Bluebird Survival Rate
Date: Fri, 21 Jul 2000 19:23:21 -0400
Date: Fri, 21 Jul 2000 21:19:03 -0400
From: "Fawzi P. Emad"
Subject: Survival Rate / BB population
Hi Stephen and all. I have heard through the grapevine (honestly, I don't recall where, may be a post by KK?) that we can count on 50% survival of what we count as "fledged". Even at that rate, just in my yard I have had 14 fledged. So, assuming that the parents also survive, then half of 14 is 7 plus four parents is 11. Where are they going to nest next year? I have space for only two families. I am sure I'll get some back, but I am going to also see if some of my neighbors would let me put up boxes in their yards (I will monitor, or they will if they prefer).
Fawzi from MD ...
Date: Fri, 21 Jul 2000 22:26:31 -0600
From: Haleya Priest
Subject: Re: Survival Rate / BB population
Hi Yes, it is 50%. I am very lucky to have all of my backyard blue babies (5) survive this first month. H ...
Date: Sat, 22 Jul 2000 07:26:17 -0500
From: "Keith & Sandy Kridler"
Subject: fledge rate
...Earlier this year we debated numbers of bluebirds that are reported fledged. Gary Springer and I and some others believe that not checking the box after the young are 12 days old will inflate the number of birds that actually fledge as often young are lost the last week and people assume all fledged. The last week is a very dangerous time as in shallow boxes these young can reach out the hole to be fed or can become food themselves to birds of prey! I doubt that a young bird sitting in a dark box like the Peterson or some other types with only small holes for ventilation can see very good because their eyes are not adjusted to bright sunlight so in effect when they see a shadow cross the entrance and thrust their head out in full sun they are momentarily blind until their eyes adjust. Seldom do monitors have problems with birds of prey but this might be the reason that some young birds "mysteriously" disappear. I am not picking on any box style because even boxes with 1/2" ventilation slots are still dark enough to create this same problem and I believe the baby birds must learn to fear predators and this may take several siblings disappearing to get the point across!
If you take the actual number of
baby bluebirds that are seen to FLY from the box and SAFELY
land in a large tree then I believe the survival rate for these
birds is about 90% the first two weeks IF there is lots of forested
land nearby. Our area is 50% fields and 50% woods (mostly hardwoods)
with seldom a field running over 500 acres. This allows the
baby bluebirds to learn to fly in a wooded environment very
unlike the country Sandy, Shawn and I drove through in southern
Wisconsin and northern Illinois. These areas were mostly farms
with 700 acres of corn being a small field! It was not unusual
to drive for miles of cultivated fields with only a few bushes
or single row of trees in the fence rows. In some of the pictures
I took looking across beautiful valleys 10 miles wide and 30
miles long dotted with barns and houses the only trees were
in the farmers yards! In this situation a young family of bluebirds
learning to fly in a tree in a
Many on this list up north report the adults driving their young away after a couple weeks. Down in my area I have NEVER seen parents drive their own young away during summer or fall months! Other bluebirds WILL attack young not their own that come into "their" territory. I can count the families of bluebirds down here as they stay in groups of 6-17 all winter until January when they break up. By watching for these groups you can tell how they fair in surviving after fledging. Using radio tracking on baby robin's it was determined that 60% die the first two weeks as they spend about the first week on and off the ground before they can fly very well. They leave the nest early and most fell prey to cats and then dogs were the 2nd largest predator. This test was conducted in a urban environment. We each have to deal with a slightly different environment and drastically different weather but we can all pick up tidbits of information that we can apply to our trails if everyone will share their joys of bluebirding but more importantly their failures. Before placing a nestbox evaluate the area the young birds will have to live in the first two weeks. I probably killed thousands of bluebirds in my early years by placing boxes where the young had little chance of reaching a tree upon fledging and I placed boxes where I wanted to monitor, NOT where it was safe for the birds. I guess this is why ALL of my posts are TOO long! I want you to enjoy your birds first but also to think and evaluate why you are placing boxes and then to sweat the details and constantly improve your boxes and most importantly to improve their location each year (or at least some of the boxes). KK
