The Darkness, Seclusion And Rustic Factors

[Steve Kroenke]

The Darkness, Seclusion And Rustic Factors

Brightness, visibility and slickness are everywhere it seems in the inside world of purple martin housing. Most modern purple martin housing seems to minimize the darkness and seclusion and maximize the brightness and visibility within the martins’ nests. This in part can be attributed to small nesting compartments with entrance holes placed around an inch above the bottom. There are slick, white plastic gourds that enhance the brightness of their interiors when light pours through the entrance holes. And of course, there are hundreds of thousands of those shiny aluminum compartments of the Trio houses scattered all over North America. Other commercial aluminum houses are painted white inside, including the Lonestar series. The sides and bottoms of both aluminum and plastic are slick and provide no rustic surface for martins to cling to and climb. And some martin house manufacturers even claim that martins prefer bright shiny compartments! Nonsense.

Okay, so why a discussion about the mysteries of interior light, darkness and roughness within purple martin housing and their possible relationship to martin nest site preferences? Do martins like it bright and slick? Or do they prefer it dark and rough? Maybe they are swingers and don’t care one way or the other? Sounds like we are talking about the secret personal life of martins! No, just exploring how such factors may or may not influence a martin’s, particularly the female’s, decision to nest in various housing types. Of course, no one knows for sure if such concepts are truly relevant, but they are certainly worthy subjects for further exploration.

So are you ready to shed some light on the darkness of martin housing interiors? Ready to slide on some rough surfaces?

Physics 101

There is a key law of physics in operation relative to the darkness/lightness dichotomy: light colors tend to reflect light, thereby brightening an area within a confined space while dark tones absorb light and reduce the brightness factor. So if you paint a room in your house white, the room will be brighter because any light striking the white wall will reflect back out into the area. In contrast, a room painted black or other dark color will be darker as light is absorbed by dark tones. This same situation will exist inside martin house compartments or gourds when light pours through entrance nest holes and strikes the interior walls, bottoms and tops. A shiny aluminum surface or white painted interior of a house compartment or gourd will be brighter than one that is darker in coloration. Physics 101.

Hole-nesting Birds Seek Darkness and Seclusion For Their Nests

For most hole-nesting birds, darkness and seclusion may mean survival while brightness and visibility may mean death. All these birds have one thing in common and it is not just breeding in a cavity. Most of their nests are constructed in relatively dark secluded areas, some of which can be a foot or more deep in dead tree cavities or even farther in bank tunnels. And why do they do this? There is a good reason for such a nesting arrangement. Predators. The darker and deeper a nest cavity is the less visible the contents are to the watchful eyes of hungry predators. And hole-nesting birds know this. Plus more secluded nests are better protected from rain soakings. You don’t see woodpeckers constructing their cavities only a few inches deep where their eggs and young are clearly visible from the outside. They dig them vertically deep and their nest cavities are like pits. Such nest cavities may be safer from predators. Additionally, these deep natural cavities have rustic interiors, so any birds nesting in them can easily climb up the walls.

The Purple Martin’s Past Was Dark, Secluded And Rustic

Before the advent of human supplied purple martin housing in eastern North America, martins nested in deep, dark abandoned woodpecker holes or similar natural cavities. The woodpecker cavities were vertically deep for the most part, sometimes as much as a foot or more. Such cavities provided the female martin in particular a dark and secluded place for her nest, eggs and young. Plus these nest chambers were rustic and provided a surface that allowed martins to cling to the sides and climb up from the bottom to the entrance hole. Martins have short legs and strong feet with sharp nails. These physical attributes are perfect for climbing vertically deep rustic substrates and martins may have evolved such characteristics in part because of their nesting behavior.

And you need to understand an important aspect of purple martin nesting behavior: while the male chooses the territory, the female chooses the nest site and the females are far more selective than the males. She is seeking the safest and best nest for her young. The darkness and seclusion of a deep cavity perhaps gave her a sense of security as her nest was not as visible to hungry predators and provided some protection from their watchful eyes.

One problem with nesting in abandoned woodpecker holes is that the nest bowl diameter tends to be relatively small. I have examined the insides of both red-headed and red-bellied woodpecker nest cavities and the bottoms tend to be oval in shape, often not more than five inches long and three or four inches wide. These cavities are a tight fit for a family of most birds. However, this has been the norm for most hole nesting birds for many thousands of years and has functioned well.

