The process of fledging in the Mountain Bluebird

Fledging is a critical event in the avian breeding cycle, but remains unstudied in almost all species. As a result, little is known about factors that cause nestlings to leave nests. We documented fledging behavior in a box‐nesting population of Mountain Bluebirds (Sialia currucoides) using radio‐frequency identification. We attached a passive integrated transponder (PIT tag) to the leg of each nestling in 40 nests. An antenna checked for the presence of a transponder signal (i.e., a nestling) at nest‐box entrances every 2 s. The time of last detection of a nestling was taken as the time that nestling fledged. We found that fledging began when the oldest nestlings were 15–22 d old. Broods that were ahead in development, as measured by primary feather length, fledged at relatively younger ages. All nestlings fledged on the same day at 33 nests (83%) and over 2 d at remaining nests. When all nestlings fledged on the same day, fledging usually began in the morning and median time between the first and last fledging was 55 min (range = 2.3 min–10.6 h). When young fledged over 2 d, fledging always began >8 h after sunrise and usually just one nestling fledged the first day, suggesting that this fledging may have been accidental. Clutches in our population often hatch asynchronously, which sets up a hierarchy within broods in developmental state, size, and competitive ability. In such situations, fledging may be initiated by one of the most‐developed and hence most‐competitive nestlings in a brood, presumably when it reaches a certain threshold state of development. Alternatively, fledging may begin when a less‐developed, less‐competitive, and probably hungrier nestling leaves the nest, presumably to gain better access to food. We used the proportion of time that a nestling was able to occupy the nest‐box entrance late in the nestling stage, waiting to intercept parents with food, as an index of nestling competitive ability. Assuming that the number of nest entrance detections reliably indicates nestling competitive ability, we found that the most‐competitive nestling fledged first at over half of all nests, supporting the notion that fledging usually begins when oldest nestlings reach a threshold state of development.     

Post‐fledging survival of Little Owls Athene noctua in relation to nestling food supply

Among the range of determinants of post‐fledging survival in altricial birds, the energy supply to the growing juveniles is likely to play a central role. However, the exact mechanisms shaping post‐fledging survival are poorly understood. Using a food supplementation experiment, we determined the effect of variation in food supply on the survival of juvenile Little Owls Athene noctua from hatching to 2 months post‐fledging. Experimental broods were food‐supplemented for 36 days during the nestling and the early post‐fledging period. The fate of 307 juveniles (95 of them provided with extra food) was determined by nest monitoring and radiotelemetry. In unsupplemented birds, the rates of survival measured at 5‐day intervals were lowest during the nestling stage, remained low during the early post‐fledging stage and steadily increased after about 2 weeks post‐fledging. Food supplementation substantially increased nestling survival, but we detected no direct treatment effect on post‐fledging survival. Instead, we found a strong indirect effect of food supplementation, in that fledglings of good physical condition had markedly higher chances of surviving the post‐fledging period compared with those in poor condition. Experimental food supplementation increased survival over the first 3 months from 45% to 64.6%. This suggests that energy reserves built up during the nestling stage influence post‐fledging survival and ultimately parental reproductive output. The low nestling and post‐fledging survival shows that the early life‐history stages constitute a crucial bottleneck of reproductive ecology in Little Owls. The strong treatment effects on the number of independent offspring indicate that natural variation in food supply is an important determinant of spatio‐temporal patterns in Little Owl demography.     

