Songbird movements within home ranges point to important habitat features
Prey availability and habitat structure explain breeding space use of a migratory songbird. Jirinec, V., Isdell, R. E., & Leu, M. 2016. The Condor: Ornithological Applications. DOI: 10.1650/CONDOR-15-140.1 View
Conservation biologists have long recognized that sufficient availability of optimal habitat is the most important factor in maintaining viable wildlife populations. It is therefore not surprising that knowledge of habitat requirements necessarily precedes successful conservation efforts. Ornithologists often concede that despite research going back several decades or more, understanding habitat requirements of even the common species can have substantial gaps. One issue is of course that most birds can fly and about 10% of species use the ability to conduct impressive seasonal movements that often involve another hemisphere. Even with focus on either the breeding or the wintering grounds, some species’ elusive behavior makes field observation difficult.
We studied habitat attributes of the Wood Thrush (Hylocichla mustelina), a migratory songbird that breeds in eastern North America. The bird’s scientific name (from Latin mustela, weasel) reflects its secretive nature, although the species is a frequent subject of ecology studies and its flute-like song is well-known to the general public inhabiting areas with adequate amount of deciduous woodland. Despite this prestige, ongoing large-scale monitoring efforts of breeding avifauna suggest that – for reasons not entirely understood – the Wood Thrush population has declined by more than half over the last several decades. Is habitat loss to blame? That assumption is unlikely to arouse controversy, but the long-standing belief the species needs large tracts of deciduous forest might be too simplistic.
The main goal of our study was two-fold: to identify important habitat attributes for thrushes on their breeding grounds, and of those features, determine whether food availability (invertebrates in the forest litter) is more important than habitat structure. We chose to tackle our quest with the tracking of 37 birds followed by intensive collection of prey availability and habitat structure data over two breeding seasons. Because of the evasive habits of this species, we attached a small transmitter backpack to each bird and tracked birds via radio telemetry. This allowed us to quantify the activity pattern (or “space use”) within bird home ranges (Figure 1) and employ its variation to identify important habitat features. Space use within home ranges is not homogenous, and we therefore assumed that areas where birds concentrated their activity should coincide with essential traits of those locations. These traits could be tree size, tree diversity, optimal foraging opportunities, or other factors that presumably aid in a successful breeding season.
Figure 1 Example home range of one of our study birds. Much like contours on a topographic map depict differences in elevation, the space use contours here depict differential utilization of various areas within the home range. We then collected invertebrate prey and habitat structure data and employed space use patterns in statistical models to identify important habitat features. [click on image for larger view]
Following our conviction that birds are complex organisms that live in a multivariate environment – one that we have limited understanding of – we related space use to biomass of five leaf-litter invertebrate functional groups (worm-like, spider, beetle, centipede, and other) and 54 habitat structure variables. We used an information-theoretic approach to identify three candidate sets of models that best explained variation in space use: prey availability models, habitat structure models, and composite prey availability and habitat structure models.
Our analyses suggested that, when compared to the prey availability model, the habitat structure model explained nearly eight-times the variation in space use. The composite model yielded only a marginal improvement over the habitat structure model, indicating that Wood Thrush space use could be mostly driven by habitat structure. Variation in space use was best explained by nine habitat structure variables and three invertebrate prey functional groups (Figure 2). Ranking 12 best variables from most to least important in Figure 2 revealed that habitat variables were most important in explaining variation in space use. For the top models, space use related negatively to number of trees within the red oak guild (Quercus spp.), mainly southern red oak (Quercus falcate), and positively to canopy height, as well as snag basal area. The top invertebrate prey variable, spider guild biomass (Figure 3), was ranked sixth.
Figure 2 Model-averaged regression coefficients (betas) and standard errors derived from our composite model of Wood Thrush space use. A coefficient below zero indicates a negative association of the variable with space use and vice-versa. Numbers under variable names indicate importance in explaining variation in spaces use, where a value of 1 indicates that a given variable was included in all models in the candidate set. [click on image for larger view]
Because we wanted to evaluate our results using an independent dataset, we examined Wood Thrush detection data collected over 5 years at 131 bird survey stations within woodlots in our study area. Following predictions generated by the composite model, survey stations where thrushes were never detected had significantly higher counts of the red oaks and pine basal areas, and significantly lower canopy tree richness than in the high-use sections of bird home ranges.
Figure 3 Biomass of leaf-litter spiders was the best-ranking food availability variable in our top models. Some of the top-ranking habitat structure variables likely relate to invertebrate prey: literature suggests dead and decaying wood (snag basal area) is linked to spider density, whereas locations with drier soils where red oaks are more likely to occur will have a lower amount of worm biomass, the second food availability variable in the top models.
In summary, our results indicate that the Wood Thrush selects areas that do contain a distinct set of quantifiable attributes. Of that set, it appears that habitat structure features reign over food availability, although we want to point out that many of the important structure variables could be ultimately linked to bird prey. Because of the difficulty to disentangle complex ecological relationships and the fact that this analysis is correlative in principle, we cannot attribute cause and effect with confidence. Though further research is needed to tease apart specific causative mechanisms, this study does provide an empirical quantification of areas that birds chose as the core sections of their home ranges. The strong link to habitat variables paves the way for rapid assessment of sites as potential high-quality breeding habitat for the declining Wood Thrush, and raises the possibility that similar patterns are found in other songbirds.
We need to highlight the fact that these results pertain to daytime behavior only. Our previous research indicates that thrushes roost in places that are often distinct from day-use areas in both habitat and location, and this mismatch is often substantial (Jirinec et al. 2015). For a complete picture of migratory songbird habitat requirements, ornithologists face the arduous task to conduct field work on the breeding grounds, wintering grounds, stop-over sites, both during the day and at night.
Tracking of the thrush weasel led to other learning experiences as well. For one, we quickly realized that birds do not respect the bounds of permitted sites and often moved their home ranges – sometimes several times during the breeding season. In fact, over half of the tagged birds have done the moving, which appeared to have been prompted by predator-induced nest failure. This vagary of the Wood Thrush breeding biology has sent us on frequent pursuits around the study area in coastal Virginia, a region that can be described as a mosaic of private ownerships and U.S. Department of Defense land. After mixed results with ground searches using pickup trucks and mountain bikes with jury-rigged telemetry gear – mainly due to access restrictions – we chartered the services of a Cessna pilot who introduced himself as Captain Fuzzzo. Fuzzzo, a gritty Vietnam War veteran, had no qualms about birdhunt forays that included buzzing the treetops of the Naval Weapons Station Yorktown and Camp Peary, a CIA training facility. This effort helped to relocate the missing birds, some of which have settled in residential subdivisions, a gated golf course resort that recently hosted President Obama, and the Weapons Station, sometimes over 4 km away from initial home ranges.
References and further reading
Jirinec, V., Varian, C.P., Smith, C.J., & Leu, M. 2015. Mismatch between diurnal home ranges and roosting areas in the Wood Thrush (Hylocichla mustelina): Possible role of habitat and breeding stage. The Auk: Ornithological Advances. 133: 1-12. DOI:10.1642/AUK-15-76.1. View
Pearson, G. These lovely birds do more than sing – they sleep around. Wired. October 21, 2015. View
Featured image: Wood Thrush Hylocichla mustelina © Christina Varian
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