The First Big Change…Indians And Natural Gourds

Then along came the Indians and natural gourds and the martins made their move. The interior color of a natural gourd, being used as a birdhouse, tends to vary somewhat but usually is a light to medium brown or beige often mixed in such tones. Some gourds may even have darker colored interiors. There may still be the dried papery light colored pulp that often clings to the sides. A natural gourd cavity is brighter inside than a deep woodpecker hole, but still retains the rustic inside and usually provides more darkness and seclusion than equivalent size white plastic gourds or aluminum houses. A natural gourd with a consistent layer of dried pulp coating most of the interior may be nearly as bright inside as the plastic gourds. A martin can climb up the side of a natural gourd because the interior is rustic and provides good toehold value. This can’t be done with slick plastic or aluminum. Natural gourds are in between the deep tree cavities and modern day housing relative to the darkness and seclusion factors. So, natural gourds were not a drastic change from cavities in dead trees. Darkness, seclusion and rustic insides are still very evident in natural gourds.

Aluminum, Plastic, and Painted Wood…Here Comes The Light, Visibility And Slickness

But oh how things have changed! Now days, commercial martin housing is not vertically deep and the entrance holes are around an inch above the bottom of the compartments, clearly exposing the insides. Commercial aluminum houses have shiny aluminum interiors or are painted white. In the small bright 6x 6 cubicles of most commercial aluminum martin houses, the contents are easily seen when looking into the hole, much to the predator’s delight when seeking a meal. The sunlight just pours through the entrance hole and brightens up the interior as the shiny aluminum reflects and spreads the light throughout the nest chamber. Even the 6 x 12 compartments of the Lonestar houses are painted white and you can see to the back of the rooms. The same situation exists with many of the plastic gourds currently on the market. They tend to be bright white on the outside and the inside, and such an interior is brighter than a similar size natural gourd. Again, light colors, particularly white, in a confined space reflect the entering light and spread it out thereby brightening the area; dark colors absorb the light and maximize the darkness. The plastic access door lids used for both plastic and natural gourds may be translucent and let the light through to help brighten up the interior. Just holding up the lid to the sun and you may see your hand silhouette pressed against the outside of the lid’s top. And the interior sides of aluminum, plastic and many painted wooden houses are smooth and provide almost no toehold value for martins to cling to the sides. Shiny aluminum and white or light colored plastic are different in texture, feel, and in the way they enhance inside brightness of martin housing from the interiors of natural gourds and unpainted wooden houses. Wooden houses with white interiors would also maximize the brightness factor.

Rustic Factor

Since most modern day martin houses have entrance holes around an inch above the compartment bottoms, martins do not need to climb or cling to get out. The rustic component of the darkness and seclusion factor may not be that relevant now days. However, rustic texture may be more important for providing a solid toehold for greater pushing power and foot traction of a martin trying to slip into a tight starling resistant entrance hole (SREH) on a slick plastic gourd. A rustic texture does provide a better platform for nesting material and keeps the material from sliding around as much, particularly in plastic gourds. I have seen this sliding nesting material phenomenon in my plastic Supergourds. When the female martins would spin their bodies to create a nest bowl for their eggs, sometimes the female would spin to the bottom and reach the bare plastic. The nesting material just kept separating because it would not adhere to the slick plastic bottom. Perhaps a rustic interior has a more natural feel to a martin.

Forget Darkness And Seclusion…Maybe It Is Just Larger Size?

Does size really matter in the world of purple martin housing? There is a temptation to say expanding the dimensions of a nest compartment or using a larger diameter gourd increases the attractiveness value to martins. That is, the size of the interior is the key factor and independent of other variables. But is it size or is it increasing the darkness and seclusion factors? Maybe it is both. Larger natural gourds either hung vertically or horizontally are readily colonized by martins. Now they are larger in size, but they are also darker and more secluded inside. Larger house compartments are also popular with martins; but again, is it just size or the increased darkness and seclusion. The interrelationship between size and the darkness and seclusion factor is interesting. You can increase the size of a nesting compartment in a martin house but paint it white and it will still be relatively bright when sunlight pours through the entrance hole. And those 10-inch diameter Supergourds, with their white interiors, are usually brighter inside than smaller diameter natural gourds with darker insides.

