Sometimes nestling or adultTree Swallows (TRES) are found mysteriously dead in a nestbox, with no apparent injuries. If there is no evidence of trauma, the nest is dry, and there are no fire ants, here are some other possible explanations. It may not be possible to determine why they died, but consider clues such as the condition of the bird, whether it is an isolated incident, recent weather, etc. The usual suspects are weather, starvation, disease, parasites, pesticides, or some combination thereof. Infanticide is well documented. It's possible there could be some mystery disease/other phenomenon causing this, as it seems to be more common with TRES than other cavity nesters.
Sometimes one nestling dies and mummifies in the nest. It may be hard to see as it is often covered with droppings by the time the others fledge.
Unable to Exit Box: Perhaps TRES have weaker legs/feet than other cavity nesters, and thus require a rough surface (toeholds) to exit a nestbox. This may be more of a problem in boxes deeper than 5.5" or it may be problem in boxes that are too narrow (e.g., less than five inches wide.) It is simple to eliminate this as a potential cause of death. Options:
Do NOT paint or varnish the interior under the entrance hole as this makes it slick.
Use rough sawn wood on the board where the entrance hole is, or roughen below the entrance hole with an ice pick, awl, screwdriver, or chisel; or use a router or hand saw to make "kerfs" (grooves) in the door below the entrance hole. See more info on how to make kerfs.
Make boxes 5+ inches wide (e.g., a floor size 4 x 5.5" used by Rusty Scalf of Berkeley CA prevented deaths, even when the interior wood was smooth. Linda Violett of CA has not had problems with Tree Swallows being trapped in her wide Two-hole mansion boxes with hole to floor depths of 7.5 - 9.5."She does have kerfs in her boxes.)
A Peterson box may be preferable to a NABs box in this regard as the door is angled, perhaps helping birds exit.
Adult deaths from this cause are probably more likely soon after migration or during very cold weather.
A common cause. On some trails, cold, wet weather (e.g., 3 days of rain and 40 °F temperatures) can result severe losses
- e.g., 50% (Sialia, Spring 1993)
Lack of flying insects due to prolonged cold/rainy weather (see starvation). Tree Swallows are aerial hunters. Cold wet weather also probably requires more energy to maintain body temperature.
Excessive heat or heat exhaustion (e.g., in temperatures of 90 degrees plus).
In one case, the only box I had on the Landfill Trail where nestlings died one week was also the only box facing West.
Starvation: Weight loss can occur during/after a long migration, if foraging is poor, or the birds are otherwise severely stressed. Heavy competition for nest sites which results in a need to spend more time defending a box and less time foraging may also be a factor. Loss of a mate may contribute as it is more work for one pair to feed themselves and nestlings. An undernourished bird is more susceptible to hypothermia, disease, etc. Also a bird that is trapped in a box (e.g., because there are no toeholds on the door) may starve to death and thus be emaciated when found.
Check the breast bone area (where the flight muscles attach) for a bony ridge down the middle called the keel. If you can feel the keel, the bird may have starved (i.e., if bone feels very sharp with no fat/flesh on it, the bird is emaciated).
Since a well fed Tree Swallow has a fairly prominent keel, this may be a difficult distinction to make. A wild bird in top form will have rounded breast muscles, gradually becoming smaller and flatter. If you blow on the breast feathers, you can see the keel and the "wishbone" directly above the keel. Birds deposit fat in the "V" of the wishbone--and you can see it as yellow deposits through the skin there. Many birds use up resources as soon as they get them, and so don't have excess to deposit as fat, unless they're gorging themselves just before migration. You might notice yellow fat deposits on nestlings (before they get all their feathers hiding their skin) if their parents are feeding them a lot.
Pesticides (perhaps they are more sensitive than other bird species?): If you have more than one nestbox, and other birds in nearby boxes at the same time have no visible problems, this makes pesticide application a less likely cause of death.
Diarrhea: Excessive/loose or runny fecal matter in feathers (turn them over and look on bottom)/nest/floor. They may have eaten something that affected their digestive system. Note: Tree Swallows parents stop removing fecal sacs around Day 14 but young may not fledge until Day 16-30, so it is not unusual for an older nest to be a tarry mess.
Disease or pathogens: ??Also see West Nile Virus (is this transmitted from adults to young?).
Parasites: ?? Not sure what type? Maybe something in insects eaten? Or from water source? Also see blow flies (nestlings only).
Death shortly after return to an area: Following their long migration TRES are low on energy. They seek shelter in a nestbox, and may either be too weak, exhausted or starved (see starvation) to exit. Often the box has no nesting material in it.
West Nile virus: The USGS has confirmed West Nile Virus in dead Tree Swallows. It may be possible to have dead birds (freshly dead [less than 48 hours], eyes not sunken) tested for West Nile. Contact your local Health Department for requirements, and be prepared to provide the exact location where the bird was found. Some health departments will only test corvids (crow, grackle, jay, magpie, raven) for West Nile. See information on handling dead birds.
