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Diet and behavior of extralimital western burrowing owls (athene cunicularia hypogea) in tallgrass prairie.

The geographic limits of a species range can be shaped by evolutionary pressures that reduce fitness at range edges as well as by dispersal capabilities and demographic rates of core populations (Gaston, 2003, 2009; Sexton et al., 2009). While unfavorable conditions beyond range edges can constrain population expansion, they also can give rise to ecological adaptations (Holt and Gomulkiewicz, 1997; Gaston, 2003; Sexton et al., 2009). Empirical studies of extralimital individuals are uncommon but necessary to understand when species can and cannot adapt to environmental change (Angert and Schemske, 2005; Sexton et al., 2009). One species for which we lack extralimital information is the western burrowing owl (Athene cunicularia hypogea), which inhabits dry grasslands in North and South America characterized by sparse vegetation (Haug et al., 1993; Sheffield, 1997; Poulin et al., 2011). Distribution of burrowing owls in the Great Plains is closely associated with the presence of black-tailed prairie dogs (Cynomys ludovicianus). Prairie dogs excavate burrows used by owls for nesting, and their vocalizations alert owls of nearby predators (Butts and Lewis, 1982; Desmond et al., 2000; Restani et al., 2001). Diet and self-maintenance behavior of burrowing owls during the breeding season are well known in undeveloped, agricultural, and urban environments throughout their core North American breeding range (Thomsen, 1971; Green et al., 1993; York et al., 2002; Moulton et al., 2005; Hall et al., 2009; Trulio and Higgins, 2012). However, we lack information on feeding habits of burrowing owls east of their core range and the behavioral mechanisms that allow extralimital individuals to cope with different environmental conditions.

Burrowing owls are recognized as a bird of national conservation concern in the United States (Klute et al., 2003) and listed as federally endangered in canada (Wellicome and Haug, 1995) due to long-term population declines (Sauer et al., 2014) attributed to habitat loss and prairie dog persecution (Sheffield, 1997; Desmond et al., 2000). Despite a strong association with prairie dog towns, burrowing owls will occupy artificial burrows and dens provided by other animals in areas without prairie dogs (Smith and Murphy, 1973; King and Belthoff, 2001; Poulin and Todd, 2006; Haley and Rosenberg, 2013). Burrowing owls are opportunistic predators that readily consume both arthropods and vertebrates, with the latter contributing the most biomass in their diet (77-99%; Poulin et al., 2011). However, most diet information comes from analyses of prey remains and regurgitated pellets (Thomsen, 1971; Green et al., 1993; Plumpton and Lutz, 1993; Moulton et al., 2005; Hall et al., 2009; Trulio and Higgins, 2012). Such methods bias diet summaries in raptors by misrepresenting proportions of prey items that are either difficult to digest, or degrade at different rates after pellets are regurgitated, or both (Simmons et al., 1991; Redpath et al., 2001; Lewis et al., 2004). Thus, direct feeding observations are needed to more accurately describe the relative importance of different prey types in the diet of burrowing owls.

Burrowing owls have developed several behavioral adaptations to cope with fitness costs associated with nesting underground and inhabiting dry grasslands with highly variable ambient temperatures. For example, burrowing owls within their core range are known to remove ectoparasites by kicking plumes of loose dirt and sand onto their bodies and ruffling their feathers while preening ('dustbathing'; Thomsen, 1971; Garcia, 2005). Burrowing owls dissipate excess heat via gular flutter or by extending their wings downward and outward while facing away from the sun ('wing-drooping'; Coulombe, 1971; Thomsen, 1971), and they collect heat by puffing their feathers away from their bodies (Thomsen 1971). However, we lack detailed observations of burrowing owl behavior in wetter grasslands where vegetation structure is relatively tall and dense and where loose substrate is not always available. Such observations are needed to understand the phenotypic plasticity of burrowing owls in different environmental conditions.

