QUICK TIPS: Mice nests are generally tall and made of grass and other fluffy stuff, with no nest cup. Do NOT let mice nest in your boxes - they carry disease, urinate in the box, will deter nesting birds, and may eat eggs or nestlings. Remove nests regularly, using a spatula or plastic bag inverted on your hand to avoid being bitten and transmission of disease. Put a baffle on the box to prevent re-nesting and move it away from rocks/shrubs they can jump off of to gain access to the box.
Sometimes you get a surprise when you open a nestbox. Not all occupants of birdhouses are birds. Mice may use a box to overwinter or breed. Roof rats may use nestboxes (including hanging boxes, and boxes mounted on trees or houses) to sleep during the day or to over winter. Rats have collapsible rib cages, and can squeeze through a 0.75" hole, and can climb vertically (even up walls and pipes).
If a new "nest" shows up in your boxes during the fall or winter, it might have been made by a mouse.
A mouse nest looks a bit like a Tufted Titmouse nest, only messier, and it does not have a cup. It will not appear to be tamped down, since the mouse nests or overwinters on the inside. (See photo above of a mouse with suckling young, and the nest.) 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. I have seen a mouse nest constructed entirely of grass clippings (see photo on left.) Like bird nests, construction depends on the individual and available materials. Rodent droppings are often evident. I have found mice living (with nursing babies) in the grass underneath an intact bluebird nest (the mouse ate the eggs though, and shell fragments were still visible.)
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
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. (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. Mouse nestboxes should be on posts, 3 to 4 feet above the ground.)
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 to bluebirders, and may eat bird eggs and young. Bluebirder Bruce Johnson noted "If mice are a problem during the off season, 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, a PVC baffle (with a cap
sitting on a hose clamp or other means to make it wobble), or a downspout 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.
Move boxes away jumping off spots like shrubs, rocks etc., that would allow the mouse to gain access to the box itself.
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.
Before removing a used mouse nest (and AFTER evicting any mice), 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 and a spatula (or a plastic bag inverted over your hand) to remove the nest. Then sweep and scrape out the box.
Clean your hands afterwards with a germ-killing hand cleaner.
Leave the box open for a day to air it out.
Afterwards, regularly remove new nesting material that appears (once or twice a week, as they tend to rebuild quickly.) This is one reason why regular monitoring is so important to successful bird nesting.
Bounce scented dryer sheets are used to deter mice in RVs. In an RV, it doesn't work more than one season. Try tacking a small square portion of one to the wall? (so any roosting birds would not be resting on it.) However, some people are concerned about residual perfumes or chemicals remaining in the box. They may also deter nesting birds, so remove when nesting season begins.
A few drops of all natural peppermint oil on a cotton ball thumb tacked to the wall or underside of the roof. (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 as a last resort. However, this is NOT recommended, in part because predators may eat poisoned rodents. Quintox is considered 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.
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 Hantavirus 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 cohabited 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, which could startle you.
MOUSE AND RAT ID
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 food, 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.)
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