An Invasion by Great Horned Owls of a Commercial Structure in Austin, Texas 2

BugsInTheNews is a VIEWER-PARTICIPANT WEBSITE. This article by Jerry Cates, first published on 5 May 2012, was revised last on 28 July 2012. © Bugsinthenews Vol. 13:05(02).


Great Horned Owl (Bubo virginianus); in nesting cavity of commercial structure: North Austin, TX --- 4 May 2012

100a. Great Horned Owl (Bubo virginianus); in nesting cavity of commercial structure: North Austin, TX — 4 May 2012

In early May, 2012, a company that manages commercial properties throughout Texas called and asked for help with an animal that was annoying the occupants of one of the structures under their care in north Austin. One tenant at this property, a research group associated with the University of Texas, conducts a specialized form of scientific analysis. Their lab, on the second floor of the structure, designs devices that make precise measurements of biological phenomena, and the introduction of extraneous noise cannot be tolerated.

However, animal invasions of structures inhabited by humans pose a number of risks beyond those associated with noise production. Those risks include fire hazards caused by damage to electrical wiring, HVAC and electronic cabling, biological contamination from urine and feces, and the introduction of parasites — such as Baylisascaris procyonis, avian mites, fleas, and ticks — into the structure. Obvious cosmetic damage to the structure’s exterior is another problem, as well.


The kind(s) of animal involved was not known to the management company, but tenants were reporting that disruptive noises had been coming from several ceiling spaces on the second floor.

The culprits:

A visit to the property — a modern, relatively new set of two-story office buildings sheathed in EIFS stucco on their exteriors — quickly confirmed the existence of a number of wild-animal-mediated access holes, in the exteriors of each of three separate office buildings at this complex. However, this article refers only to one of these structures, referred to as building C.

Details of the wild animal incursions affecting the other structures at this site will be discussed in future articles. All of the structures here should be fully investigated to determine the extent of the animal nesting there. Exclusionary procedures carried out in one offers a high probability that the displaced animals (particularly the owls) will move to the others, making a bad situation worse for them. Such an outcome can be prevented with a site-wide exclusionary program, developed and carried out in a fully-coordinated project.


Animal Nest 1

PRIVACY NOTICE: The exact location of this site is not divulged in this article, in order to protect the privacy of the tenants and the property management companies involved.


The animal nesting holes in the walls of the structure are located directly under the eaves at the highest point of elevation on the exterior walls, and are obviously being exploited by wild animals of various kinds. Tell-tale signs of contamination and claw marks caused by the regular ingress and egress of the animals are conspicuous at each hole (see, also, the fecal staining on the exterior wall below nesting hole 1, in fig. 102, below). At another nest (fig. 105) the nesting hole appeared, at first glance, to be a large mud-dauber’s nest, until, directly under the hole, a large accumulation of bird droppings was noticed, indicating the dark “marking” was in actuality a nesting hole; on closer examination, it became clear the  hole was being used by birds, most likely European starlings (Sturnus vulgaris), a social species that is also a notable synanthrope.

Some of the holes in the exterior of this building appeared, at first glance, to be man made, as they seemed rather uniform in size, shape, and position. Each was also located in close proximity to the apex of the affected eave, as though placed there by the building’s architect to serve an important function. It is not uncommon for wild animals, including birds, squirrels, and others, to appropriate man-made orifices for their own use.

Wildlife exclusion from human-occupied structures usually involves sealing the animal’s entry ports, typically using a one-way door that enables the animal to exit, but not to return, then — once the animal has departed, is not able to get back in, and ceases further attempts to regain access — applying a more-or-less permanent seal that is more durable and lasting than the original EIFS sheathing itself. In the process, the wildlife specialist must always be alert to the possibility that the port involved serves a valuable function. Permanently sealing a hole that provides necessary drainage or ventilation to a building creates unintended consequences, often on a scale rivaling or exceeding those related to animal invasions.

But, if these holes are manmade, and serve a useful function, what function might that be?

Great horned owl (Bubo virginianus); nest, in context; north Austin, TX --- 4 May 2012

101a. Nests 4 & 5 in context.