Date: Mon, 24 Jul 2000 10:27:15 -0500
From: Nolan/Hunter Family
Subject: Re: Survival Rate
Hot Springs, Arkansas
I thought that it was interesting that you should ask this question just as I was pondering what so far has been a dismal season for the pair of Eastern Bluebirds in my backyard. I feel pretty confident in my numbers because the nest box is within sight as I sit at the computer, and the adults and fledglings (after about a week staying in the trees) come for mealworms a couple of times a day. I also see the parents carry mealworms to the trees where the fledglings are during the first week or so post fledging. The female has just built a 4th nest - a first for us here - they are usually content to have 3 nestings - with the heat of the summer upon us, I hope it is successful. Anyway, nest #1: 4 eggs laid, 4 fledged and survived a couple of weeks. Then there were 3, then there were only 2 fledglings. They took off as "independent adolescents" at 7 weeks of age (I think the leaving influenced by fact that their mother had a new mate). Nest #2: 5 eggs, 3 fledged (witnessed), after 3 days, there were no signs of parents' feeding behavior, parents were hanging around nestbox busy with nest #3, never any sightings of fledglings. Nest #3: 5 eggs, 3 fledged (presumed), 1 week post fledging, parents are feeding only 1.
In summary, 14 eggs laid, 10 fledged, 3 survived so far. I hope that it is just a bad year that has been balanced by much better years in the past. I have seen what I think is a Sharp Shinned Hawk twice for the first time this year, and we don't really have roaming cats or dogs, so it is puzzling as to what has happened to the fledglings. Any ideas out there? ...
Date: Fri, 20 Jul 2001 08:09:12 -0700 (PDT)
From: Horace Sher hjsher1"at"yahoo.com
Subject: EABL fledglings/juveniles
Hi everyone...my neighbor's EABL box had the 4 nestlings in
the box a day or so before they fledged. Everything was fine
then. I know because I saw them then when I monitored the box.
Since they fledged hopefully about 2 weeks ago, I saw 3 fledglings
in the trees a few days ago. Didn't see the 4th. Today I saw
only 1 fledgling(juvenile) show up with the father for food
nearby the EABL feeder. Question? Can I assume that only 1 of
the 4 fledglings has
Date: Fri, 20 Jul 2001 10:22:13 -0500
Hi Horace---If you continue to see only one fledgling and the parents are still nearby, I think it is safe to assume there is only one fledgling left. That would be consistent with the first year mortality rateof ~70% (1 in 4 birds survive the first year).
Date: Fri, 20 Jul 2001 11:28:47 -0400
Horace and all--Similarly, this morning at the feeder I saw only 2 of four young blues that fledged a couple of weeks ago, whereas last Monday I saw three of them at the feeder. I'm going to watch the feederover the weekend to see if their numbers improve.
Date: Fri, 20 Jul 2001 11:43:31 -0400
Am I correct in thinking that the mortality rate of songbirds in the first month after fledging is 50 percent?
Date: Fri, 20 Jul 2001 11:01:14 -0500
I'm not sure about songbirds in general but recently read about a couple of studies done with Eastern Bluebirds where the mortality rate in the first year is 70%. I assume that the majority of these mortalities occur during the first month or so when the fledglings are more vulnerable. If I recall correctly, if they survive the first year the mortality rate drops to 20% after that.
From: Seward, Elizabeth D. Elizabeth.D.Seward2"at"usdoj.gov
Sent: Friday, July 20, 2001 9:25 AM
Subject: Re: EABL fledgins/juveniles
Date: Sat, 21 Jul 2001 11:34:58 -0700
Frank Gill has a very lengthy section in his book, Ornithology, about demography. This is a recap of what he say, with quotes in quotation marks:
LIFE HISTORY PATTERNS
"Several patterns characterize avian life histories. First, mortality rates are initially high among young birds less than 1 year old and then decline to nearly constant levels among adults."