But Martins Have Adapted To The Light And Have Thrived

And yet, martins have readily colonized and thrived in the bright, visible and slick nesting compartments of commercial houses and plastic gourds. In one way, you can make the argument that humans have forced martins to adapt and nest in such housing because the commercial aluminum houses are probably the most common houses erected in the United States.

So is there even an issue here to discuss? Does it really make any difference? Do martins prefer the darker secluded interiors of natural gourds or unpainted, rustic wooden houses or do they prefer the brighter compartments of modern day plastic gourds and aluminum houses? And maybe martins really don’t care one way or the other?

There Is Still Something Here To Explore More Thoroughly

I am guilty of having an inquisitive mind and like to meddle in martin behavior and biology. I am not satisfied with generalities and like details. So I am highly intrigued by the darkness and seclusion factor’s possible influence on a female martin’s selection of one type of housing over another for her nest. A further exploration of the role of rustic interiors, darkness and seclusion may yield insights into improving the attractiveness of current housing and particularly helping those folks who are experiencing year after year of failures to establish a martin colony. We should always be in a research and explorative mode of operation. Keep a pioneering spirit! Never pass on a chance to look at a subject in detail and question so called standard practices. Insight is gained through taking chances at times and perhaps delving into subject areas that are often given just cursory reviews. I am amazed how much I have learned over the years about purple martin behavior and how much I didn’t know! Sometimes, I have to re-think or completely change a viewpoint because I learn something new through research, detailed observation, or insights provided by other martin landlords.

Are There Any Formal Research Studies?

I am not aware of any research studies that have been done to test the darkness, seclusion and rustic factors independent from other variables like size of compartment/gourd, type of housing, and housing design configurations. Sometimes multiple factors are so interrelated that you can’t extrapolate one from the other with any true precision or reliability. The relationship between size and darkness/seclusion is a good example. Several factors may jointly operate and produce a combined effect. Drawing definite conclusions one way or another requires objective data that has been carefully collected and analyzed using scientific methods.

However, all that said, we can still look at and consider anecdotal or just common sense viewpoints. If we can see significant trends or make consistent observations over a period of time, then we may be on to something that can be further explored on a more scientific level. Backyard observations from serious martin landlords have an important place in purple martin colony management and can lead to significant improvements in housing designs that ultimately benefit martin survival.

Possible Indications Of The Importance Of The Darkness, Seclusion and Rustic Factors

One possible indicator that martins may prefer darker, secluded and more rustic nesting interiors comes from reviewing occupancy levels recorded in various housing types. This involves studying martin housing preferences during various stages of the nesting season such as territory establishment by the males, female selection of the male and his territory, and the female’s ultimate choice of a site for her nest, eggs, and young. Do the territory seeking males gravitate toward one type of housing first? Most importantly, do the mate seeking females consistently select males occupying a certain kind of housing and stay to breed?

Huge Natural Gourd Colonies

There is no question about it. Natural gourds are perhaps the most attractive housing you can use, particularly in parts of the Deep South. Research by the PMCA has shown that natural gourds are highly attractive to martins. These colonies can be enormous in size, often over 100 pairs. But you can’t say this is because gourds are rustic, darker and more secluded inside. Just shows that there may be something to it, but not a direct correlation as there are only natural gourds in the colony, and there is no comparison with plastic gourds or houses with lighter interiors. However, if an area contains both natural gourd colonies as well as colonies of other housing types, you can compare occupancy levels and see if there are any patterns. But again, you really can’t say for certain how much effect, if any, the darkness, seclusion and rustic factors are exerting on the martins’ preference for one type of housing over another. Other factors could be involved, too.

Colonies Offering A Variety Of Housing

What about colonies using a variety of housing? Are there differences in martin preferences among houses and gourds? If so, can you narrow the selective criteria to the darkness, seclusion and rustic factors? Are natural gourds with their darker interiors more attractive than equivalent size plastic gourds with light insides? What about houses? Are houses with unpainted interiors or ones painted dark more attractive than ones that are painted white or are aluminum? Many of these questions can only be reliably answered in controlled studies where controls are in place to minimize or eliminate other variables that could also influence martin preferences.