Drain Spout

101b. Drain Spout

Drainage of rain water from the roof seemed the most likely possibility, if the holes are part of the building’s design. However, free discharge of captive roof effluent from an eave would be unconventional, to say the least. On the roof, which is a flat, built-up arrangement covered with gravel over rubberized sheeting, a number of drains were observed, but these were found to be connected to individual internal wastewater downspouts that empty into the grounds on the outside of the structure, using spouts that emerge from the exterior walls about four feet above the ground (fig. 101b).

Not one of the holes in the eaves was fitted with a drain spout extending beyond the eaves, as would be expected from a functional drain, and none showed signs of extensive staining that would be expected — on the building’s exterior below each orifice — from a primary or secondary rainwater or condensate discharge port. Whatever staining was noted appeared directly beneath each hole and is likely confined to natural accumulations of urine and other animal excretia emanating from the nests themselves.

It seems clear, then, that the holes in the eaves have nothing to do with normal drainage. To investigate their origins further, photographs were taken of each one from the ground, and later examined at the EntomoBiotics Inc. lab. That examination proved that although the holes appeared uniform in size and placement when viewed from a distance, they displayed a range of variation in size and shape fully consistent with animal nesting activities. A number of different animals appeared to be involved, including starlings and similar birds, squirrels, opossums, raccoons, and owls.

Strong evidence of squirrel incursions into the attic was found on the roof, and — during a second inspection on 5 May — a squirrel was actually observed entering one of the holes (fig. 104; animal nest 3) after jumping from a nearby tree limb and climbing up a short section of the roughly textured stucco exterior. Although it is unlikely that an animal would naturally scale the stuccoed walls in a vain attempt to gain access to the roof (the eave overhang would prevent that), the existence of nesting ports in the exterior wall would lead to active scaling of the stuccoed walls by mammals nesting there, i.e., after the ports were in place but not before.

No firm evidence of opossum or raccoon nesting was observed in the attic spaces surveyed, but two animal burrows were found in the soil on the perimeter of the structure’s foundation, and one of those burrows (fig. 106) showed signs of recent usage. This is likely the burrow of a skunk, an opossum, or an armadillo, or all three. These animals often have several burrows within their territories whose usage — except when nursing young — is rotated opportunistically. They have been known to share their burrows with one another, though generally not at the same time. Raccoons, on the other hand (and to a lesser extent, opossums), tend to prefer aerial nests which, in urban and suburban settings, are often inside the attics of accessible manmade structures. Thus, the likelihood of opossum or raccoon incursions into this structure cannot be ruled out. Further, as long as animal nesting — within the walls of this structure — continues unchallenged, the greater the likelihood that raccoon and opossum nesting will become a significant factor.

Raccoon invasions of man-made structures often pose serious problems for their human occupants. The challenges involved in evicting these charming, highly intelligent animals from such habitats are exemplified by a raccoon exclusion project I was asked to carry out in Denton, Texas, in April 2010.

Although no birds had been noted during the initial inspection, a later analysis of the photographs revealed the presence of a great horned owl (fig. 100a) that had momentarily poked its head out of one of the holes, attentively watching as its photo was being taken. Below that hole, but just around the corner from it, another hole appeared to be stuffed with leaves and other loose nesting detritus (fig. 101a, above). This lower nest is likely that of a family of squirrels that has closed off the outside portal to protect against predation by the owls; it is obviously still active, as the detritus plug would quickly be eroded by wind eddies unless constantly replenished by the squirrels.

Additional processing of the photographs taken of the nesting holes revealed what was initially thought to be a second family of owls in nesting hole 1 (fig. 100b). Additional photos were taken on several different days, the most recent on 9 May 2012, and these are inconclusive regarding the identity of the animal nesting in hole 1. It appears to be the most active nest at the site, but beyond that a firm conclusion on its occupants awaits further investigation.

The best conclusion from all this evidence is that all the nesting holes observed were originally made by squirrels. They were made, not for the squirrels to gain access to the attic, but as a direct result of the attic nesting behavior exhibited by these animals. The squirrels’ nesting holes only later became roosts for other animals, chiefly birds, including at least one parliament of at least one, and possibly two families of great horned owls.