In Kansas, burrowing owls are uncommon summer residents in western shortgrass and mixed-grass prairies where prairie dogs have not been extirpated (west of the 100[degrees] meridian; Busby and Zimmerman, 2001; Klute et al., 2003), and it appears they have never been common in eastern tallgrass prairies (Sheffield, 1997; Klute et al., 2003). At least three nests have been reported in the Flint Hills of eastern Kansas since the late 19th Century, but details on habitat, brood production, diet, or behaviors associated with these records do not exist (Thompson et al., 2011). Here, I used still and video photography to document the diet and behavior of burrowing owls at an extralimital nest in the Flint Hills at Tallgrass Prairie National Preserve, Chase County, Kansas (38.4328[degrees]N, --96.5589[degrees]W). The aims of this study were to (1) quantify the relative importance of different prey types in burrowing owl diet based on both frequency of capture and estimated biomass, and (2) quantify the timing and prevalence of a novel sunbathing behavior observed in this study and discuss its possible adaptive significance. Avian sunbathing can function in collecting heat to thermoregulate (Clade, 1973; Clark and Ohmart, 1985; Clayton et al., 2010), drying wet feathers (Clark and Ohmart, 1985; Clayton et al., 2010), and shedding ectoparasites (Blem and Blem 1993, 2000; Moyer and Wagenbach, 1995; Clayton et al., 2010). The practice of combating ectoparasites by exposing skin and feathers to intense ultraviolet light, while preening with uropygial gland oil, has been documented in violet-green swallows (Tachycineta thalassina; Blem and Blem 1993, 2000) and black noddies (Anous minutus; Moyer and Wagenbach, 1995) but never in raptors, to my knowledge. If burrowing owls in tallgrass prairie sunbathed to shed ectoparasites, I predicted the timing of sunbathing would be associated with hot ambient temperatures during midday and coincide with preening. Alternatively, if sunbathing functioned in collecting heat to thermoregulate, I predicted owls would sunbathe during the coolest periods of the day. If owls sunbathed to dry wet feathers, I predicted owls would sunbathe after rain events.

On 9 May 2013, I flushed a pair of burrowing owls from an inactive American badger (Taxidea taxus) den at Tallgrass Prairie National Preserve. The 4,400-ha preserve is located in the central Flint Hills and manages vegetation structure via rotational prescribed fire and grazing treatments ('patch-burn grazing'; Fuhlendorf and Engle, 2001). The nest burrow was 2 m from an unpaved road in a pasture burned the previous growing season (2012) and within a larger 1,500-ha pasture stocked with one head of cattle per 2.65 ha. Vegetation surrounding the burrow consisted primarily of Bromus japonicus, Hordeum pusillum, Artemisia ludoviciana, and Ambrosia psilostachya and ranged in mean height from ~10 cm in early May to >40 cm in mid-July.

I deployed a motion-detecting trail camera (8MP Trophy Cam HD, Bushnell Outdoor Products, Overland, Kansas) equipped with an 8GB secure digital memory card directly behind the burrow entrance from 9 May 2013 until a lone owlet fledged on 15 July 2013. Adult burrowing owls were not noticeably deterred by the camera and used it as a perch. I programmed the camera to capture either three sequential photos or one 60-s video for each motion detection and alternated between still and video settings approximately weekly while replacing memory cards. Stills required less memory (allowing for more-frequent motion detections), and provided higher resolution images for identifying prey, while videos provided more time to observe owls per motion detection. I programmed the sensor to pause for 5 min (stills) or 15-30 min (video) after each trigger; therefore, the camera did not capture all activity at the burrow. The camera recorded date, time, and ambient temperature ([degrees]C; only for stills) at the time of each trigger. I reviewed stills and videos for the presence of owls and categorized the following behaviors: (1) feeding or food delivery when owls had prey in their bill or grip, (2) sunbathing when owls laid face down or bowed toward the ground and exposed fully spread wings and tail feathers (Fig. 1a; recordings 486778 and 486779, Macauley Library, Cornell Lab of Ornithology), (3) dustbathing (Thomsen, 1971; Garcia, 2005), (4) wing-drooping (Fig. 1b; Coulombe, 1971; Thomsen, 1971), (5) copulating, and (6) dung-lining when they held or entered the burrow with dung.

I identified prey to species or the lowest classification possible with help from mammalogists and entomologists at Kansas State University. I estimated prey biomass based on measurements of the same or related species of similar size from previous studies and unpublished data (Table 1). I calculated the frequency of capture (percent of total items) and the percent of biomass represented by each prey type during each of three breeding stages. I defined the three stages as follows: (1) preincubation as the first observation date to 1 d prior to the incubation initiation date, (2) incubation as the incubation initiation date to 1 d prior to the hatch date, and (3) brood-rearing as the hatch date to the fledge date. The duration of incubation and brood development are likely variable among regions and individuals, so I based my estimates of breeding stage lengths on the mean of those observed in other published studies: I estimated hatch date by subtracting 14 d from the date one owlet first emerged from the burrow and incubation initiation date by subtracting 28 d from the estimated hatch date (Poulin et al., 2011).

