Why do birds fledge

Cats are an insidious threat to millions, and in rural gardens even weasels may join the ranks of those gorging on the fast food. And in recent years, the omnivorous magpie, having become a common and conspicuous feature of the garden scene, has come to be regarded, unfairly, as an arch executioner of suburban youth.

Magpie eating a bird chick. And it only takes one or two experiences to turn the enraged gardenwatcher into a fully committed member of the culling lobby. The trouble is, magpies do their worst out in the open, in public, while cats are more private — and more prolific. And there is remarkably little quantifiable evidence as yet that magpies, despite appearances, actually seriously reduce wildbird populations of their own accord.

Yet the magpie, like its relative, the carrion crow, is a master of the art of bad publicity. The parent birds of fledglings naturally do all they can to protect their charges against the activities of such predators, but the truth is, if detected, there is little a small bird can achieve. Young birds face many dangers, including this female sparrowhawk. In truth, the actual commitment of parents to their newly-promoted fledglings is quite variable.

The majority of garden birds, including wrens, dunnocks and blackbirds, feed their young for about two and a half weeks before insisting they fend for themselves. But young starlings join flocks of their peers after only a few days, and young tits, moving up to a canopy that may be literally dripping with caterpillars, may similarly become independent after a week or so.

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.

Home Biology Ecology. Cavity-nesting birds, like this mountain chickadee about to feed its young, have safer nests that allow young to stay in nests longer and develop their wings for improved flight at leaving. Credit: T. Parent gray-headed junco enticing young to leave the nest. Parents hold food away from nest and young come out to get it and this picture captures a young that was just fed out of the nest.

Explore further. More information: Thomas E. Martin et al. Age and performance at fledging are a cause and consequence of juvenile mortality between life stages, Science Advances DOI: This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission.

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Nov 12, Covid vaccines - CDC report on associated mortality Nov 12, Implications of Covid becoming endemic Nov 12, Related Stories. The loss of a parent is the most common cause of brood failure in blue tits Jun 12, May 07, Martin, doi Major life changes can be dangerous, even fatal. Probably the most dangerous life transition is when young animals, such as fledgling birds, begin to move about on their own and to make their own decisions.

Predictably, when baby birds -- nestlings -- transition from dependency to their new life as fledglings living outside of the nest, their first few weeks of exploring the landscape and learning to fly are fraught with extraordinary dangers. When nestlings leave the nest too early, they fly poorly, or not at all, because their wings are small and underdeveloped. This is especially true for birds that build open-cup nests on or near the ground.

A young gray-headed junco Junco hyemalis caniceps is captured leaving the nest, with its sibling Predictably, predation plays an important role in driving the evolution of optimal fledging times for birds. Songbirds that experience higher daily rates of predation -- species like towhees and juncos that build open-cup nests on the ground or in low bushes -- have evolved younger ages of fledging to deal with this pressure.

In contrast, this pressure to fledge early is relaxed for birds that enjoy a relatively low risk of nest predation -- as seen in cavity-nesting birds, like chickadees and bluebirds. Cavity-nesting birds, like this mountain chickadee Poecile gambeli , about to feed its young, have Professor Tobalske was a co-author of the recently published study. This is typical: similarly high or highly variable mortality rates due to predation in the first weeks of juvenile life are common across a wide variety of other animal species, too ref.

A research team, headed by avian ecologist Thomas Martin, Assistant Unit Leader and Senior Scientist in the Montana Cooperative Wildlife Research Unit at the University of Montana , investigated how predation influences the transition from nestling to fledgling in different species of songbirds. These songbirds included species that build open-cup nests either on the ground, low down in bushes or higher up in trees, as well as species that nest in cavities.

Martin and his colleagues measured nest predation rates, wing growth rates, fledging ages and they used high-speed videography to record and examine flight performances of newly fledged birds of 11 songbird species to see if this may explain differences in their fledgling mortality rates. As expected, Dr.