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Scholer, Micah N. – Land cover and topographic effects on cavity-nesting owl occurrence and the role of species interactions in structuring cavity-nesting owl communities. 2011.

scholer on snowy hill at nightThis thesis consists of two chapters describing results of my research on landscape scale habitat associations and community interactions of three cavity-nesting owls: Boreal Owls (Aegolius funereus), Flammulated Owls (Otus flammeolus), and Northern Saw-whet Owls (A. acadicus).  My objectives were to (1) model the effect of land cover and topographic features on cavity-nesting owl site occupancy, (2) spatially depict the probability of occupancy in a geographic information system, (3) determine whether interspecific interactions influence site occupancy, and (4) assess the effect of snag characteristics on cavity-nesting owl occupancy.  I carried out fieldwork from February – June of 2009 and 2010 in the Boise National Forest (BNF), which is located in southwestern Idaho.  The BNF is approximately 10,600 km2 in size and has a diversity of habitats influenced by forestry, fire, recreation, and other influences under a multiple-use mandate for land management.  The habitat association and co-occurrence data I collected will serve as a reference in future studies seeking to understand site occupancy for cavity-nesting owls.  Cavity-nesting owls are susceptible to anthropogenic changes (Imbeau 2001).  Therefore information contained in this thesis should be of interest to forest managers who guide restoration and management of public or private lands suitable for owls as well as ornithologists and other scientists interested in the biology of cavity-nesting owls and the factors that shape their community structure.

Background

With the exception of a small number of species of conservation concern, little is known about the distribution, abundance, and population trends of North American owls (Takats 2001, Duncan 2003).  Understanding cavity-nesting forest owls in particular frequently poses special challenges because of their small size, secretive behavior, and in some cases rarity.  Therefore, knowledge of their occurrence and distribution is needed.  Species occurrence and distributions are determined by a range of factors, including land cover, topography, and species interactions (Franklin 2009).  Generally, studies that describe owl distributions and occurrence are accomplished by modeling characteristics of the habitat.  Larger, landscape-scales studies of owl-habitat relationships are useful as they can be modeled within a geographic information system (GIS) to create spatial representations of the probability of occupancy for species of wildlife.  However, the potential to model species distributions as a function of species interactions has been largely unexplored (but see Heikkinen et al. 2007).  For instance, we know that species interactions play an important role in shaping the distribution and occurrence of cavity-nesting owls (Martin and Eadie 1999).  Cavity-nesting owls engage in both facilitation interactions with woodpeckers (woodpeckers excavate cavities in living and dead trees, which provide nesting sites for secondary cavity nesters such as owls; Martin and Eadie 1999, Virkkala 2006) as well as competitive interactions with sympatric owl species (McCallum 1994, Vrezec and Tome 2004, Rasmussen 2008).  New advances in occupancy modeling now allow for such species interaction to be assessed along with identifying important habitat characteristics that explain owl occurrence.  Furthermore, as owls in particular can be logistically difficult to monitor because nighttime surveys are required, there is potential benefit in identifying possible surrogate species (e.g., woodpeckers) whose occupancy may provide an index of owl occurrence or abundance.

Overview of Chapters

In chapter one, I developed occupancy models to evaluate the landscape scale habitat associations of Boreal Owls, Flammulated Owls, and Northern Saw-whet Owls (hereafter Saw-whet Owls) in the BNF.  I modeled owl occurrence using land cover and topographic variables by selecting the best representative scale (400-m, 1-km, or 3-km-radius plots centered on point-count locations) for each owl species.  As I ultimately detected no Boreal Owls, I developed predictive habitat models for Flammulated Owls and Saw-whet Owls and incorporated these results into a GIS to create spatial maps depicting suitable habitat for each.