I reviewed a total of 4,230 events (motion detections) from 11,325 photos and 455 videos. The camera did not operate from 18-19 May, 25-26 May, 24-29 June, or on 12 July 2013 due to insufficient memory or malfunction. The pair copulated 32 times adjacent to the burrow from 9-21 May and up to eight times in 1 d (10 May). Fifteen copulations occurred during daytime and 17 during nighttime. Adults brought most dung to the nest during preincubation and incubation (11 times during each period) but only three times during brood-rearing. At least one coyote (Canis latrans) visited the burrow three times during preincubation and one time during broodrearing (always at night). During all four visits, the coyote approached the burrow and left before triggering a second video (<30 min) or second series of photos (<5 min). One owlet first emerged on 4 July and apparently fledged on ~15 July. On 13 July, the owlet had well-developed wings and made multiple attempts to fly away from the burrow. I last saw the owlet at 2055h on 14 July, stretching its wings and poking through vegetation surrounding the burrow. I did not observe any owls at the nest site on or after 15 July. I removed the camera on 15 July following fledging and deployed it to an adjacent burrow 160 m away that at least one adult had periodically occupied during the breeding season (M. R. Herse, pers. obser.). I visited both burrows 34 d per week for the following 3 mo and never observed the owls during subsequent visits.

I recorded 142 prey items the adults brought to the nest burrow. Of these prey items, I identified 140 items to at least order, including at least 18 species (Table 1). My observations provide only a sample of all prey items the owls delivered to the burrow, and I discuss them as such. Owls delivered 37 prey items (biomass = 120.1 g) to the burrow during preincubation, 71 (biomass = 284.6 g) during incubation, and 34 (biomass = 190.5 g) during brood-rearing. Owls frequently entered the burrow with prey but, because still photos typically only captured owls approaching the burrow, I could not determine which prey were taken into the burrow. Arthropods represented 90.8% (129 of 142) of prey items and 39.2% of biomass. The owls consumed more grasshoppers than other prey types (54.2% of items; 30.7% of biomass). Owls brought most prey (90.8% of items) to the burrow during the daytime. They only brought rodents to the burrow at night and only brought birds and reptiles during the daytime. Owls consumed more arthropods than vertebrates during all three breeding stages, but biomass of vertebrates increased across successive stages (preincubation = 39.1%, incubation = 58.0%, brood-rearing = 78.7%; Fig 2). Owls brought one unidentified carrion item to the burrow during preincubation and one during brood-rearing (Table 1).

Adult owls sunbathed 21 times from 17 May-14 July (May: n = 1; June: n = 10; July: n = 10). Owls always sunbathed at midday (mean time of day = 1343h [+ or -] 27.1 min, range 1106-1744h) in direct sunlight when temperatures were at or near the daily maximum (mean temperature = 33.9[degrees]C [+ or -] 0.82, range 27-38[degrees]C). The camera did not record temperature during videos, but 15 of 21 sunbathing events were captured by stills. During events captured by video, the duration of sunbathing ranged from ~10 s to the entire 60 s and always coincided with preening and scratching. The sunbathing posture was never preceded or followed by an encounter with a predator on video. Owls never sunbathed during wet or cool (as low as 5[degrees]C) daytime conditions. Owls never dustbathed and always dried themselves by ruffling and puffing feathers away from their bodies. They always sunbathed alone, but I could not identify which owl was the incubating parent. I could easily distinguish between sunbathing, stretching, and wing-drooping (the latter observed one time on 17 May, 1240 h, 28[degrees]C; Fig 1b).

I provide the first information since the 1930s (Errington and Bennett, 1935) on habitat, brood production, diet, and behavior for an extralimital breeding pair of burrowing owls in the Great Plains and document a novel self-maintenance behavior in raptors. Similar to reports based on indirect observations of burrowing owl diets, owls in tallgrass prairie consumed more arthropods than vertebrates, with the latter contributing the most biomass in the diet (Poulin et al., 2011). However, arthropods represented far more of the total prey biomass (39.2%; Table 1) than others have reported for owls in their core range (1-23%; Thompson and Anderson, 1988; Moulton et al., 2005; Poulin et al., 2011). Thus, either indirect observations underestimated the importance of arthropod prey in burrowing owl diets, or owls in eastern Kansas are more dependent on arthropod prey than owls in the core breeding range, or both. The total biomass of prey delivered to the burrow decreased between incubation and brood-rearing stages. This difference could be explained by adults consuming more prey away from the burrow while brood-rearing, or caching items during incubation for later consumption, or both (Wellicome, 2005; Poulin et al., 2011). Owls tended to consume more vertebrates and fewer arthropods as breeding progressed. Pardalophora species--which represented at least 41 of 77 grasshopper prey items and 60.6% of grasshopper biomass--overwinter in late nymphal stages and only occur as adults from early April to early July (Smith, 1981; Otte, 1984). Thus, declines in grasshopper biomass delivered to the burrow between incubation and brood-rearing could have been a functional response to decreasing availability. Alternatively, burrowing owls could be more selective predators while caring for nestlings that require specific nutrients during development (Wellicome et al., 2013).

