 2A) whereas extent of midstory canopy cover increased with age since reclamation and was similar to midstory cover of natural stations by 30 years post reclamation (Fig. 2B). Extent of upperstory canopy cover among reclaimed stations also showed little relationship with station age up to 30 years, as compared to values derived from matured natural stations (Fig. 2C). Remote-sensed, EVI values increased with years since reclamation, with the index equating to those of natural stations up to about 20 years since reclamation (Fig. 2D). When we compared values among the five reclaimed stations and the remaining 30 stations, the EVI index also correlated with both year since reclamation and proportion of a station’s habitat considered natural (Fig. 3).

The mean yearling proportion of the 29 species over all 6 years and 35 stations combined was 0.407, ranging from 0.178 in red-eyed vireo and northern waterthrush to 0.613 in swamp sparrow (Table 1). The probability of a captured adult being a yearling was negatively related to EVI (Fig. 4A), indicating that yearlings overall were found in less-forested habitats. This relationship was strongest for birds typical of forested habitats, as reflected by our capture data of adults and weakest for species inhabiting earlier successional habitats (Figs. 4B and 4C). The probabilities of capturing yearlings for species preferring mature forests, such as magnolia warbler, cedar waxwing, northern waterthrush and mourning warbler, were lower in those forested habitats and higher in successional habitats, whereas species preferring successional habitats, such as song sparrow, clay-colored sparrow, alder flycatcher, savannah sparrow and chipping sparrow, had lower yearling probabilities in those habitats and higher probabilities in forested habitats (Figs. 4B and 5). We also found significant relationships between EVI coefficients, EVI mean and EVI range among our 29 species, including a slightly positive correlation between EVI coefficient and range (Fig. 5), suggesting that more specialist species, those found in a narrower range of habitats, tended to have more negative correlations between EVI and yearling probabilities than species found in a wider range of habitats.

Discussion

Among our 35 stations, habitat-development patterns indicate that ground and understory cover do not change markedly after initial planting, that midstory cover develops within the first 30 years after initiation of reclamation and that upperstory canopy does not noticeably develop until >30 years after initiation of reclamation activities. This is consistent with other studies on boreal-forest habitat succession, in which trees reach about 40% of their maximum height within 50 years and attain maximum height and canopy development (closure) at about 75 years of age (Bartels et al., 2016). EVI values will increase as midstory canopy develops within the first 20 years, but then may not change substantially as upperstory canopies mature and obscure the lower strata. It thus appears that EVI can track habitat characteristics, structure and development for 20–30 years following vegetation planting, but that in-situ habitat data or data based on remote imagery that can distinguish canopy layers will be needed once upperstory canopy closure commences.

Mean yearling proportions among the 29 species examined show wide variation, from 0.178 in red-eyed vireo and northern waterthrush to 0.613 in swamp sparrow. We believe this variation reflects biological factors as opposed to age-determination or capture bias; following increased precision of age determinations though examination of digital images, mean error rate in age-determination was estimated at only 8.1%, with 20 of our 29 species showing age-error rates <10%. The variation in mean yearling proportions may reflect factors related to species-specific variation in (1) the ability or proclivity to exclude yearling birds from higher quality habitats by adults, (2) age-specific variation in capture probabilities, and/or (3) habitat variables among our 35 MAPS stations.

Despite this variation in yearling proportions, forest-dwelling landbird species (cf. Rodewald, 2015), those captured at higher rates in forested stations of our study, showed lower probabilities of yearling capture in stations with higher EVI, whereas species captured more frequently in successional breeding habitats showed higher capture probabilities of yearlings in forested habitats. Both of these results are consistent with hypotheses suggesting that yearling birds are excluded through despotism to suboptimal breeding habitats (Holmes, Marra & Sherry, 1996; Bayne & Hobson, 2001; Rohwer, 2004; Haché & Villard, 2010). Alternatively, yearlings may also be selecting poorer habitats due to lack of breeding experience or other factors. In either case, it is possible that stations with high yearling proportions may occur in sink as opposed to source habitats (Pulliam, 1988; Donovan et al., 1995; Faaborg et al., 2010), presuming that yearling landbirds also average poorer reproductive success than older birds (Holmes, Marra & Sherry, 1996). Our study also indicates that despotic exclusion or other factors affecting yearling dynamics may operate selectively and specifically among different species within a forest landbird community. Maintenance of high yearling proportions in suboptimal habitats for the species may promote emigration of yearlings to more suitable habitats for their second year of breeding (Greenwood & Harvey, 1982; Rohwer, 2004), underscoring the need to model population age structure and habitat dynamics at the meta-population level over multiple years (Faaborg et al., 2010).

Using capture data from the same study areas reported on here, Foster et al. (2017) found significant correlations between habitat covariates and captures of adult birds, young (HY) birds and/or the probability of capturing a young bird (productivity). They also found that positive responses to reclamation age from obligate forest-dwelling species took more years to become evident than those for species preferring early successional-stage habitats. Our analyses further support relationships between demographic variables and habitat structure, including those related to habitat succession, suggesting that recruitment into the breeding population by yearling landbirds could be an initial indication of successful boreal-forest reclamation. We infer that yearling proportion can become an important vital-rate measure of both species-specific habitat suitability and the progress of reclamation efforts.

Captured yearlings may be “floaters”, or non-breeding individuals not holding territories that are simply passing through the territories of breeding individuals (Sherry & Holmes, 1989; Barber & Robertson, 1999; Bayne & Hobson, 2001). Demographic analyses using capture data, for example those examining correlations of yearling proportion with population density or productivity (Graves, 1997; Sillett, Holmes & Sherry, 2000; Bayne & Hobson, 2001; Rohwer, 2004; Haché & Villard, 2010), should account for the possible presence of floaters. We suggest that breeding condition data, for example, the extent of brood-patch or cloacal-protuberance development (Pyle, 1997) or recapture data confirming length of stay, may be useful in assessing the occurrence of yearling and older floaters in capture data sets (Barber & Robertson, 1999). Habitat selection processes by age may also be subject to interannual or density-dependent effects (Rodenhouse, Sherry & Holmes, 1997). By incorporating all demographic and breeding-condition variables in population models, that in turn include multiple species and multiple years of data, we suggest that yearling proportion may eventually be useful in estimating first-year recruitment and survivorship, the latter widely considered to be much lower than survivorship of older adults but is difficult to estimate due to low natal fidelity in most landbird species (Hobson, Wassenaar & Bayne, 2004; Anders & Marshall, 2005; Cooper, Daniels & Walters, 2008; Faaborg et al., 2010). In any case, presuming that yearlings show lower reproductive success than older birds, our results indicate that habitats with high yearling proportions could be located within sink as opposed to source habitats.

Conclusions

To our knowledge, this study represents the first attempt to model the probability of capturing yearling landbirds against habitat factors using a multi-species approach. For many landbird species, age can only be accurately determined in the hand, so our dataset provides the only large-scale, long-term data set that we are aware of that permits estimation of yearling age-structure of both sexes combined. Key to deriving accurate yearling proportions is observer training in age-determination criteria and validation through a procedure like the photographic review and aging confirmation processes used here.

Our results suggest that yearling landbirds are excluded from optimal breeding habitats by older adults, although they might disperse into more optimal habitats the year following breeding as yearlings. The probability of capturing a yearling of forest-dwelling species was higher in successional-stage habitats and vise versa. In conjunction with other vital rates estimated using MAPS data (i.e., productivity, survivorship), yearling proportion may help identify sink habitats and estimate juvenile survival, on a species-by-species basis.

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