ResearchResearch Priorities: Cavity Nesters

Research Priorities: Cavity Nesters

What burning questions about bluebirds and other small cavity nesters are still unanswered?

There are many areas where research is needed to enhance our knowledge base. The goal of this page is to be a clearinghouse for potential cavity-nesting bird research projects, suggested by people who are actually engaged in frontline conservation activities across North America.

The North American Bluebird Society funds projects! They seek research proposals that apply to and will help small cavity nesters like bluebirds and their landlords.

All research projects should be conducted in a responsible manner. They should build on existing knowledge, and not duplicate research that has already done unless the goal is replication or testing under different conditions.  Projects should follow NABS guidelines for nestbox design, predator protection and monitoring protocols. See NABS fact sheets for more information.

And remember – all experiments must be ethical, have required permits, and DO NO HARM to native species!

Annually, faculty and students in institutions across the continent want their work to make a difference. We can provide them with real world problems and an opportunity to have their work directly applied to furthering cavity-nesting bird conservation.  Researchers should be very specific in crafting their question and indicating how the results will contribute to cavity-nester conservation. If you have any ideas as to the overall parameters of the testing methodology, or potential sources of data, feel free to include them as well. Remember, we are ultimately seeking to provide targeted ideas to researchers who can help us to better do our job … so the more thought we put into it on the front-end, the more useful the results will be! If you know of research that has already been conducted in an area of interest I’ve listed below, please provide a link or source.

A tremendous amount of valuable research has been conducted by CITIZEN SCIENTISTS – some of whom didn’t even graduate from high school.  Just because you don’t have a pHD doesn’t mean you don’t have something to contribute to our knowledge base!