Injury: Engaged in an aerial battle with another TRES or competitor, returned to nestbox and died (possibly of exhaustion or injuries.) Early in the breeding season, both sexes will grapple with competitors inside cavity, in air, on ground, or even on water. Such fights usually involve one bird vigorously pecking head and back of another of same sex. (Leffelaar and Robertson 1985). Possibly signs might be female dead on the floor of the box, flat on stomach with wings spread out to her sides; head tilted up and back; forehead efeathered and covered with dried blood and puncture wounds (Lombardo 1986). (Also see HOSP Attack).
Ingestion of contaminated water.
Egg bound: (Female found on nest.) The hen is unable to pass an egg that has formed. The egg may be stuck near the cloaca, or further inside. Egg binding is a reasonably common, and potentially serious, condition that can lead to infection or damage to internal tissue. Also it is possible an egg could break inside in the oviduct in response to a non-fatal blow (batted by cat, picked up roughly by a person), causing an infection that can be fatal within a week.
Note: Two dead birds I found were on a nest containing distinctly pink eggs. Keith Kridler speculated that the illusion/appearance of eggs being pink might be associated with a female that is old or sick or unable to steal enough calcium from her body to create a eggshell with normal thickness that would hide the insides of the egg.
No toeholds (see unable to exit): Unable to fledge from a box because the surface did not offer purchase for under-developed legs. One or two nestlings may successfully exit by standing on siblings, but since the parents may cease to tend remaining nestlings, they starve.
Note: Per Keith Kridler, in the early 1980's when testing the PVC pipe nestboxes for NABS, Robert M. Patterson found that healthy Tree Swallows could exit these slick pipes even when the nest was 10" below the entrance hole.
Hypothermia associated cold steady rain, day time temperatures in the 50's and night time temperatures in the 40's or below. Parents are absent more often because flying insects are scarce. Particularly a problem after female stops night time brooding (around Day 6 or 7) and before babies can thermoregulate (before 10 days old.)
Infanticide: Male Tree Swallow moves in to replace a dead or missing male and kills nestlings of widow. (Robertson and Stutchbury 1988, Robertson 1990). Also, subadult (Shelley 1934) and adult females may kill nestlings at another female’s nest site to hasten the availability of a breeding opportunity (Chek and Robertson 1991). Nestlings may have a puncture wound behind the eye (Lombardo 1986). In 1934, a young female TRES entered a colony in MA and was observed killing 19 nestlings from five broods over an 8 day period. (Shelly 1934.) In one study of Barn Swallows, this always happenened when nestlings were less than 5 days old (Weisheit and Creighton 1989)
Vitamin or mineral deficiencies, legs are not strong enough to hold weight (similar to rickets.)
Parents abandoned nest - could be for a variety of reasons. Some reports of second nestings being abandoned when group migrates. Nestlings starve or succumb to hypothermia.
Blowfly infestation (serious - more than 10 larva per nestling.) May weaken nestlings enough to make them more susceptible to other problems such as hypothermia. According to Terry Whitworth, blow fly infestation in and of itself is unlikely to cause death.)
Runt undernourished - less able to compete (see starvation).
??Juvenile TRES fledged from nearby nests fly around other nestboxes (or may even enter the nestbox?) to steal some of the food the parents bring for their own nestlings. Some of the young still in the nestbox then starve (see starvation).
?? in a horizontal box (Carrier/Tree Branch style) perhaps the parents can not see the gapes of all nestlings (as they are not looking down into the open mouths) and thus do not feed them equally, and one or more may starve or turn into a runt that is left behind.
Wasp stings (e.g., yellow jacket)
Choked on food: Sometimes a nestling is unable to swallow an insect provided by a parent and chokes. (Other times the parent pushes it down, or the nestlings spits the item out.) Photo.
Handling Dead Birds: If you do find a dead bird, use rubber gloves or a plastic bag to pick it up. Seal it in a plastic bag (if you do not have gloves, insert your hand into a plastic bag, grasp the bird and invert the bag over the bird) and dispose of it. Birds submitted for West Nile Virus testing should be double-bagged. Sanitize your hands before you go home and handle any pet birds.
If there are other live nestlings in the box, remove them (gently cup your hand around them and place them in a bandana/handkerchief in a hat/box/bag) and take out any corpses. Sometimes nestlings that have been dead for a while are "mummified" and not discovered until the box is being cleaned out. If the nest is infested with maggots, you may want to do a nest change.
Note: In six years on my small trail, where TRES nestings are common, I have seen TRES deaths even though all boxes have kerfs.
one dead (mummified) nestling in a possible second nesting.
one dead adult on May 9, 2005 after extended cold and rainy weather in box regular NABS with kerfs under entrance hole with nesting material, in an unusual position - beak first in the nest. Feathers were in excellent condition, possible starvation but bird looked very healthy
one dead adult on landfill trail on top of 5 eggs, some feather loss (see photo above), eggs clean, keel evident. No pesticide use in this area.
In 2005, 40% nestling losses on landfill trail, at various stages of development (all partially or fully feathered). Possibly weather-related.
If you experience problems with the website/find
broken links/have suggestions/corrections, please contact me!
The purpose of this site is to share information with anyone interested
in bluebird conservation.
Feel free to link to it (preferred as I update content regularly), or use text from it for personal or educational
purposes, with a link back to http://www.sialis.org or
a citation for the author. No permission is granted for commercial use. Appearance of automatically generated Google or other ads on this site does not constitute endorsement of any of those services or products!