Johnson et al. Hereafter, we refer to this as the maximum time hypothesis and it has 2 implied assumptions. The second assumption is that nighttime represents a period of increased risk for young because darkness may preclude them from moving to a safer location and may increase the chances they lose contact with their parents, both of which could increase mortality.
More time available to reach preferred fledgling habitat is particularly important considering that, relative to adults, the mobility of young fledglings is extremely limited due to a lack of coordination and an inability to sustain flight e.
In this study, we test the maximum time hypothesis by suggesting that the time of day when fledging begins and the time period over which broodmates fledge is influenced by nest predation risk.
Specifically, our research on shrub-nesting birds has revealed that nest survival is positively correlated with nest site vegetation density and nest height Chiavacci SJ, unpublished data , the latter of which other studies have also found to have a positive influence on nest survival e.
We predicted that, in nests facing a high risk of predation, 1 nestlings would initiate fledging earlier in the day and 2 the time between the fledging of broodmates would be shorter i. We conducted this study in shrubland habitats in Illinois. Eight study sites were located in northeastern Illinois and 4 were located in east-central Illinois.
Habitats consisted primarily of herbaceous vegetation such as common goldenrod Solidago canadensis and blackberry Rubus spp. Such habitat characteristics offered open areas with low-lying shrubs and dense thickets composed of shrubs several meters in height i. We searched for nests from approximately 1 April through 1 September during — We located nests using systematic searches and adult behavior.
We monitored a subset of nests with time-lapse video recording systems Cox, Pruett, et al. We camouflaged cameras with paint and vegetation and placed them within 0. We recorded nest activity on digital video recorders at 6 frames per second.
We connected cameras to recorders with to m-long cables and placed recorders in a camouflaged container at the extent of the cable. In most cases, we did not deploy cameras until nests contained full clutches to avoid abandonment. Following nest termination, we measured vegetation at all nest sites. We used a Robel pole Robel et al.
To do this, we placed the Robel pole at the nest and estimated the percent of the lower 0—1 m and upper 1—2 m portions of the pole covered by vegetation while standing 5 m away and at a height of 1 m; we did this in 4 cardinal directions and averaged these to obtain a single estimate. We also used the Robel pole to estimate the percent of the nest concealed by vegetation. We estimated the percentage of these 3 decimeter bands obscured by vegetation while standing 5 m away and at a height of 1 m in each cardinal direction; we obtained a single estimate for each nest by averaging the estimates.
If nests were greater than 1. We measured nest height as the distance between the ground and the bottom of the nest. When reviewing video, we recorded the time of fledging as the moment when a chick left the nest and did not return. To obtain fledging initiation time, we calculated the difference between the time that the first chick fledged from a nest and the onset of dawn civil twilight.
We did not use sunrise time because birds are known to be active before sunrise e. Because the timing of dawn twilight varied over the course of the season and geographically, we recorded it for each day of the season in each year for each study site.
We did not account for the potential influence of overcast skies on visibility during dawn twilight, as we had no reason to suspect such conditions were temporally or spatially consistent enough to influence our results in a consistent manner. To calculate fledging interval lengths, we measured the amount of time that elapsed between subsequent fledging events and averaged these within broods.
Force-fledging due to natural disturbances i. Such behavior is in contrast to typical fledging behavior in the absence of such a stimulus. In cases where the first chick fledged naturally, but at least one remaining chick was force-fledged, we used the nest in our analysis of fledging initiation time, but not in our analysis of fledging interval length. We also excluded all nests where the first chick to leave the nest was force-fledged as we were unsure how such an event affected the timing and span of subsequent fledging events at a nest.
Lastly, we excluded nests that suffered partial brood loss due to predation, as we did not know how this affected the fledging behavior of remaining nestlings. To test the maximum time hypothesis, we developed a priori models containing variables we hypothesized would explain variation in fledging initiation times and the time interval length between broodmate fledging events. We examined the effect of nest site characteristics on fledging because of their potential to influence nest predation risk.
Specifically, we examined the influence of nest height because it is often negatively related to nest predation, with higher nests suffering lower predation rates e.
Also, nest height is a particularly reliable determinant of nest survival in our system, as nests higher above the ground have a lower probability of being depredated by several dominant predators, including black ratsnakes Pantherophis obsoletus , raccoons Procyon lotor , and white-tailed deer Odocoileus virginianus ; Chiavacci SJ, unpublished data.
We also examined the influence of nest concealment, as this has been found to positively relate to nest survival e. Additionally, we included vegetation density within 5 m of nests as a predictor of fledging time, as we surmised denser vegetation around the nest site improved the survival probability of fledglings e.