From what I have read, observed and heard from other martin landlords, when martins are offered a choice of a variety of housing, martins generally, but not always, select natural gourds more often. This has always been the case in my colonies and I have used natural and plastic gourds, as well as commercial and homemade aluminum and wooden houses. My natural gourds were nearly always occupied first and in my largest super colony no martins would nest at the site until I added natural gourds. The martins shunned the fancy aluminum houses with the bright, visible interiors for years. So many factors come into play and rarely does a multi-housing colony site have scientific controls in place to minimize the influence of other variables. But it is still interesting that natural gourds, with their dark, secluded and rustic interiors, are so attractive to martins when compared to other housing.

Modified Trio Houses…A Clue That Darkness And Seclusion Are Important

The Trio aluminum martin houses are probably the most common commercial ones around. Martins will colonize in these houses and breed successfully. However, something interesting occurs when you start messing with the insides of the houses. When the Trio houses are modified to create double nest chambers, any martins selecting these houses, invariably build their nest in the isolated, darker and secluded compartment that is AWAY from the entrance hole. It is important to understand that the actual nest compartment has NOT been enlarged and is still only 6” x 6” in dimensions. What has been created is a no bird foyer that martins enter first through the entrance hole. This area is open to light and highly visible. Then, another entrance hole is cut in one of the room dividers to the nest chamber, so the actual nest area is secluded and not flooded with as much light, thereby making it darker. If brightness and visibility were not issues, then martins would also probably nest in the foyer area of these double nest chambers. But they usually don’t. So the darkness and seclusion factors may be the key variables in operation here.

Okay, So What Does All This Talk About Darkness, Seclusion and Rustic Interiors Mean Now?

Purple martins are hole-nesting birds and they seek out the nest sites that may offer the safest place for their offspring. Their babies are the parents’ genetic legacy and continuation of the species is what it is all about. Keeping their young away from predators is a primary concern. So breeding in a dark and secluded place makes the nest and young less visible to the eyes of hungry predators. If given a choice or opportunity, martins may instinctively gravitate toward gourds and houses with interiors that maximize darkness and seclusion and minimize brightness and visibility. This is particularly true of the females that select the nest site. Perhaps the darkness and seclusion factor is in the purple martin’s genes. The rustic component doesn’t seem to be as relevant now days since martins in Eastern North America no longer nest in deep abandoned woodpecker holes. However, the texture of natural gourds and wood does provide a substrate for climbing and better holding the nest foundation together. This may be intrinsically important to female martins in selecting a male’s territory.

So what does this imply for martin housing? Well, it may imply nothing, but it sure makes me wonder. The attractiveness of natural gourds and the obvious preference of martins for double-chambered house compartments may be strong anecdotal evidence that female martins are seeking nest sites that maximize darkness and seclusion. The females may be doing this so that their young are not as visible to predators and other dangers.

Suggestions To Enhance the Darkness, Seclusion And Rustic Factors

For those folks who are interested in incorporating the darkness, seclusion and rustic factors in their plastic gourds and aluminum houses, here are some ideas you may want to consider. Maybe try it on a few gourds or house compartments. Again, no one knows for sure if martins are influenced one way or the other. But if you are having difficulties attracting martins, then this might help. However, it is undeniable that most cavity nesting birds seek dark and secluded areas for their nests to perhaps hide their young from predators. This may just apply to martins as well. Here are some suggestions:

(1) Make sure any plastic access lids on your natural or plastic gourds are not permitting sunlight to enter. Hold the lid up to the sun or a light and see how bright the underside of the lid appears. Can you see the silhouette of your hand pressed against the top of the lid? If so, paint the inside of the lid black or other dark color to block out the light.

(2) Paint the insides of your white plastic gourds a darker color such as medium to dark brown to somewhat replicate the interior color of an equivalent size natural gourd. Again, dark colors absorb light while lighter colors reflect and spread it. I have also used black and tan for plastic gourd interiors and martin readily nested in these gourds. Tan or medium brown is probably more similar to the coloration found in natural gourds.

(3) If you are using Supergourds, you may want to eliminate the semi-circle of light from the interior of the outer rim of the rain canopy that may appear above and around on the inside of the entrance hole. The plastic in this area may not be sufficiently opaque on some Supergourds. This can be done when you paint the gourd insides or you can paint the outer perimeter of the rain canopy.