Why, and how, were these holes made?

Animal nest 1

102. Animal nest 1

Animal nest 2

103. Animal nest 2

Animal nest 3

104. Animal nest 3

Animal nest 6

105. Animal nest 6

2012 May 5 Active Animal Burrow

106. Active Animal Burrow

Exactly how or why squirrels made holes in the sides of this building, high above the ground level, is a matter of conjecture. However, it is possible to arrive at a plausible explanation based on what is known about the behavior of the animals involved.

Owls are opportunistic nesters. They typically exploit existing nesting sites previously made by other animals. Squirrels, by comparison, are active nest builders adept at modifying less-than-ideal habitats, and converting them — over time — into more suitable nesting arrangements.

The attic spaces of these buildings are spacious and uncluttered. They provide relatively good nesting sites for squirrels entering the attic from openings on the roof. Under normal circumstances such nests would never result in portals extending to the exterior of the structure, but the structures in this complex are built with EIFS exterior walls, which provide a relatively soft substrate through which a squirrel can easily chew.

Numerous openings on the roof were found that could easily be exploited by squirrels (figs. 200, 201, below). An inspection of the chases for high and low pressure freon piping from A/C condensers on the roof revealed that each one, practically without exception, provides ample space for a squirrel to pass from roof to attic. Some of these chases showed clear evidence of regular animal activity (e.g., the insulation damage to the low pressure freon return line displayed in fig. 200).

Once inside the attic, the nesting squirrels would naturally nest against an outside wall, preferentially at a corner that provides insulated walls (the stucco exterior) on at least two sides. One thing leads to another:

1. Squirrel nests generate recurring litters of young squirrels, two litters a year, averaging — for the fox squirrel (Sciurus niger) native to this part of Texas — three young per litter; 2. The young squirrels require chewable media on which to try out their rapidly developing gnawing teeth (a pair of incisors at the anterior maxilla), and the nearest chewable media just happens to be the interior surface of the stucco wall the nest is built up against; 3. Eventually — after enough litters have been produced in the nest — a sufficient portion of the wall becomes chewed away to the extent that a small hole is produced; 4. The adult squirrels find the hole useful as a source of diurnal lighting and thereafter actively enlarge it; 5. Before long the hole is large enough to serve as an ingress/egress portal that, in concert with the rough EIFS exterior of the building, enables the squirrels to scale the exterior wall to come and go with ease; 6. At this point the nesting hole becomes the primary portal for the nesting squirrels and the rooftop access reverts to a secondary status; 7. Later, predators of squirrels such as great horned owls easily purloin the squirrel nests for their own use.

It is important to note that the EIFS walls of these structures are most likely not the only things these squirrels have been chewing on. Every cable, electrical wire, etc., accessible by these animals in the areas they have access to is at risk. Squirrel damage to the electrical and electronic infrastructure of office and residential buildings is a serious problem that should be corrected as quickly, and effectively, as possible. Time is of the essence, but slipshod, temporary repairs are a bad investment. Only permanent corrections, conducted in a stepwise process that progressively excludes the squirrels and the birds from the building, using methods guaranteed in writing and backed up by experienced and knowledgeable wildlife specialists, will provide an acceptable solution to an active multi-species animal invasion of this nature.

A/C Chase in context

200. A/C Chase in context

A/C Chase portal

201. A/C Chase portal

But what about the great horned owls?

Great horned owls are adept at taking over the nests of other animals (they prey on practically all the animals involved, and are formidable adversaries), so any holes originally fashioned by squirrels would be viewed by these and other birds, by virtue of their locations — high on the structure, beyond the reach of terrestrial predators like humans — as highly valuable nesting sites. If a great horned owl decides to take over such a nest, it does so without encountering much resistance. The nest’s original inhabitant either quietly departs, or simply becomes the owl’s next meal.

