Saturday, June 27, 2009

Dead bluebirds in shelters.

The other day the following email crossed my screen.


Today(Th., 06/25/'09), I found another dead bluebird in one of our cone shelters at Raystown. It must have died a horrible death.

The Raystown folk are looking at getting mesh covers for the cone shelters. This is dead bluebird#02 and other bluebirds have been rescued out of the cone shelters. I encourage everyone to keep their shelters wide and I discourage the use of cone shelters without mesh covers. For some reason, bluebirds like to go down into the cone shelters. They don't realize the peril.

--Rick Entrekin
Huntingdon, PA.

Plantra responded with the following offer.

Plantra will supply free nets to anyone who requests them for any brand of tube. This is an important topic.

Yes, this is an important topic and I want fill in some history, offer some alternative solutions and ask for any ideas others might have for ways to keep bluebirds out of tree tubes without interfering with tree growth.

At the risk of giving away the ending, we are now recommending installing Plantra tree tubes with stakes on the inside so the birds just walk out.

In 1989, we had the first reports of bluebirds in tubes. (I co-founded Treessentials. We sold Tubex brand.) Other species also end up in tree tubes, but bluebirds do so more than others and since bluebirds suffered significant population loss, losing a single bird is a tragedy to avoid.

The first place we went looking for help was the foreign manufacturer. Initially, they denied experiencing any problems in the UK and treated us as though we were over-reacting. They were no help at first. Later, we learned they knew of the problem. Even The Royal Society for Protection of Birds knew.

Much to my surprise there was the equivalent of a gentleman’s agreement to ignore bird death in tubes. They suppressed the problem by reasoning the tree tubes were creating needed habitat for birds to use for centuries to come. The sacrifice of a few birds was a price worth paying.

I believe they were afraid that if the public knew, there would be an outcry against tubes and tree tubes were the only effective method of ensuring the establishment of young trees in the English forest. Believe it or not, in the UK over half of all broadleaved oaks and other hardwoods are planted with four or five foot tree tubes. The reason is that they do not tolerate planting failure. They expect every tree they plant to live and provide the protection required to get the job done. Why would anyone do any less? Tubex tubes sold in America were the first to supply a free net with every tube 4 feet and over. That policy continues to this day.

From a strictly biological perspective, I cannot argue with the logic of ‘sacrificing’ a few birds for the long-term benefit of the entire bird population, but we will never be satisfied with that solution.

Once it became obvious this was a serious problem and we were not going to get help from overseas, we had to find our own solution. With great trepidation, I called Sadie Dorber who was then president of the North American Blue Bird Society (NABS). Sadie listened carefully as I described how we introduced treeshelters to the US to protect oak seedlings from deer and generally ensure a future for oaks in America. She well understood the problems of plants and animals that evolved survival skills in habitat that no longer exists. She listened and to my surprise was very understanding. Sadie agreed there was a serious problem and offered her help. Bluebirds were dying needlessly in tree tubes.

Sadie told me they had a similar problem with the ventstacks on tobacco drying sheds. The solution was a simple wire mesh over the opening. Sadie put us in touch with Jack Finch. Jack had started the non-profit corporation Homes for Bluebirds, Inc. to distribute bluebird boxes. Jack was known nationally and Dan Rather had recently profiled his efforts. Jack Finch was the “go to” guy for bluebird problems. He once built a pit under bluebird nesting boxes and filled it with snakes to develop methods to keep snakes out of the nest boxes he was producing.

We sent Jack some tree tubes. Chris Siems, one of my two partners at Plantra and the first employee of Treessentials remembers how Jack set up his test.
“He put a wood slat across the top of the tube to reduce the opening size and prevent accidental entry (falling in) by birds, then attached the tubes to stakes so the bottom of the tube was 2-3’ off the ground, and attached a collection bag to the bottom on the tubes. He monitored the tubes/bags hourly, and captured bluebirds in the bags – meaning that the birds had to be trying to get into the tubes rather than falling in.”

Based on Jack’s tests, we decided to exclude the birds. Flexible mesh tubes slipped over the top were the easiest solution. We began supplying a free flexible mesh with every tube four feet and taller. We have not had much trouble with shorter tubes. I am somewhat surprised by the incident mentioned in the email at the beginning since it was in a shorter conical tube. It is rare to hear of a bluebird in a shorter tube.

There are additional theories for why bluebirds enter tree tubes.
  1. Jack Finch’s explanation is that bluebirds are cavity nesters and tubes are attractive cavities.
  2. Some folks say bluebirds eat bugs and enter the tube in pursuit of dinner.
  3. I think some bluebirds actually fall into the tubes

Elements of all three might be true. Let us explore each reason

1. Bluebirds are cavity nesters and tree tube openings trigger an instinct to explore cavities even though the cavity faces the sky.

This seems odd in the sense that we expect a nesting cavity opening to be perpendicular to the ground, not open to the sky and rain the way a tree tube does. The instinct to explore cavities probably does not have an orientation requirement since tree cavities facing the sky can be quite spacious and provide adequate cover.

To my surprise, the NABS FAQ page has an entry near the bottom about open top nesting boxes. This style is discouraged because the nestlings get wet and die of hypothermia. The important point here is that the bluebirds nested in a box with a top open to the elements. That fact strongly supports Mr. Finch’s explanation the birds are looking for a nesting site when entering vent pipes and tree tubes. Even if unwise, cavities open to the sky are acceptable to the bird.