Lastly, we examined the influence of daily nest survival rate hereafter DSR on fledging behavior. We calculated DSR as a function of the interactive effects of study site and day of year using the logistic-exposure method Shaffer We did this to account for the fact that predation risk often varies spatially and throughout the breeding season e. Specifically, we knew a priori that DSR varied among our 12 study sites and that predation by dominant predators varied seasonally Chiavacci SJ, unpublished data.
Thus, we sought to account for this variability when modeling the influence of DSR on fledging behavior. We included species as a random effect because we were interested in detecting generalizable patterns irrespective of nesting species. We used an information-theoretic approach Burnham and Anderson to determine the relative support for models. We evaluated all models for their inclusion of uninformative parameters i.
All analyses were performed in SAS, version 9. We documented fledging events at nests of 17 species Supplementary Appendix ; we excluded 50 of these nests from all analyses due to force-fledging or partial brood loss see Methods. The earliest date of fledging was 24 April, whereas the latest was 7 September. Forty-one nests fledged a single nestling, whereas 73, 63, 24, and 1 fledged 2, 3, 4, and 5 nestlings, respectively.
Mean fledging initiation time i. The second- and third-ranked models were weakly supported but had confidence limits that excluded 0, suggesting that they explained some variability in the data.
The third-ranked model indicated that fledging initiation was positively related to nest site vegetation density i. Model selection results from a priori candidate models describing the time of fledging initiation at bird nests in shrubland habitat in Illinois, — AIC c. The mean interval length between subsequent fledging events in the single nest fledging 5 nestlings was 2. Mean time interval length between fledging events at these nests was best predicted by a model including nest height, year, and nest site vegetation density Table 2.
However, our top model was not overwhelmingly supported, as it received only 1. Thus, we generated model-averaged predictions based on variables within our competitive model set. Year was also a strong predictor of mean time interval length; nestlings fledged most rapidly in model-averaged predicted estimate 1. Model selection results from a priori candidate models describing the mean time interval length between broodmate fledging events within bird nests in shrubland habitat in Illinois, — Fledging earlier in the day may be a strategy that gives young fledglings more time to reach a relatively safe location before dusk i.
We expected that the timing of fledging, like the age at which fledging occurs, would vary in response to nest predation risk, due to the potential costs e. Indeed, we found that fledging began later in the day and broodmates remained in the nest longer under lower nest predation risk conditions, suggesting that the timing of nest departure represents a strategy by which birds might mitigate predation risk.
Several nest site features known to influence nest predation risk explained patterns in fledging behavior. First, we found a weak relationship between fledging initiation time and nest concealment, such that fledging began earlier at more concealed nests. Although counter to what may be expected if greater concealment reduces predation risk Horie and Takagi , the earlier initiation of fledging among better concealed nests was likely the result of nests nearer to the ground i.
In contrast, nest height had a much stronger influence on the timing of fledging initiation and the rapidity with which broodmates fledged. Alternatively, the relationship between nest height and fledging behavior may have been due to the strong influence of nest height on nest survival.
Specifically, nest predation rates in our system are greatest among nests nearest to the ground Chiavacci SJ, unpublished data , a finding similar to studies elsewhere e. Further, we found that nestlings fledged later and over longer time periods among nest sites containing denser vegetation, a habitat feature positively correlated with nest survival in our system Chiavacci SJ, unpublished data as well as fledgling survival elsewhere e.
Thus, the decision of when and how quickly to fledge may be influenced by both the relative safety of the nest itself and the safety of the area around the nest i. Additionally, the timing of nest departure may have also been influenced by predation risk at broader spatial scales, as is evidenced by the relationship between site-specific DSR and fledging time interval length. This relationship, although weak, suggests that sites with greater overall predation risk led to earlier fledging.
How might young benefit from staying in their nests later into the day and longer after fledging begins? One possibility is that fledging after being fed for several extra hours may reduce the need for adults to feed hungry fledglings during travel to postfledging habitats, as adults tend to intensively feed fledglings until begging ceases Lemel Young that are satiated on leaving the nest may emit fewer begging calls, thereby reducing feeding activity, both of which are cues used by predators to locate vulnerable young e.
Additionally, Vitz and Rodewald found that fledging at a lower mass—tarsus ratio, a proxy for condition, resulted in fledglings moving shorter distances during their first days out of the nest. Thus, obtaining greater mass at fledging may enable young to reach preferred postfledging habitat more quickly. Greater mass may also buffer young from unexpected conditions outside the nest that may preclude or reduce food delivery by adults e.
