Sometimes you get a surprise when you open a nestbox. Not all occupants of birdhouses are birds. Mice may use a box to over winter or breed. Roof rats may use nestboxes (especially hanging boxes?, or those mounted on trees or houses) to sleep during the day or to over winter.
It is hard to tell deer mice (Peromyscus maniculatus) and white-footed mice (Peromyscus leucopus) apart, but deer mice are better climbers, have a longer tail, and are distinctly bicolored. Both are fairly common, with as many as 11 or 12 per acre in some areas. In northern areas, mice breed from about March through October. They may have 2-4 litters per year. Litters consist of 1-11 young, typically 4-6. For deer mice, eyes open on the 15th day, and the young are weaned between day 25 and 35.
Roof Rats (Rattus rattus, also called black, tree or ship rats) are smaller than Norway (Brown) Rats. These nocturnal rodents are attracted to pet and bird feed, and thus are most common near homes. They breed throughout the year, but primarily in February and March and again in May and June. The number of young per litter average seven. Young are weaned when about 3-4 weeks old, and are able to reproduce when they are approximately 3-5 months old. Females can produce 5 litters per year.
Note that Woodrats (Allegheny woodrat is Neotoma magister) are native and part of the natural environment, and may be protected by law. They are vegetarian and pose no danger to bluebirds. Woodrats are brown above and white below, like an overgrown deer mouse, and have fur on their tails, unlike the scaly tails on introduced rats. (They are no longer found in CT.)
Rats have collapsible rib cages, and can squeeze through a 0.75" hole, and can climb vertically (walls and pipes).
Nest ID: Mice usually make their nest from a variety of materials, such as grass, leaves, hair, feathers, milkweed silk, shredded bark, moss, cotton, or shredded cloth. Rodent droppings are generally evident.
A mouse nest looks a bit like a Tufted Titmouse nest, only messier, and it does not have a cup, and does not appear to be tamped down, since the mouse nests/over winters on the inside. (see photos to the left of a mouse with suckling young, and the nest.) I have seen a mouse nest constructed entirely of grass clippings (see photo on the right). Like bird nests, construction depends on the individual and available materials. If a "nest" shows up in your boxes in the fall or winter, it might be a mouse.
Roof rats generally use a bare nestbox, or sleep on a nest made by a bird. They may collect pine branches. They may chew the entrance hole and top edges of the nestbox roof. Where they do breed, they build a disheveled nest that may have bits of nut shells, hair and fur.
Deterring Mice and Ratsfrom a Bluebird Trail: Some people intentionally place nestboxes for deer or white-footed mice on fence posts/fence lines near their cabins or at nature centers. A wren house is appropriate for use by both species. nestboxes should be on posts, 3 to 4 feet above the ground.
Mice are more likely to use boxes mounted on T-bars, wooden posts, or fence lines, and boxes in tall grass or near shrubs. Some sources say mice can't climb metal conduit, but I have found mice in boxes mounted 5 feet high on conduit. Mice can also climb 10 feet or more up a tree trunk 10. Rats can enter a 1" hole.
On one trail, 75% of boxes were occupied by mice during the winter months. They leave an odiferous urine-soaked mess behind. Mice and rats also pose several hazards. Bluebirder Bruce Johnson noted "If mice are a problem during the off season, the bluebirds are not safe during the nesting season."
To prevent mice or rats from using nestboxes, try these tips:
On pole mounted boxes, use a stove pipe guard with wire mesh covering the top, PVC baffle (with a cap
sitting on a hose clamp or other means to make it wobble), or a down spout sleeve over the pole. Make sure there is almost no gap between the baffle and the pole. Put the guard/baffle/sleeve a few inches below the floor of the nestbox.
Plug up the box (e.g., with a plastic or styrofoam plug), or leave the door open until the ground freezes solid (at which point the mice have hopefully hunkered down elsewhere.) Of course this will make the box unavailable for other birds (bluebirds, chickadees, woodpeckers) that might use it to roost.
Do not mount boxes low on wooden posts. If you do use wooden poles, wrap thin flexible tin around the pole directly underneath the box down to the middle of the pole.
Build a mouse-sized, cheap, unobtrusive predator guard. Using scissors, cut a 7" diameter circle from a piece of plastic (like a salad bar container lid), and cut a hole in the center (3/4" for metal conduit). Then cut a straight line from outside edge to center hole. Duct tape the disk to the pole below the box, pulling the two cut edges together some to form a cone with the wide end down. Staple the seam.
Remove any nesting material you encounter. To prevent infection with very rare but potentially deadly Hanta Virus:
Before removing a used mouse nest, use a spray bottle to thoroughly soak the nest and box (to control dust) with a 10% bleach solution (water if no bleach is available).
After 15-20 minutes, while standing upwind/wearing a dust mask, use gloves or a plastic bag to remove the nest, and then sweep and scrape out the box.
Wash your hands afterwards.
Leave the box open for a day to air it out.
Repellants:
Bounce scented dryer sheets are used to deter mice in RVs. Try tacking a portion of one to the wall? (so any roosting birds would not be resting on it.) Some people are concerned about residual perfumes or chemicals remaining in the box. In an RV, it doesn't work more than one season. Another deterrent that might work is a pouch of natural Fresh Cab air freshener. Either of these may also deter nesting birds, so remove when nesting season begins.
A few drops of all natural peppermint oil on a cotton ball thumbtacked to the wall. (Just in case, avoid using peppermint oil in such as way as might result in direct contact with the bare skin of nestlings, as it could be an irritant.)
Some folks poison rats in nestboxes. Quintox is recommended because it is less likely to cause secondary poisoning to hawks/owls that may eat poisoned rats. Take rat bait pellets and embed them in silicone to make little patties, and place them underneath the nest. Any dead rats should be picked up with a plastic bag and disposed of in the trash.
Hazards:
Deer mice and several other common mouse species can carry Hantavirus Four Corners (also known as Muerto Canyon) virus, which causes a rare but deadly pulmonary syndrome. The virus is transmitted by infected rodents through urine, droppings, or saliva. Breathing in airborne particles contaminated with mouse droppings can cause infection. The onset of HPS begins with a flu-like illness. People may experience a fever, sore muscles, headache, nausea and have shortness of breath. Chills, vomiting, diarrhea, and abdominal pain have also been reported. Most cases reported a dry, non-productive cough. As the disease progresses, fluid builds up in the lungs, making breathing very difficult. Approximately 60-70% of infected people will die.
Mice may carry deer ticks which in turn transmit Lyme Disease.
Mice may evict or attack birds that attempt to nest in a box they are using/want to use. They may destroy eggs and nestlings in nestboxes, as do roof rats, eastern chipmunks (Tamias striatus) and red squirrels (Tamiasciurus hudsonicus). However, Dean Sheldon reports several occasions where mice have actually cohabitated in a nestbox underneath a bluebird on a nest without problems.
Roof rats plays can transmit human diseases such as endemic typhus, rat bite fever, and bubonic plague.
When boxes are opened in the spring, be aware that a startled mouse might jump out at you.