The timing and prevalence of sunbathing by burrowing owls suggests they may have done so to shed ectoparasites rather than to thermoregulate or dry wet feathers based on three lines of evidence. First, owls only sunbathed during midday when temperatures were high but never during cool conditions to collect heat or following rain events to dry their feathers. One limitation to my study is that I could not identify which adult owl was sunbathing, and I did not measure ambient temperature or humidity inside the burrow. If either the ambient temperature or ambient humidity, or both, was greater inside the burrow than outside during midday, the posture I observed could be explained by the female dissipating excess heat during incubation recesses. However, nearly half of the sunbathing events (10 of 21) occurred in July after incubation had ended. Second, sunbathing coincided with scratching and preening with uropygial gland oil, which could have been done to remove pests subdued by exposure to intense ultraviolet light (Clayton et al., 2010). Third, I never observed owls dustbathing. The soil surrounding the nest burrow was not loose and was overgrown with vegetation during June and July (Fig. 1a), which likely would have prevented the owls from dustbathing at the nest site. Thus, burrowing owls may sunbathe as an alternative strategy to dust-bathing for shedding ectoparasites in tallgrass prairie.

Seemingly small changes in environmental conditions that influence fitness can result in abrupt range edges, and it is often difficult to determine what factors prevent a species from expanding geographically (Gaston, 2003, 2009). Burrowing owls in tallgrass prairie exhibited feeding habits and nest-maintenance behavior (dung-lining) similar to conspecifics in the core breeding range. Although burrowing owls have been well studied throughout their core breeding range, they have never been documented sunbathing as I describe here. Owls in tallgrass prairie exhibited the capacity to alter their behavior in response to environmental change; however, a potentially high ectoparasite load and unfavorable conditions for maintaining pests could be one of several selective pressures that prevents burrowing owl populations from expanding eastward in the Great Plains. Tallgrass prairies are wetter than shortgrass prairies, and regions with higher ambient humidity support more ectoparasites, which can influence the life history strategies of their avian hosts (Moyer et al., 2002). Additional research is needed to explicitly test whether burrowing owls sunbathe to shed ectoparasites when dustbathing is not possible or to dissipate heat after enduring extreme conditions inside the burrow. Future studies should also consider whether factors other than environmental conditions, such as dispersal capabilities and demographic rates of core populations, influence geographic range limits of burrowing owls. Investigators should utilize rare opportunities to directly observe individuals beyond range margins and compare their habits to conspecifics in core ranges. As human activity is projected to drastically and rapidly change global climate and habitats during the next century, information on extralimital individuals can provide valuable insight into species abilities to cope with environmental change and advance our understanding of geographic range limits (Sexton et al., 2009).

I thank E. Welti, A. Ricketts, K. Waselkov, and A. Laws for assisting with prey and plant identification and for providing biomass data. A. Boyle, B. Sandercock, K. Waselkov, and an anonymous reviewer provided valuable comments that improved earlier versions of this manuscript. I thank K. Hase of the National Park Service and B. Obermeyer and P. Matile of

The Nature Conservancy at Tallgrass Prairie National Preserve for allowing me to monitor the owls.


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Submitted 6 February 2016.

Acceptance recommended by Associate Editor, Michael Scott Husak, 12

September 2016.

Caption: FIG. 1--Photos of adult western burrowing owls (Athene cunicularia hypogea) captured at a nest at Tallgrass Prairie National Preserve, Kansas, 2013. (a) Sunbathing during intense midday heat, (b) wing-drooping (Thomsen 1971), and (c) delivering Sialia sialis, Eastern bluebird and (d) Sigmodon hispidus, hispid cotton rat prey.
TABLE 1--Number of individuals (n) and estimated biomass (g) for prey
items documented for Western burrowing owls (Athene cunicularia
hypogea) during three breeding stages (PI, preincubation; Inc.,
incubation; and BR, brood-rearing) at Tallgrass Prairie National
Preserve, Kansas, 2013. I include the percentage of total prey items
and the percentage of total estimated prey biomass brought to the
burrow in parentheses. References indicate the sources for biomass
estimates. (-) indicates no observations.