Flammulated Owls occurred in areas with lower diversity in land cover composition at the 1-km scale and a higher proportion of Douglas-fir forest at the 0.4-km scale.  At the 3-km scale, Flammulated Owls tended to occur on more south-facing aspects.  At the 1-km scale, Saw-whet Owls occupied locations containing larger proportions of non-forested area situated on more southerly-facing aspects.  At the 3-km and 1-km scales, areas containing a higher proportion of ponderosa pine and Douglas-fir, respectively, were more frequently occupied by Saw-whet Owls.  Saw-whet Owls also occurred in relatively flat landscapes (low terrain ruggedness) at the 0.4-km scale.  An independent data set confirmed that the final occupancy models I developed accurately distinguished between occupied and unoccupied sites for Flammulated Owls (X2=26.03, d.f.=3, P<0.0001). External validation of the Saw-whet Owl model was not possible as an independent set of occurrence data for this species in the Boise National Forest was not available.

I examined patterns of co-occurrence between woodpeckers and sympatric cavity-nesting owls in chapter two using two-species occupancy models.  As cavity-nesting owls require natural cavities or, more commonly, cavities that woodpeckers create for nesting and shelter, their occurrence may be dependent on the presence of primary cavity excavators (i.e., woodpeckers).  Specifically, I tested the hypothesis that cavity-nesting owl occupancy was conditional on the presence of one or more common species of woodpecker: Hairy Woodpeckers (Picoides villosus), Lewis’s Woodpeckers (Melanerpes lewis), Northern Flickers (Colaptes auratus), Pileated Woodpeckers (Dryocopus pileatus), and Red-naped Sapsuckers (Sphyrapicus nuchalis).  Additionally, I examined the pattern of co-occurrence between cavity-nesting owl species to help understand the nature of their possible competitive interactions.

I surveyed for owls during nighttime hours and returned to these same locations during daytime hours to conduct surveys for woodpeckers.  As snags may also be an important component of cavity-nesting owl occurrence, I modeled owl occupancy as a function of snag number and density and pattern of species co-occurrence while accounting for imperfect detection (i.e., the possibility that an individual may go undetected during surveying even when present).

The average number of snags per hectare (12.5 ± 0.2, N = 150) and mean diameter at breast height of snags (35.17 cm ± 0.08, N = 150) had no effect on cavity-nesting owl occupancy.  There was no support for the hypothesis that cavity-nesting owl occupancy was conditional on the presence of woodpeckers.  Likewise, presence of Saw-whet Owls neither excluded nor facilitated Flammulated Owl occupancy (i.e., they occurred independently of one another).  Therefore, despite the potential value in understanding the occupancy of the nocturnal owl community by examining the diurnal woodpecker community, relationships between woodpeckers and either Flammulated Owls or Saw-whet Owls do not appear strong enough to warrant such an approach.

Chapter One:  Factors in influencing site occupancy of cavity-nesting forest owls in the Boise National Forest, Idaho: A spacial application using GIS

Developing effective management plans requires an understanding of species distribution and the factors affecting their occurrence.  Spatially explicit models depicting the probability of species occupancy therefore offer a useful conservation tool for land managers.  These models can be generated from remotely sensed data of land cover type, composition, and other topographic features.  However, knowledge of the scale at which these factors act to influence occupancy is often lacking.  I examined landscape level habitat associations of Flammulated Owls (Otus flammeolus) and Northern saw-whet Owls (Aegiolius acadicus) in the Boise National Forest, located in southern Idaho.  I considered land cover and topographic variables by selecting the best representative scale from 0.4-km, 1-km, or 3-km-radius plots centered on point-count locations (N = 150).  Using occurrence data collected between 2009 and 2010, I developed habitat models for Flammulated Owls and Northern Saw-whet Owls and incorporated these results into a geographic information system to create a spatial map depicting suitable habitat.  Flammulated Owls (N = 27 occupied point-count locations) occurred in areas with a higher proportion of Douglas-fir at the 0.4-km scale and less diverse land cover composition at the 1-km-scale.  At the 3-km scale, Flammulated Owls tended to occur on south-facing aspects.  Saw-whet Owls (N = 45 occupied point-count locations) were found in relatively flat landscapes (i.e., low terrain ruggedness) at the 0.4-km scale.  At the 1-km scale, Northern Saw-whet Owls occupied locations containing larger proportions of non-forested area and with more southerly aspects.  At the 3-km scale, areas containing a higher proportion of ponderosa pine were more frequently occupied by Northern Saw-whet Owls.  An independent data set confirmed that the final model accurately distinguished between occupied and unoccupied sites for Flammulated Owls (X2=26.03, d.f. = 3, P<0.0001). External validation of the Saw-whet Owl model was not possible as occurrence data for this species was not available.  Land managers can use these models to guide future surveys or management focused on owls.