Despite the obvious tension between the owls and their natural prey, it is not uncommon to observe both occupying robust nesting sites in close proximity to one another (note fig. 101, which depicts an owl’s nest only inches away from an active squirrel’s nest). Such phenomena are often found in nature, with — for example — ratsnakes occupying the burrows of mice and rats without the former fully dislodging (not to mention consuming) the latter. Ironically, such relationships tend to benefit both animals, though at a price (as demonstrated, for example, by the Red Queen Hypothesis of selective adaptation). The point, as it relates to this discussion, is that the presence of the owls will do nothing to blunt the activities of the squirrels at this site, even though their predation on the squirrels may, from time to time, reduce their numbers. Both sets of animals must be dealt with separately, and decisively, to bring the animal invasion of this structure to a close.

Excluding owls from this site first involves fully excluding the squirrels from gaining entry to the attic from rooftop portals by sealing all such access ports on the roof. All but one will be sealed with permanent seals, but one — the portal showing evidence of use — will be fitted with a one-way door the squirrels can exit through but cannot reenter.

Afterward, the squirrels and other animals, including the owls, will be prohibited from using the nesting holes in the sides of the building by installing permanent, durable seals (in the form of hard over-caps) that occlude those holes after taking steps to ensure no nesting birds are inside the attic space. Adult squirrels that retreat into the far reaches of the attic will be able to escape via the one-way door on the roof, but imprisoned birds — especially owls — will not be able to use such escape routes and must be excluded via one-way doors on the nesting portals themselves.

This work should be scheduled after all nesting young have left the affected nests (i.e., no earlier than mid-June, and no later than mid-August, to take advantage of the period between the two litters normally produced by the fox squirrel, Scirurus niger), by wildlife specialists familiar with the behavior of the animals involved. Even though the nests should, in the interval described, no longer contain young, adult owls will continue to defend their nests from intruders, necessitating the wearing of protective gear capable of withstanding their defensive attacks.

A complete plan of action for safely excluding all these animals from this site, using methods that will not cause injury to any of them, including the adult or juvenile owls nesting here, was submitted to and accepted by the property management company. As already mentioned, human safety must be given careful attention. These birds are possessed with sharp, forbidding talons that, in the adult, reach 1.5 inches in length. They are able to sink their talons into an adversary with up to 300 lbs. of crushing strength (compared with the grasping strength of an ordinary man’s hand of 60 lbs.), and have been known to blind and take huge chunks of flesh out of humans who attempt to violate their nesting territories.

The great horned owl is relatively common throughout the U.S. It is not a migratory bird, but remains within its established territory throughout the year. Mating occurs in the late fall, and eggs are often being incubated by late December. The eggs hatch in January or early February, and the young develop slowly and typically remain in the nest until early to mid-June. This means that any attempt to remove these birds earlier than that date will likely be fraught with daunting (and largely unnecessary) challenges.