                            PI   Inc   BR
Prey item                   n     n    n

  Sigmodon hispidus         --    1    --
  Microtine rodent (b)      --    1    --
  Rodent--unidentified      --   --    1
  Sialia sialis             --   --    1
  Passerine;                --    1    1
    unidentified (b)
  Crotophytus               1    --    1
    collaris (c)
  Phrynosoma cornutum       1    --    --
  Colubridae species;       --    1    --
    juvenile (c)
  Unidentified              --   --    1
    vertebrate (d)
  Unidentified carrion      1    --    1
Total vertebrates           3     4    6
  Scarabaeidae              2     9    3
  Unidentified (e)          3     3    2
  Acrididae (f)             23   42    12
  Gryllidae                 --    1    2
  Lepidopterans (g)         1     4    5
  Hemipterans               1    --    --
    (Cicadidae) (h)
Unidentified insects        1     6    3
Unidentified larvae (h)     1    --    --
Arachnids (Hogna species)   --   --    1
Gastropoda (snail) (h)      --    1    --
Crustacea (crayfish)        2     1    --
Total arthropods            34   67    28

Prey item                     N (%)        Mass (%)     Reference

  Sigmodon hispidus           1 (0.7)      100 (16.8)       6
  Microtine rodent (b)        1 (0.7)       40 (6.7)        6
  Rodent--unidentified        1 (0.7)       35 (5.9)        6
  Sialia sialis               1 (0.7)       31 (5.2)        3
  Passerine;                  2 (1.4)       32 (5.4)      8, 10
    unidentified (b)
  Crotophytus                 2 (1.4)       57 (9.6)        8
    collaris (c)
  Phrynosoma cornutum         1 (0.7)       24 (4.0)        8
  Colubridae species;         1 (0.7)       19 (3.2)        8
    juvenile (c)
  Unidentified                1 (0.7)       24 (4.0)        8
    vertebrate (d)
  Unidentified carrion        2 (1.4)       --
Total vertebrates            13 (9.2)      362 (60.8)
  Scarabaeidae               14 (9.9)      5.6 (0.9)        9
  Unidentified (e)            8 (5.6)      2.8 (0.5)        9
  Acrididae (f)              77 (54.2)   182.7 (30.7)     4, 5
  Gryllidae                   3 (2.1)      2.1 (0.4)        7
  Lepidopterans (g)          10 (7.0)       10 (1.7)        2
  Hemipterans                 1 (0.7)      1.0 (0.2)
    (Cicadidae) (h)
Unidentified insects         10 (7.0)        5 (0.8)
Unidentified larvae (h)       1 (0.7)      0.5 (0.1)
Arachnids (Hogna species)     1 (0.7)      3.0 (0.5)        1
Gastropoda (snail) (h)        1 (0.7)      1.0 (0.2)
Crustacea (crayfish)          3 (2.1)     19.5 (3.3)        6
Total arthropods            129 (90.8)   233.2 (39.2)

References: (1) Amaya et al. (2001); (2) Casey (1976); (3) Hector
(1985); (4) Landa (1992); (5) A. Laws, pers.comm.; (6) Marti (1976);
(7) Simmons (1987); (8) Steenhof (1983); (9) E. Welti, pers.comm.;
(10) E. Williams, pers.comm.
(b) Microtine rodent likely Microtus ochrogaster; unidentified
passerines include nestling and small adult.
(c) Crotophytus collaris includes large and medium adult; Colubridae
likely Lampropeltis calligaster.
(d) Likely P. cornutum.
(e) Likely Scarabaeidae; includes both large and small individuals.
(f) Includes Oedipodinae (Arphia, Hadrotettix, Pardalophora, and
Brachystola magna) and Melanoplinae (Melanoplus packardii).
(g) Lepidoptera includes Hyles lineata; unidentified insects all
non-Orthopteran items.
(h) I assigned a value of 0.5 g to unidentified insects and larvae and
1.0 g to Cicadidae and Gastropoda.

FIG. 2--Percent (%) frequency and estimated biomass of three prey
types of Western burrowing owls (Athene cunicularia hypogea)
observed during three breeding stages at Tallgrass Prairie National
Preserve, Kansas, 2013. The number of individuals (n) and total
estimated biomass (g) are indicated above each bar.

                 % Biomass   % Frequency

Pre-incubation   37          120.1
Incubation       71          284.6
Brood-rearing    34          190.5

Note: Table made from bar graph.
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Author:Herse, Mark R.
Publication:Southwestern Naturalist
Article Type:Report
Date:Dec 1, 2016
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