(4) Coat the wet painted interior bottom surface of the plastic gourds with sawdust or similar material to create a more natural texture. You could also streak the bottoms with some kind of adhesive to produce multiple ridges. This action may reduce the sliding nesting material phenomenon that sometimes occurs when the female martin spins to create her nest bowl in a plastic gourd. She may literally be sitting on the bare plastic when the material continues to separate and slide. The nesting material may better adhere to a rustic surface. A sub-floor or nest tray that is NOT slick should help, too.

(5) Create double nest chambers with existing Trio and similar aluminum houses. This action greatly increases the darkness and seclusion factor and martins invariably nest in the nest chamber that is AWAY from the entrance hole.

(6) If you do not modify your aluminum martin house that has the 6 x 6 dimension compartments to create double nest chambers, consider painting some of the room interiors a dark color, like medium or dark brown to maximize the darkness and seclusion factor. Even if you are using deeper nests, such as those in the Lonestar series, you can create a darker and more secluded environment by painting the inside rooms a darker color. I painted the insides of one my 14-room Alamo Lonestar house with a light film of black spray and martins readily accepted the compartments. After doing this, I couldn’t see at night inside the compartments when my flood and porch lights illuminated the house. This may help to reduce visibility to owls, too. The inside compartments, which were white initially, became visible when light struck the house at night. Owls may be able to see all the way to the back of horizontally deep but vertically shallow compartments that are painted white or have shiny aluminum sides.

(7) Add pre-built nest to your gourds and house compartments to create a close replica of a natural martin nest. Such nesting material will absorb light rays and help increase the darkness and seclusion factor. I use dry pine needles to create a solid foundation. Then I add a handful of dry oak leaves to a depressed bowl area at the back of the nest compartment or gourd.

(8) Add tunnel extenders to the entrance holes of plastic gourds. You can attach PVC pipe pieces of various lengths, usually three or more inches, to the gourd entrance holes. These tunnels can be either straight or angled. This creates seclusion and darkness in the nest chamber, as sunlight does not directly enter the nest hole.


Purple martins are similar to other hole-nesting birds in one critical way. They seek out nest sites that will offer the best chance for their young to survive and not be eaten by predators. To keep their nest and young protected, martins may instinctively choose when available those sites that maximize darkness and seclusion and minimize brightness and visibility. We might be able to increase the attractiveness of current aluminum houses and white plastic gourds through darkening the interiors and creating double nest-chambered compartments. Let’s keep the light out.

Steve Kroenke

[Guest]

I enjoyed your post, Steve. I will be offering a coates watersedge and natural gourds this year. It will be interesting to see which one the martins might prefer. I've had a hard time attracting martins; this year I will add the naturals to see if that will make a difference. There are 2 other wood houses around the lake (and several aluminum off the lake), but I have not seen natural gourds, or plastic ones, up there. I have prepared my gourds, but have left them natural. The more I read on this forum about heat, I may have to consider painting them white. I really do like the natural look, but looks aren't everything. I probably need to do what is best for martins, not me.

Thanks again for a great post.


Lanell

[Don Ricketson]

That's an interesting artical Steve. Would there be a certain kind of paint. It looks like some paint chemicals might be harmful to the birds. I have 2 modified G8Trios and 4 super gourds. Might try it thes year. Thanks for the tip. Don.

[Nanette]

Hi steve,
You mentioned in the article that the female martin sometimes has trouble making the nest bowl in the slippery SG. Would it be toxic to the birds if I put a circle or two of silicone caulk around the inside perimeter of my SG's??

[John Atteberry]

Hello Steve,
Great reading again!!!! Was wondering if I should put some traction tape or something similair on the porches of my trio houses to give them some traction to stand on the porch, especially in windy conditions? Also I was wondering if I should get 2 inch or 3 inch PVC pipe to make tunnels in the supergourds at an angle or straight out? Is that big around enough? And can you put traction tape inside of those tunnels too? Is 3 inches long enough out from the supergourd? I was going to try to do this to some of my supergourds. And one more question, Can I leave the round hole in the pipe or do you have to use a SREH type entrance? Thanks Steve! John!

[Steve Kroenke]

Hey Lanell,

I am glad you liked the article. Natural gourds are usually well liked by martins. In my area of northwest Louisiana, I have seen almost no gourd colonies. However, my natural gourds are the BEST occupied of all my housing types! I have used both painted and unpainted natural gourds and both were well occupied. My unpainted gourds did deteriorate more rapidly than the painted ones. My gourds that are painted white may be a little cooler in the summer because of the reflective power of the white color. I prefer to paint mine.