  • Kingdom Animalia (ahn-uh-MAYHL-yuh) — first described in 1758 by the Swedish taxonomist Carolus Linnaeus (23 May 1707 – 10 January 1778), using the Latin word animal = “a living being,” from the Latin word anima = “vital breath”, to refer to multicellular, eukaryotic organisms whose body plans become fixed during development, some of which undergo additional processes of metamorphosis later in their lives; most of which are motile, and thus exhibit spontaneous and independent movements; and all of whom are heterotrophs that feed by ingesting other organisms or their products;
  • Phylum Chordata (kohr-DAY-tuh) — animals that have, at some point in their life cycle, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail.
  • Subphylum Vertebrata (vurr-tuh-BRAY-tuh) — chordate animals with backbones and spinal columns;
  • Class Aves (AAH-vays) — Birds, i.e., vertebrate, endothermic (warm blooded), bipedal, tetrapod, egg-laying animals covered with feathers and fitted with wings that (usually) confer the faculty of flight. With around 10,000 living species, they are the most speciose class of tetrapod vertebrates, regarded by paleontologists as the only clade of dinosaurs to have survived the Cretaceous–Paleogene extinction event 65.5 Ma ago.
  • Subclass Neornithes — modern birds having feathers, a beak without teeth, that lay eggs with hardened shells, exhibit a high metabolic rate, have a four-chambered heart, and a strong, lightweight skeleton. The forelimbs of all modern birds are modified to function as wings and most are capable of aerial flight. The digestive and respiratory systems of all modern birds are specially adapted to the necessities of flight. Many birds, e.g. corvids and parrots, are highly intelligent animals capable of the manufacture and use of tools, as well as transmitting cultural knowledge from one generation to the next;
  • Order Strigiformes (Wagler, 1830) — Owls, most of whom are solitary and nocturnal, that typically hunt small mammals, insects, other birds, and fish; characterized by small beaks, wide faces, large binocular eyes; divided into two families: typical or true owls (Strigidae) and barn-owls (Tytonidae); the order was first described by the German ornithologist Johann Georg Wagler (1800-1832) in 1830 while he was director of the zoological museum at the University of Munich, a position he’d held since 1926; he worked extensively on avian collections from Brazil, and died in 1832 from an accidental but self-inflicted gunshot wound while in the field collecting specimens; apparently Wagler crafted the order’s name using the same Greek word, for screech owl, στριξ (pron. “strix” or “strige”) that Vigors had applied to the family name of the true owls five years previously;
  • Family Strigidae (Vigors, 1825) — typical (true) owls, presently represented by 25 genera and 189 species; these vary significantly in size, from the smallest Elf Owl, to the Eurasian Eagle-Owl, the latter being a hundred times larger; all typical owls share a similar body plan, with large heads, short tails, cryptic plumage and round facial discs surrounding the eyes; most are arboreal, though some burrow into the ground; all secure their food on the wing; their wings are large, broad, rounded and long; as with other birds of prey most exhibit reverse sexual dimorphism, wherein females are larger than males; the family name was first described in 1825 by the Irish zoologist/politician Nicholas Aylward Vigors (1785-1840), who merely applied the Greek word for screech owl, στριξ (pron. “strix” or “strige”); the name is today juxtaposed with the Tytonidae (crafted by the American ornithologist Robert Ridgeway in 1914 from the Greek τυτω “tyto”, meaning a night owl), the barn owls; Vigors co-founded the Zoological Socety of London in 1826, and later founded what eventually became the Royal Entomological Society of London in 1833.
  • Genus Bubo (Duméril, 1805)  Horned owls; classically this genus includes all the North and South American horned owls and Old World eagle-owls, but the number of species within the genus is in dispute; the generic name was first described in 1805 by the French physician, zoologist, and ornithologist André Marie Constant Duméril (1744-1860), applying the Latin name Bubo, which simply means “owl”;
  • Species Bubo virginianus (Gmelin, 1788) — the large Great Horned Owl or Tiger Owl native to and the most widely distributed true owl of the Americas; body length 18–27 inches (46–69 cm); wingspan 40–60.5 inches (101–153 cm); weight 0.72 to 2.55 kg (1.6 to 5.6 lb); adults exhibit large ear tufts, reddish, brown or gray faces with white throat patch; iris of eye is yellow; horns adorning head crown are tufts of feathers; underparts light in color with brown barring; upper parts of mottled brown; legs and feet entirely feathered to talons.


Anatomy: in process

Behavior: in process

Common Names: tiger owl, hoot owl, cat owl, chicken owl, eagle owl, great horned owl, horned owl, king owl, Virginia horned owl;

Distinguishing Characteristics: in process

Distribution: in process

Physiology: in process

Mythology: in process

Similar Families: in process




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2 thoughts on “An Invasion by Great Horned Owls of a Commercial Structure in Austin, Texas

  1. David James May 12,2014 12:50 am

    Wow, the writer really gave the beautiful and valuable rodent hunter both barrels here.

    “fire hazards, damage to electrical wiring, HVAC,electronic cabling, biological contamination, urine and feces, fleas, tick,damage to the structure”

    What bird or animal is any different and the true dangers of the animals, we have shared the globe with for millennium, are well known. This is presented as a danger separate and vastly more catastrophic than every other living thing. And this is offered near to the very start of the article. I sense a person on a mission here. Hmmmmm.

  2. admin Dec 13,2014 12:02 pm

    David: Owls in the forest are beautiful to behold; when they invade commercial structures and bring bugs with them that are capable of producing disease in the human inhabitants of that structure, they are anything but beautiful. Please notice I do not recommend measures that cause injury to the owls. Exclusion merely prevents them from inhabiting man-made structures where they become nuisances and pests.

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