ANNUAL SURVIVAL AND MORTALITY
"Rates of annual survival, a main ingredient of demographic analyses, change conspicuously with age in the first years of life. Survival rates may differ between the sexes, among species, and in various geographic regions. ... In general, large species survive longer than small species, seabirds survive better than land birds, and tropical species survive better than temperate zone species. ... A young bird's annual chance of survival from fledging to breeding age is typically about half that of an adult. Small land birds are especially vulnerable in their first year. Mortality during the first few weeks out of the nest is generally high. In the case of Yellow-eyed Juncos in the Chiricahua mountains in southeastern Arizona, nestlings and fledglings experienced two early episodes of high mortality: only 11 percent of banded fledglings reappear the following year. (Sullivan, 1989) Grown nestlings and fledglings incapable of extended flight easily succumb to predators, which take about 50% of the available young juncos in a 9-day risk period. Survivorship then improves for 3 weeks while parent juncos care for their mobile fledglings. With independence comes a second episode of high mortality due to starvation. Newly independent young find insects slowly and inefficiently and spend almost all day feeding. Approximately 42% of them die, most by starvation. It takes about two weeks for the juveniles to develop adequate foraging skills.
In general, the more physically developed the young are when they leave the nest, the greater their chances of survival. This is one of the advantages of longer nestling periods and of the fast growth in atricial nestling. A fledgling's chance of survival (measured by ornithologists in terms of future recaptures) increases in proportion to its mass at fledgling. Food availability, the quality of parental care, the number of siblings competing for that care, and the timing of fledging are also important factors.
Once birds reach adulthood, their chances of survival increase and stay essentially constant. Survivorship of Florida Jays, for example, is extremely low during the first few months after they leave the nest. (Woolfenden and Fitzgerald, 1984) At ages of 2 to 3 months, their mortality rate is still four times that of breeding birds. Only 40% of Florida Jays survive their first year, after which they 'graduate' to the high adult survival rates.
Males and females often differ in their survival rates. Only
20% of fledglings Great Tits survive their first year, but 48%
of breeding females and 56% of males survive each 12 month interval
after that. (Bulmer and Perrins, 1973)Male birds generally survive
better than do females, a situation that leads to a male-biased
sex ratio in many populations. The causes and timing of greater
female mortality and thier relations to parental investment
remain uncertain (British, 1989) but these results support the
long-held viewpoint that females have higher costs of reproductions
than males do."
"To study the life history of a particular kind of bird, the ornithologist follows the annual progress of a cohort of eggs, nestlings or fledglings until the last one dies. The proportion of the cohort that survives each year defines the annual survivorship. The probability of survival to a particular age, therefore, is the product of the preceding annual survival rates. The number of young produces each year by adults in the cohort defines age-specific fecundity. The product of this specifies an individual's expected annual fecundity, which is to say fecundity at a certain age discounted by the chance of dying before reaching that age." For those that understand that, he gives a statistical table of Eastern Screech Owl survivorship.
That said, I would add my own comment- and opinion!- about starvation among young birds. I do not feel that this is license to feed young mealworms constantly because, as Gill states, " It takes about two weeks for the juveniles to develop adequate foraging skills." Since they may not develop these skills as quickly if the "restaurant" is open constantly, they will be in danger longer.
I believe that, while the information on juncos may not exactly predict what will happen to small and birds, it is probably indicative of the fate of many species.
From: Tina Wertz [mailto:tinawertz"at"bellsouth.net]
Sent: Monday, May 16, 2005 3:15 PM
Subject: Success rate to adulthood
Does any one know the statistics in bluebirds as to the success rate from fledgling to adulthood. My pair of bluebirds had 2 babies at the feeder these past few days, and now there is only one. Last year they successfully fledge 7, and when they returned to my feeders in December, there was only one of their offspring with them. I would hate to think that only 1 in 7 make it to adulthood.