Research is needed on small cavity nesters

    • Minimum spacing distances for boxes for cavity nesters other than bluebirds and Tree Swallows.
    • Are there any studies on oval holes for species other than bluebirds?
    • Monitoring: What is the difference in productivity for a monitored vs. unmonitored trail in similar habitat? Would probably require two sets of boxes matched as best as possible (maybe on the same golf course). Both could be monitored with 24 hour video. Would not only give us information on the relative benefit of monitoring, but also give us some idea of any possible difference between our perceptions of reality based on once or twice a week monitoring and what actually happens in our nestboxes.
    • Dispersal from natal sites.
    • Does Bergmann’s Rule specifically apply to the HOSP (or BCCH, or other cavity nesters) and if so, to what quantifiable extent? Are there any studies correlating HOSP-excluding maximum hole size to the local minimum annual temperature, latitude, continental climate, coastal climate, etc.? (look at Bird Banding data?)
    • Are white eggs the result of a recessive gene? Analyze data on banded birds?
    • Is there a direct correlation (over time) between bluebird populations and those of starlings/House Sparrows/House Wrens?
    • Migration – Why do some birds in the same area overwinter while others migrate? Extent, causes, residency, timing, routes, distances.
    • For Mountain Bluebirds, what happens with pair formation before arrival at breeding grounds?
    • Assessment of subspecies of WEBLs, including identification of point of origin for wintering birds
    • Vocalizations of WEBL – calls and songs (systematic study), time of day of song
    • Hardiness of eggs against temperature stress (heat and cold, esp. WEBL.)
    • Are bluebirds with low body fat more likely to roost communally?
    • Winter kills
  • HOUSE SPARROWS (HOSP)– also see Experiments on Deterring HOSP
    • Why are House Sparrows deterred by Sparrow Spookers and monofilament and Magic Halos? Is it related to vision? Sound (e.g., of the taut monofilament)? Flight pattern? (e.g., wings spread upon landing?) Understanding the mechanism would aid in developing effective deterrent tactics.
    • Which is most effective as a live decoy: a male HOSP, female, a pair, or juvenile or some combination? What is the optimum number of decoys? This could increase trapping effectiveness. To test, 5 identical traps (style, bait and placement such as elevation) could be put out in an infested area – one with food only, one with no food or decoys, one with 1 M decoy, one with 1F, one with MF pair. Order should be random. All captured birds should be immediately removed to avoid affecting results.
    • Do House Sparrows attack the eggs, nestlings and adults of other House Sparrows?
    • Does removal of nests with a nestcup or eggs result in increased House Sparrow aggression on neighoring nest sites?
    • Do HOSP nest in abandoned woodpecker holes in trees, and if they do, are they less likely to choose this type of nest site, and why? Is it because of the shape of the interior cavity?
    • How prevalent is hybridization between HOSP and Eurasian Tree Sparrows, and is it affecting aggression?
    • What nestbox designs and locations are most likely to be used by Brown Headed Nuthatches? (Could review TBN data.)
    • What type/style of nestbox deters House Wrens? A House Wren resistant box could prevent destruction of eggs or filling up boxes with dummy nests which renders them unusable.
    • Is forest fragmentation increasing House Wren populations?
    • Almost nothing is known about the biology of HOWR at their wintering grounds. Are some southern HOWR non-migratory? Do they migrate in flocks or alone? What migration routes do they take?
    • Reasons for decline in some areas, including interactions between House Wrens and Bewick’s Wrens (e.g., effect of nest site selection, affect of abundant nestboxes)
    • Tree Swallow young are often found dead in nestboxes. Is some disease involved? Collect data (weather, temperature, nestbox type, age at death). Collect corpses and test dead birds?
    • What type of nestbox/set up is favored or avoided by Tree Swallows? This would be useful in areas where Tree Swallow populations are high and swallow nesting appears to negatively impact bluebird nesting (also see Pairing). Would also help monitors that wish to attract Tree Swallows.
    • Do chickadees prefer Gilbertson boxes painted like birches over a standard NABS box?
    • Do chickadees that use nestboxes or re-use existing holes (created by woodpeckers or chickadees) nest earlier than those that excavate the holes themselves?
    • What is the most effective nestbox treatment to deter European and Native Paper Wasps for an extended period of time? An effective method could increase nestbox use, prevent abandonment of boxes and monitor injury, and save monitors time.
    • Impact of habitat loss on migrating and wintering populations.
    • Size, weight of runt eggs found vs. normal, position in the laying sequence (e.g., first egg), position of clutch (first brood, second etc.), the size of clutch containing the abnormal egg, the age of females laying such eggs if known. More.
  • DIET
    • Which insects are the primary source of natural bluebird food being brought to the nestbox, and how does diet change as nestlings age. Idea: Set up nestcams in bluebird boxes (where mealworms are NOT being fed), capture snapshots of parents feeding young, and have an entomologist identify the insects.
    • What impact does feeding mealworms have on bluebird nestling health and survival? Dried mealworms? E.g., are calcium levels lower? Do birds fledge earlier or later? Do they weigh more? Are they less likely to die of hypothermia? Do bluebird adults fed mealworms have more broods? Results could inform recommendations on mealworm feeding.
    • Did past tests of 1.5″ round holes to larger (Peterson) oval holes reflect a preference for a larger hole opening or a preference of an oval hole shape? Using a larger 1&9/16″ round hole in future tests would help identify whether any preference for the oval hole is because larger bluebirds can’t enter 1.5″ holes comfortably.” (EABL and WEBL)There is always a need to mine EXISTING data sources for preferences (and fledging success) for various nestbox styles, heights, etc., by species.
    • How much (if any) of a bird’s nestbox style preference is related to fidelity (i.e., born in that style, used to it, drawn to it for breeding)
    • Will all three species of bluebirds use PVC boxes?
    • How does floor size impact brood size, nestling survival to fledging, and/or time to construct nest prior to first egg
    • How does hole depth (e.g., use of wooden blocks) affect nestbox preference by various species?
    • Hole size tests – what size is PREFERRED by various species.
    • Does Eastern Red Cedar (aromatic) pose toxicity concerns to developing nestlings, or is it a preferred building material because it may help repel/control nuisances like bird mites and blow fly larva? (test with non-native House Sparrows?)
    • Mountain bluebird: Data on preferred nestbox height, entrance diameter and orientation, and interior dimensions
    • Does a nestbox made of PVC offgas any materials that are harmful to birds/eggs/nestlings, especially when exposed to high outdoor temperatures?
    • Will a PVC box with sides made of a “foam sandwich” (two pieces of hard plastic on the outside with a layer of insulating foam between them) provide even better insulation and lower maintenance than wood?
    • Carefully measure the distance between the bottom of the new nestcup and the bottom of the nestbox. Then measure again after the eggs hatch to determine how much the nest compacted during incubation. Then again how much compaction after the babies fledge. Again it would not be hard to set up a software program to track this nest size and even weight of the nests in various sized nestboxes. (Dry the nests and then weigh them. Do they change in size and weight as summer temperatures climb? Are second or third nests the same volume as first nests for multiple broods?)
    • Evaluate impact of western red cedar boxes on nestlings (using HOSP) with regard to plicatic acid. Human occupational exposure can result in respiratory problems, probably as a result of exposure to plicatic and abietic acids.


If we knew what we were doing, it wouldn’t be called Research.
– Albert Einstein


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