Predators And Problems On The Trail

It would be great if all we had to do is put up a bluebird house and presto! – at least six weeks of pleasure watching a loving bluebird family, and pride in helping to increase bluebirds. But tragedies do occur. Here are some clues to trouble and some well-tried solutions by-experienced bluebirders.

The Perpetrators

A=ants, B=blowflies, CT=cats, E=English(House) sparrow, GN=gnats or blackflies, MI=deermice, P=pesticide, RN=raccoon, RS=red squirrel, S=starling, SN=snake, TS=tree swallow, W=wren, bb=bluebird

  • Clue: Little sticks in box, often up to top, maybe over bb nest: W
  • Clue: Feathers (usually white) on top of bb nest: TS
  • Clue: Debris, paper, weeds along with grass nest, often circled high over hole: E
  • Clue: Shredded bark, leaves, etc. filling box: RS
  • Clue: Shredded milkweed, seed and conef lower seed heads: MI
  • Clue: Coarse grass mixed with green weeds. Eggs robin-size but lighter blue: S
    Note: Nests with moss and fur-lined (chickadee); large grass with bits of shed snakeskin (crested flycatcher); shreds of inner bark, fur, feathers, hair (nuthatch); should be welcome if there are boxes enough.
  • Clue: bb eggs suddenly disappear, nest undisturbed: W,E,SN, possibly RN
  • Clue: bb eggs disappear, nest partially pulled out of hole: RN or CT
  • Clue: bb eggs shattered on ground or in nest: W,E,RS,Chipmunk
  • Clue: Tiny holes pecked in eggs: W
  • Clue: Nestlings gone, nest intact, parents still around: CT,SN,E
  • Clue: Nestlings gone, nest partially pulled through hole. Grass below may be trampled, feathers around: RN,CT
  • Clue: Nestlings dead outside box, sometimes 1/day: E
  • Clue: Nestlings suddenly dead in box, bodies unmarked, stomachs full, parents still around: P
  • Clue: Young nestlings dead in wet nest: H
  • Clue: Nestlings dead in nest, emaciated: Predator got both parents.
  • Clue: Nestling dead in nest, little blackflies around, red welts on bodies, especially under wings: GN
  • Clue: Nestlings dead in nest, large red welts over bodies, black pupae or gray maggots under nest: B larvae
  • Clue: Nestlings mutilated in nest: E, or possibly MI,RS
  • Clue: Female dead in box, head feathers and/or eyes pecked out: E

Bluebird Recovery Program
Audubon Chapter of Minneapolis
P.O. Box 3801
Minneapolis, Minnesota 55403


Some Solutions

A=Ants. Slip a paste or liquid ant poison under nest. Or staple a Q-tip soaked in p liquid ant poison under nest and bottom of house. Spray with Flys Away II.

B=Blowflies. If small black cigar-shaped pupae or gray maggots are found near bottom of nest, lift nest out with young and shake out as many insects as possible. Brush off floor. Spray inside of box with Flys Away II. If nest is wet and highly infested, construct a new one of dry grass and replace nestlings.

CT=Cats. Usually leave some feathers around. If cat is known and cannot be confined during nesting period, trim front claws. Trimmed and declawed cats can still climb. Use cat/coon guards on box, inverted cone on post. Dispose of itinerant cats.

F=English (House) sparrows – bluebirds worst avian enemy! Will kill adult bluebirds and nestlings. Must be eliminated. Let sparrows start to nest, then trap with Peterson sparrow trap. Destroy nest & eggs. Where feasible, use baited ground trap.

GN=Gnats or blackflies(tiny). Use Flys Away II for heavy swarms and infestation. Spray inside of box regularly until nestlings are 12-13 days old.

H=Hypothermia. Check nest boxes after heavy, slanting rains. Replace wet nest with new dry grass. Young nestlings may die if left in wet nest. Dampness also attracts blowflies. Extended rain with cold endangers both tree swallows and bluebirds.

MI=Deermice. Often nest over winter in closed boxes. Leave boxes open during winter if they become a problem. Continue to throw out.

P=pesticide poisoning. Usually a problem at second nesting only. Try to determine if poisons will be used nearby by farmer, groundskeeper, gardener lawn-care company; close boxes to prevent use. Take freshly-dead nestlings to nearest DNR Nongame Specialist for autopsy. Do not put boxes near lawn-treated areas.

RN=raccoon. Use separate smooth metal posts if possible, or smoothly-wrapped tin on wood posts; try cone guards on post, cat/coon guards on box.

RS=red squirrel. Keep nest boxes sealed in winter, or leave open. Climb-proof posts as above, and keep away from trees and saplings squirrels could leap from.

S=starling. Use starling trap. EIiminate. Be sure box entrance hole is minimum size for bluebirds. (1&3/8 x 2&1/4″ or 1&1/2″ round).

SN=snake. Climb-proof separate posts or heavily grease.

TS=tree swallow. Pair boxes 25-30′ with 300′ between pairs. Remove used bb nests promptly for maximum nesting opportunities.

W=wren. Keep sticks removed. Move bb boxes away from brush, trees, at least 200-300 feet if possible. Do not try to solve problem with putting up wren boxes. This only produces more wrens eventually will go further into open areas to take bluebird boxes or prey on eggs and young.


Always place boxes high (5-6′) on separate posts; never on trees or on fencelines. In cat/raccoon areas, use cat/coon guard around holes; wrap wooden posts high with smooth tin or, preferably, use smooth narrow metal posts. Grease posts regularly or use inverted cones. Flys Away II is a natural insecticide (pyrethrin) made to spray on open wounds of horses. It has been tested extensively on small birds.