Tina Wertz, Woodstock, Ga.
From: Keith & Sandy Kridler [mailto:txbluebirder"at"sbcglobal.net]
Sent: Monday, May 16, 2005 11:08 PM
Subject: Re:Success rate to adulthood
Keith Kridler, I am going to cheat on this one:-) Scott pretty well answers your question in his article this past week so I just copied it from the Pittsburg paper. I'll be back in a week or so from the NABS meeting!
Cavity-nesting birds find shelter from many predators
THE WILD SIDE
The first of what will surely become many pleas for help arrived recently. A bluebird nest had been plundered - the eggs were gone, and the nest itself torn apart. A raccoon was probably responsible.
In nature, more often than not, life is brief and death comes violently.
Fewer than half of most bird nests actually produce any chicks that leave the nest. Among open-nesting birds that build their nests on the ground or in trees and shrubs, as few as 20 percent of the nests fledge any young.
Cavity-nesting birds, on the other hand, are usually more successful.
But even cavity-nesters suffer predation, particularly by black rat snakes.
Despite the toll that snakes take on cavity-nesters, cavities are still a much safer place to nest than open nests in trees and shrubs. So why don't more birds, or all birds for that matter, nest in cavities?
It's a matter of supply and demand. Cavities are in short supply, so relatively few species use them. In North American north of Mexico, only 85 species of breeding birds nest in cavities.
Competition for old woodpecker holes among bluebirds, wrens, chickadees and titmice, to name just a few examples, is fierce. Add house sparrows, starlings, mice, squirrels, snakes, bees, wasps and spiders to the list and you begin to appreciate the price birds pay for the security of a cavity.
Birds cope with heavy nest predation in one of two ways. Open-nesters and some cavity-nesters (bluebirds and wrens, for example) offset frequent nest failure by nesting two or even three times each year. This increases the chances that at least one nest will succeed.
Other cavity-nesters such as woodpeckers, chickadees, titmice and nuthatches raise only one brood each year. But they improve their chances for success by nesting early in the breeding season. This puts eggs and chicks in the nest before snake activity peaks and before other predators have babies of their own to feed. And most renest if their first effort is destroyed by predators.
What does all this mean for long-term population trends? To keep a population at a stable level, each individual need only replace itself in the course of its lifetime. That means each pair of birds need only produce two offspring. And since most birds that reach breeding age live at least a few years, chances are good they will eventually replace themselves, and the population will remain stable.
Despite nest losses that may appear catastrophic to us, many populations remain relatively constant. We shouldn't begrudge predators their fair share. Their habits are not wrong or bad; it's just what they do.
On the other hand, if you maintain a few nest boxes for cavity-nesters, don't let them become predator feeders. Protect each with a predator baffle (an 8-inch metal or PVC pipe) beneath the box to discourage climbing snakes and raccoons. For a copy of plans for a simple baffle, send me $1 and a self-addressed, stamped envelope.
West Virginian Scott Shalaway has written three nature books, including "Butterflies in the Backyard." Send questions and comments to Shalaway in care of The Wild Side, Journal Star, 1 News Plaza, Peoria, IL 61643, or send e-mail to sshalaway"at"aol.com.
From: bookfanaticef-bluebird"at"yahoo.com [mailto:bookfanaticef-bluebird"at"yahoo.com]
Success rates vary from species to species, and from region to region. From what I've heard in various wildlife classes & from different professors & ornithogists, there are some *very* general "rules" for success in passerines ("songbirds"), though I would guess that rates tend to be lower in more urbanized areas (I've read a couple scientific journal articles that suggest this). As nearly every professor I've had has said in all things relating to Nature and ecology: "It depends!"