Lastly, young that remained longer after a sibling fledged may have further improved their condition via continued feedings in a safe nest site, as feeding rates to remaining nestlings may remain constant even after several have fledged Johnson et al.
In contrast to fledging later, fledging early in the day may offer benefits to young facing high predation risk conditions. For example, fledging sooner may help improve fledgling development if young can feed themselves or rates of parental feedings increase once outside the nest. Increased feeding opportunities outside the nest may be particularly important if high nest predation risk reduces feeding rates to nestlings e. In addition, leaving the nest earlier maximizes the daylight available to reach a safer location before nightfall Johnson et al.
This behavior could be critical if young leaving as soon as possible do so at a lower mass, resulting in them travelling relatively shorter distances Vitz and Rodewald Lastly, snakes are a dominant predator of nests nearer to the ground in our system Chiavacci SJ, unpublished data , but they are typically inactive until later in the day Stake et al.
Thus, fledging earlier from risky nests i. If the process of fledging can be explained by predation risk, as our results suggest, then how might birds both adults and nestlings be evaluating risk and how does this inform the decision of when in the day to fledge?
We recognize several nonmutually exclusive possibilities. One is that parents evaluate risk through direct exposure to predators and alter their own behavior to influence that of their young. For example, parents could encourage young to fledge by holding food beyond their reach or by reducing feeding rates to remaining nestlings, though the few studies to examine this behavior have found little or no support for it Nilsson and Svensson ; Michaud and Leonard ; Johnson et al.
It is also possible that females nesting in high risk areas laid eggs with higher levels of circulating stress hormones that, via maternal affects, led to faster wing growth rates and a propensity in nestlings to fledge as soon as possible Coslovsky and Richner ; Cheng and Martin In contrast to adults, nestlings have likely had no or only minimal direct exposure to predators, thereby limiting their knowledge of predation risk and, consequently, the best time of day to fledge Lima This lack of direct experience suggests that nestlings may be relying on indirect cues to evaluate risk.
For example, nestlings could use the frequency of parental alarm calls as a gauge of risk sensu Lima Indeed, on hearing parental alarm calls, nestlings become silent and remain still Platzen and Magrath ; Caro , illustrating their ability to associate such calls with risk. Nestlings are also able to identify and respond with silence to the presence of a nearby predator Magrath et al.
Another possibility is that young birds instinctively seek habitat features that offer protection from predators and if such conditions are present at the nest, nestlings may be less inclined to fledge as soon as possible. Specifically, young fledglings often move to areas with both dense vegetative cover, a feature that improves their survival e. The presence of elevated perches seems particularly important, given that young, uncoordinated fledglings are especially vulnerable to terrestrial predators e.
In fact, the behavior of seeking safer locations during periods of vulnerability is a widespread behavior in animals Caro and may explain the patterns in fledging behavior we identified. Our results reveal that the situations faced by nestlings preparing to fledge may be more complex than we currently recognize and the timing of nest departure is likely the first of many critical decisions birds must make in an effort to maximize their fitness.
Information on such fledging behavior in birds should continue to accumulate, given the increasing use of camera systems to document nesting activity Cox, Pruett, et al. Further study into the mechanisms underlying fledging behavior in different habitats would greatly expand our understanding of the generality of the predation risk influence we found, particularly if nest site features such as nest height vary among habitats in their effect on predation risk Martin In addition, studies exploring the process of fledging in open-cup nesting birds would advance our understanding of the variability in fledging behaviors, as the vast majority of our knowledge has been derived from cavity-nesting species Lemel ; Nilsson ; Nilsson and Svensson ; Johnson et al.
Future research should evaluate the relationship between fledging behaviors e. Specifically, comparing nestling condition and postfledging survival among nests facing varying degrees of predation risk may reveal interactions among nest site riskiness and the costs and benefits of fledging at different times of day. For example, does fledging earlier in the day convey a survival advantage to chicks fledging from high risk nests?
Likewise, does fledging later in the day from relatively safe nests lead to higher postfledging survival? Also, given that the age at which nestlings fledge may vary in response to predation risk Bosque and Bosque ; Martin ; Martin et al. We also recommend researchers conduct experimental studies to explicitly test our hypotheses about the factors driving the timing of nest departure.
For example, nest site characteristics e. This nest holds 4 females and one male. By this stage, babies are strong enough to cling to the entrance of the nestbox to look out. They have a narrow ring of white feathers around each eye, and their breasts are speckled with gray.