Chapter Two: Patterns of co-occurrence between cavity-nesting owls and woodpeckers in the Boise National Forest, Idaho

Boreal Owls (Aegolius funereus), Flammulated Owls (Otus flammeolus) and Northern Saw-whet Owls (A. acadicus) are small, cavity-nesting birds that require natural cavities or, more commonly, cavities that woodpeckers create for nesting and shelter.  I investigated co-occurrence between woodpeckers and sympatric cavity-nesting owls in the Boise National Forest, Idaho.  Specifically, I tested the hypothesis that owl occupancy was conditional on the presence of one or more common species of woodpecker.  Moreover, I examined the pattern of co-occurrence between cavity-nesting owl species to help understand the nature of their possible competitive interactions.  I located owls by broadcasting conspecific vocalizations during nighttime hours at point-count locations (N = 150) during 2009 and 2010.  I surveyed for Hairy Woodpeckers (Picoides villosus), Lewis’s Woodpeckers (Melanerpes lewis), Northern Flickers (Colaptes auratus), Pileated Woodpeckers (Dryocopus pileatus), and Red-naped Sapsuckers (Sphyrapicus nuchalis) at these same locations during daylight hours in a similar manner while broadcasting their respective woodpecker vocalizations.  As no Boreal Owls or Lewis’s Woodpeckers were detected, my analyses focused on relationships among the remaining owl and woodpecker species.  I modeled occupancy as a function of snag number, diameter at breast height, and species co-occurrence while accounting for imperfect detection (i.e., the possibility that an individual may go undetected during surveying even when present) using the software program PRESENCE.  Average number of snags per hectare (12.5 ± 0.2, N = 150) and mean diameter at breast height of snags (35.17 cm ± 0.08, N = 150) had no effect on cavity-nesting owl occurrence.  There was no support for the hypothesis that cavity-nesting owl occupancy was conditional on the presence of woodpeckers.  Likewise, presence of Northern Saw-whet Owls neither excluded nor facilitated Flammulated Owl occurrence (i.e., they occurred independently of one another).  Therefore, despite the potential value in understanding the nocturnal owl community by examining the diurnal woodpecker community, relationships between woodpeckers and Flammulated Owls and Northern Saw-whet Owls do not appear strong enough to warrant such an approach.

Thesis Abstract

Where Are They Now?

 

Publications:

Scholer, M.N., B. Martín, A. Onrubia, M. Ferrer, M.J. Bechard, G.S. Kaltenecker, and J.D. Carlisle. in press. Variable shifts in the autumn migration phenology of soaring birds in southern Spain. Ardea.

Scholer, M.N. 2015. Novel behavior in the parental care of a House Wren (Troglodytes aedon): post fledging use of an old nest during cold nights. Wilson Journal of Ornithology 127: 545-547.

Scholer, M.N., M. Leu, and J.R. Belthoff. 2014. Factors associated with Flammulated Owl and Northern Saw-whet Owl occupancy in southern Idaho. Journal of Raptor Research 48: 128-141.

Smith, J.A. and M.N. Scholer. 2013. Nest components of Crested Caracaras (Caracara cheriway) breeding in Florida. Florida Field Naturalist 41: 42-48.

Scholer, M.N. and A. Onrubia. 2012. Depredación de Tarentola mauretanica por una mantodea en la Península Ibérica. Boletín de la Asociación Herpetológica Española 23: 24-25.