On average, only ~50% of passerine nests make it to fledge at least 1 young (for some cavity-nesters, it is sometimes slightly higher) under natural conditions. Much depends on weather, availability of food, experience/age of the parents, geographic region, urban vs. rural areas (and therefore population levels of predators), etc.
The period immediately after fledging, when the young birds are learning to fly, avoid predators, & find food, is extremely dangerous, more so even than when they were in the nest. Basically, they are very stupid at this stage (think human teenagers). Fledglings tend to be noisy & conspicuous, which attracts predators. Sometimes adults can't always feed all their brood enough if they start spreading out too much, and once a fledgling is newly independent, it takes a while for it to learn to forage efficiently enough for itself, and may die of starvation. The chances of surviving this period after fledging are typically lower than at any other time in their life. If birds migrate, that places a further strain on their chances of survival, especially in 1st-year birds who must make both journeys to & from on instinct alone--many species do not stick with their parents long enough to "learn" the route from them, though they may be ! lucky enough to find other members of their own species to migrate with.
For many passerines, the chances of a bird surviving through their first year to adulthood/breeding age is ~50% or less. After they become adults, their chance of surviving each year thereafter typically becomes higher & tends to stay relatively steady, and for small land birds ranges (on average) from ~30-65% each year. However, some species have a rate of return (of birds banded as nestlings that came back as breeding adults the next year) to the area they were born as low as 10-15%, while others have a >50% chance of becoming a year older. As I said before--It Depends.
I'm sorry I don't have more concrete numbers, or specifically for bluebirds, but I hope all this will help in making a rough guess. Many of the bluebirds on the trail I help monitor were banded as nestlings by people at the nearby university doing various projects (the boxes are on univeristy property meant for research purposes). This is the 4th year the boxes have been up, and the ratio of banded to unbanded adult breeding birds is roughly 50:50 from what I've seen this year--but some of these were nestlings 2-3 years ago, so it is possible that the return rate of birds banded here as nestlings only 1 year previous is slightly less than 50% each year. However, as there are also many snags to choose from, I'm going to optimistically say the return rate is 50%--I only know about the birds that came from/come back to the boxes, so others may be born/return nearby in a natural cavity, and I might never know.
So, all this means that it is not unlikely that of a brood of 7, only 1 survived to return as an adult. But keep in mind that even in birds that stay in one place year-round and return to the area they were born (like bluebirds), they may travel a bit, and find a place more to their liking not far away (or sometimes very far away), and you might never know.
On the other hand, survival rates might be lower than the "natural average" if you live in an urban/suburban setting where birds have to contend with not only the usual predators and the hardship of finding their own food, but they also must run the gauntlet of cats & dogs, high numbers of human-wise raccoons & squirrels, cars, possible poisoning (as from pesticides/herbicides), buildings with reflective windows (which birds often mistake for more open air instead of solid surfaces), and sundry other man-made obstacles.
While it is very sad to contemplate that so few birds make it to adulthood, that's the way Nature is. We can help reduce mortality from human-induced causes by doing little things like keeping cats inside, etc, but it will never ensure that all the young birds, or even most, become adults.
May you have many fledglings (and hopefully more will come back next year)!
From: Evelyn Cooper [mailto:emcooper"at"bayou.com]
Sent: Tuesday, May 17, 2005 5:35 AM
Subject: FW: Success rate to adulthood
All my bluebird books say that 70% of Bluebird babies do not make it through
From: Tina Wertz [mailto:tinawertz"at"bellsouth.net]
Sent: Tuesday, May 17, 2005 8:14 AM
Subject: RE: Success rate to adulthood
Thank you to everyone that responded. It's hard to want to believe the numbers, but as I watched yesterday, I can now believe it. I heard the little fledgling calling for his parents, and when I spotted him, he was on the eve of my roof. Now any quick flying sharp shinned hawk could have made a quick meal of him in an instant. I opened my window and he flew into the trees. I hate to think that they leave themselves so vulnerable. I guess nature knows what it's doing and we as humans must accept it.
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