Notice the cobalt blue feathers of the male. Nestlings exercise more, and may stand on the edge of the nest and look out of the nest cavity. When I took these photos since they were after Day 13 I blocked the entrance hole and moved quickly and quietly. Premature fledging is less likely in a top opening box. Bluebirds generally fledge days after hatching, typically Development depends on food availability. The parents often get more defensive around the box at this time, and may divebomb passersby.
Nestlings are able to hop. Western and Mountain Bluebird s typically stay in the nest longer than Eastern Bluebirds - days, depending on the weather. All 5 babies are still in the nest, eyes open, fully feathered. The runt is on the bottom of the pig pile. By early morning, four have fledged. At fledging, the babies weigh slightly more than their parents. The runty girl has not fledged. The parents continue feeding her mealworms. As I approach the box I hear her calling to them. The runtess is still in the nest!
I probably should check to make sure that her feet are not tangled in anything, and that her wings are okay. There is some fecal material in the nest, as the parents have probably stopped removing it. Empty nest syndrome begins. The nest is typically flattened and soiled. A dead mealworm lies in the nest. Young stay in cover for several weeks.
Parents will continue to feed them until they are about 30 days old. Helping bluebirds is a deeply satisfying hobby, and a unique adventure in conservation. May all your blues be birds! The purpose of this site is to share information with anyone interested in bluebird conservation. 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!
Photo in header by Wendell Long. Please honor their copyright protection. See disclaimer , necessitated by today's sadly litigious world. Last updated August 24, Design by Chimalis. Bluebird and Small Cavity Nester Conservation. The female is more gray, and has a white eye ring. She makes the final choice for a nest site. Occasionally an Eastern Bluebird will add feathers to a nest. I offer mealworms to help the parents feed the hungry nestlings.
I'm raising my own , since they get expensive. Soft gray down is now along the edges of wings, the head and spine. The skin beneath looks blue-black as feathers begin to develop beneath it. One of the babies the last to hatch probably is clearly smaller than her nest mates. Feather sheaths begin to emerge on wings. The adult bluebirds begin feeding the young immediately after hatching, starting with soft insects and graduating to courser food as the nestlings grow.
The adult bluebirds continue to care for the young and teach them to forage for food. The male bluebird will continue this job while the female begins her second or third nest. After nesting season is over, bluebirds give up their territories and flock together. South Carolina bluebirds do not migrate. They are joined by migrant northern bluebirds and roam the area looking for berries.
In winter, bluebirds will roost in pine tree stands and nest boxes to avoid cold weather. To better understand the hatching and fledging process of bluebirds, here are some important questions that almost all bluebird enthusiasts have. As early as February — April for the first nesting. Sometimes later if first-time nesters or multiple nesters. Bluebirds typically use woven grass, pine needles, straw, hair, and sometimes feathers. Typically there will be no seed heads, cigarette butts, strings, sticks, or other junk in a Bluebird nest.
The eggs are laid one each day until the entire clutch is complete, usually powder blue or, occasionally, white eggs. Incubation lasts approximately days for Eastern, days for Mountain, and days for Western Bluebirds.
Incubation usually begins right after the last egg was laid, but sometimes the female will wait a day or two. After hatching the chicks will remain in the nest for days for Eastern , for Mountain, and days for Western Bluebirds.
You should not open the box for monitoring after the 13th day after hatching or the chicks might fly from the box prematurely. The fledglings look surprisingly unlike the adult birds.
They are speckled with touches of blue on the wings. Bluebirds are quite tolerant of human monitoring of their nests. Sometimes the eggs or babies must be touched to resolve a dangerous problem from a predator or pest. Of course, unnecessary handling of the nest, eggs, and babies should be avoided. Both parents feed the babies. Often fledglings from a previous nest also help with the feeding.
I saw one of the adult birds carrying something white out of the nest box. What was it? The parents are very meticulous about cleaning up the nest box. When the eggs hatch, the female usually eats the eggshells to help with her egg production or disposes of them far away from the nest box.
While the babies are growing, the parents will carry away the waste products from the nestlings. Yes, take the nest out, and if no parasites, save it in a plastic bag for emergency nest changes. Microwaving the nest briefly. This will kill any undesirable bacteria or hidden bugs. Wash inside of nest box with a mild solution of bleach and water.
Some sources say 1 part bleach to 10 parts water, other sources suggest a ratio. In either case, flush with plain water after disinfecting. Leave the door open until it dries. Females have been known to start a second nest within a day of fledge day and occasionally will start a second nest in another box even before fledging occurs in the current nest. For this reason, the box should be cleaned the day after the last baby fledges or sometimes, that very day.
The old nest should always be removed and disposed of far away from the nest box.
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