If your site is open enough, you should be able to attract martins in your area. You are located near Houston which appears to have a large martin population.

Good luck in 2006 and keep us posted.

Steve

Hey Don,

I am glad you liked the article. I have used Krylon Fusion paint with good success on the insides of my Supergourds. Other folks have used it, too. Just make sure you clean the surface of the Supergourds to remove any dirt, oils, or other substances that could interfere with the paint adhesion.

Good luck in 2006.

Steve

Hey Nanette,

Silicone adhesive caulk should be ok and not toxic to the martins. Before applying the caulk, I would recommend you clean the Supergourd surface to remove any dirt, oils, mildew and other materials that could interfere with the caulk adhering to the surface.

Good luck in 2006.

Steve

Hey John,

You probably don?t need to add any traction tape to the porches of the Trios. The guard rails on these houses provide good perches. Just my opinion.

Yes, you could add tunnels to some of your Supergourds. I have added PVC couplers to my naturals with great success. I haven?t tried them on the Supergourds yet. I used couplers that join two inch diameter PVC pipe together. The hole is about 2 and ? nches in diameter. These couplers have been the perfect size for the martins in my colonies. My couplers are both straight and angled at 45 degrees. I guess they protrude out about 3 to 4 inches in length. I sanded the insides of them and then smeared adhesive caulk in the area where the martins enter. This caulk creates a surface that martins can easily cling to and walk on. You could probably use some kind of traction tape. You can leave a round hole in the pipe. However, on my PVC couplers, I attached wire perches made from No. 10 plastic coated copper wire. I wanted to make sure the martins could easily enter the PVC extenders. Martins seem to enjoy sitting on these wire perches in front on the PVC extenders. I didn?t add any wire perches to my natural gourds with funnel necks because the gourd surface is rustic. If you go back and look at my old postings on the funnel gourds and the modified PMCA Deluxe gourd rack, you will see my natural gourds with the PVC couplers and wire perches.

Good luck in 2006. I always enjoy your enthusiasm for martins!

Steve

[TIGERPITT]

Hi Steve,

Very interesting information. Your theories sound plausible. I have always been a believer in natural gourds and what you have said may be the reasons they are so effective at attracting PMs. If anyone is having trouble starting a colony I believe natural gourds will help. If they do not work I believe the PMs are just not around. If you have man made houses and natural gourds they will nest in the gourds first and the late comers will get the houses. That is the set-up that I have now and the gourds are always full. This works fine but you need the gourds to get the colony started. I have 2 Trio houses which are ok but I sure like some of the newer types that everyone seems to be putting up now. I have had these two for a long time and the birds are doing fine in them but I sure would like to up-date soon. I guess I should add a new one next year. If my colony grows this year I may. Good luck everyone. It will not be long now!!!!!! I can not wait.

[Steve Kroenke]

Hey Tigerpitt,

I do believe that natural gourds with their dark, secluded and rustic interiors are highly attractive to martins. This internal nesting environment somewhat replicates that found ?naturally? in nature and martins like most cavity birds look for the safest place to raise their young. What worked for martins in the ?past? still works for them in the present. In my colonies, my natural gourds are nearly always occupied first and ALWAYS have the highest occupancy rates relative to other housing types. In fact, I could not get one of my super martin colonies started until I used natural gourds!

Your area in south Alabama is ?martin rich? territory! I have seen many martin colonies in your area, including ones around Gulf Shores along your coastal areas. I have seen HUGE natural gourd colonies in many areas of Alabama, Mississippi, Georgia and the Carolinas.

Good luck and your martins will be arriving soon.

Steve

[Steve Kroenke]

Since the first martins will be returning to southern Florida soon, this is a good time to consider modifications to one's existing housing. Modifications can include updated housing, different cavity types or revising what you have. The internal environment of the nesting cavity may be an important attribute for female martins which actually select the nest site. I am re-activating this older posting that discusses in detail three factors relative to the nesting cavity: darkness, seclusion and rustic surface. I believe all three are important to consider. We may be able to increase the attractiveness of our housing to martins by using cavities that maximize darkness, seclusion and rustic surfaces.

Steve