It could be that the ideal nest height triggers the instinct to explore a cavity. NABS says bluebirds will tolerate a nest as low as three feet, but prefer 4 or 5 feet. The bluebirdsforever website says: “The bottom of the nest box should be at least 3 feet above ground. Ideally, it should be mounted 4 to 5 feet above ground.” The reason given is that anything lower is more susceptible to predators. This strongly parallels our experience with tree tubes. Dead bluebirds are found in 4 and 5-foot tall tree tubes and rarely 3 foot. In over twenty years, I do not recall a single instance of a bluebird in a tube shorter than 3 feet.

Jack Finch’s explanation of birds looking for nesting sites probably covers the vast majority of cases. There are two other possibilities often put forth that may explain some cases.
2. Bluebirds eat bugs and enter the tube in pursuit of dinner.

According to insects make up 80% of the bluebird diet during spring and summer. A University of Michigan site states: Eastern bluebirds eat a variety of foods depending on the season. In summer months, eastern bluebirds consume mostly beetles (order Coleoptera), crickets, caterpillars, grasshoppers, and other insects. These are exactly the sort of insects found crawling on a leaf in a tree tube.

Given the obvious fact that bluebirds have no fear of entering a tree tube, it seems plausible a bluebird would drop into a tube to grab a beetle.

3. Bluebirds fall into the tubes

It is hard to imagine a bird falling into a tube. Still, we have all seen wildlife videos of botched landings by birds. (Click the link to the left and watch the video if you think birds always land perfectly) The tree tube in a field is a suitable perching structure and thereby creates bluebird habitat. The tree tubes become convenient perches from which to pursue flying insects or swoop down to the ground and grab a grasshopper. The hunting bird takes off and lands hundreds of times in a day. Not all landings are perfect and some tree tube designs make landing riskier.

The bluebird has three toes forward and one toe back somewhat like a thumb. Think of the rear toe as analogous to the tailhook on a jet landing on an aircraft carrier. If the tailhook misses the arresting wire, the plane hurtles forward and must take off or fall into the sea. Perching birds evolved to land on round twiggy branches covered with soft bark. The classic Tubex tube has a hard surface with a flair curved like a funnel into the tube.

When landing on a flared hard plastic rim the three forward toes have nothing to grab and can slide forward. To land successfully the rear-facing toe must catch the edge of the hard rim flare. If the single rear toe slips, the downward momentum of the bird will pitch it head first into the tube. Once in the tube it cannot open its wings to fly out and it cannot walk up the smooth inside.

This happened very quickly, but I remember standing in a field and saw a bluebird land on the tube rim and pitch forward into the tube. At least that is what I thought I saw. In retrospect, I might have been witnessing a nest search, a bug pursuit or a botched landing. Whatever I saw we have to keep the birds out of the tubes.

Soft un-flared rim
Plantra tubes are made with soft low density polyethylene (LDPE). Since it is soft, it does not abrade tree bark. The Plantra tree tube does not need a bird sucking flare in the rim. You can bend or roll up an LDPE tube without damage. The softer more flexible rim might allow the claws a better and safer grip. The nested style tubes are made with a similar but hard plastic called polypropylene (PP). PP tubes resist bending and turn white if forced to bend. I think eliminating the hard curved rim helps, but it is not the complete answer. Some customers report fewer bird problems with Plantra tubes, but birds still enter our tubes. Eliminating the flare is not a complete solution.

Pinch the top mostly closed

Inch and a half entrance holes are recommended by NABS for eastern bluebird nesting boxes. That means the opening at the top should be smaller than 1.5 inches to exclude bluebirds.

This should only be attempted with a ventilated tree tube. Without ventilation, the unchecked photosynthesis in a tree tube depletes CO2 below the level photosynthesis needs to keep growing. Also without ventilation, moisture levels in the tube reach levels that cause fungal problems in some plants.

There are some other potential benefits of this approach. Birds do what birds do. We have seen cases where a tube becomes an extremely popular perch and the plant and tube become encrusted in feces. Any fertilization effect is lost if the plant becomes coated. The small opening should limit the amount of feces deposited in a tube.

Additionally, this configuration may produce a straighter stem. The apical meristem or terminal bud seeks blue light. For this reason, we make the sidewalls red to suppress branching in the tube and channel growth toward the blue light that naturally propagates down the tube from the strongly blue middle of the sky. The reduced opening at the top will create a smaller and more focused source of blue light for the terminal bud to aim for. I am speculating here, but that might create a straighter stem.

Put the stake on the inside of the tube.

Sadly, we find many growers do not use the mesh. Recently we have been working on another approach to prevent bluebird death. There is a video on youtube of birds in the UK nesting in a tree tube and walking up and out at will because the tree has emerged. Birds know how to walk up a tree stem. We always knew the problem went away once the tree emerged. This led to an obvious idea. Put the stake on the inside and the birds can walk out. So far, this seems to be working.

Traditionally, we have recommended putting the stake on the outside to maximize the available volume for leaf expansion. We feel if the stake is not overly large, putting the stake on the inside is a great idea.

There are several other benefits. If the stake is on the inside:
1. A buck will not rub the stake with his antlers.
2. The bird can walk up and out on the stake
3. The tube is more wind stable and has a reduced profile in the wind.
4. Rodents cannot use the stake to climb up to the tie hole and gain entrance.

If you have any other ideas, please send them to me at and we will share them.

Again, If you need net we would be happy to supply it free of charge, but consider putting the stake on the inside.

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