Behavior of adult and young grassland songbirds at fledging

The behavior of adults and young at the time of fledging is one of the least understood aspects of the breeding ecology of birds. Current hypotheses propose that fledging occurs either as a result of parent‐offspring conflict or nestling choice. We used video recordings to monitor the behavior of nestling and adult grassland songbirds at the time of fledging. We observed 525 nestlings from 166 nests of 15 bird species nesting in grasslands of Alberta, Canada, and Wisconsin, USA. Overall, 78% of nestlings used terrestrial locomotion for fledging and 22% used wing‐assisted locomotion. Species varied in propensity for using wing‐assisted locomotion when fledging, with nestling Grasshopper Sparrows (Ammodramus savannarum) and Henslow's Sparrows (Centronyx henslowii) often doing so (47% of fledgings) and nestling Song Sparrows (Melospiza melodia), Common Yellowthroats (Geothlypis trichas), and Chestnut‐collared Longspurs (Calcarius ornatus) rarely doing so (3.5% of fledgings). For 390 fledging events at 127 nests, camera placement allowed adults near nests to be observed. Of these, most young fledged (81.5%) when no adult was present at nests. Of 72 fledging events that occurred when an adult was either at or approaching a nest, 49 (68.1%) involved feeding. Of those 49 fledgings, 30 (62.1%) occurred when one or more nestlings jumped or ran from nests to be fed as an adult approached nests. The low probability of nestlings fledging while an adult was at nests, and the tendency of young to jump or run from nests when adults did approach nests with food minimize opportunities for parents to withhold food to motivate nestlings to fledge. These results suggest that the nestling choice hypothesis best explains fledging by nestlings of ground‐nesting grassland songbirds, and fledging results in families shifting from being place‐based to being mobile and spatially dispersed.     

Do parents play a role in the timing and process of fledging by nestling Mountain Bluebirds?

What causes young birds to leave nests remains unclear for almost all altricial species. For many years, the assumption was that parents often controlled the time of fledging by coaxing young from nests, e.g., by holding food within view, but out of reach, of nestlings. This assumption, though, was based solely on scattered anecdotal reports of such behavior. We used continuous video‐recording of nests to assess the role of parents, if any, in the timing and process of fledging of cavity‐nesting Mountain Bluebirds (Sialis currucoides). We placed perches ~50 cm in front of nest‐box entrances to give parents ample opportunity to display food to nestlings. We found no evidence that parents routinely initiated the fledging process. On the day of fledging, parents did not perch on supplemental perches with food more often, or for longer periods of time, than on the day before fledging. Also, after going to nest‐box entrances, parents never held food away from a nestling reaching for the food. Parents were usually absent (16 of 19 cases) when the first nestling fledged. In the remaining three cases, a parent perched with food in view of a nestling for 8, 15 and 65 s, respectively, just before that nestling fledged. Although these might have appeared to be attempts at coaxing, in each case, the parent was encountering, for the first time, a nestling partially emerging from the nest entrance. Parents may simply have hesitated to approach nests because the nestling's position prevented parents from delivering food in the normal manner. Finally, the rate at which parents fed nestlings on the day of fledging did not differ from the rate the day before, suggesting that parents do not try to use hunger to induce fledging. Our results are consistent with previous research suggesting that, in Mountain Bluebirds, it is a nestling that initiates fledging, typically when it reaches some threshold state of development.     

Scaling nestling energy requirements

The total energy metabolized (TME) by nestling birds, from hatching to fledging, scales as M ^1.06, with body-mass at fledging (M) explaining 97% of the variation in TME. After statistically removing the effects of body-mass, multiple-regression analysis reveals that 69% of the variation in TME is explained by nestling developmental rate, expressed as the time to fledging ( t _fl, days). Together t _fl and M explain 99% of the variation in TME for the 30 species considered ( M range 9.7 to 3700 g).
Peak daily metabolized energy (DME, kJ/day), the maximum rate at which parents must provision their nestlings, scales with fledging mass as M^0.78. Body-mass explains 96% of the variation in nestling peak DME, with 37% of the residual variation being attributable to the time taken to fledge ( t _fl). Together, t _fl and M explain 97% of the variation in peak DME. An examinination of residuals indicates that adaptive modifications in nestling energetics are attained principally through changes in growth rate, which affect TME and peak DME oppositely. Doubling nestling growth rate increases peak DME by 61%, but decreases total nestling energy demand (TME) by 77%. This opposing effect complicates evolutionary interpretations of avian reproductive patterns.     

Brood provisioning rates and fledgling behavior of Cordilleran Flycatchers in southwestern Colorado

The behavior of young songbirds after fledging is one of the least understood phases of the breeding cycle, although parental provisioning rates and movement of fledglings are key to understanding life history evolution. We studied Cordilleran Flycatchers (Empidonax occidentalis) at two sites in southwestern Colorado, USA, from 2012 to 2017. We banded and sexed breeding adults to determine the relative contributions of males and females to nestling and fledgling care, and attached radio‐transmitters to nestlings to facilitate observations of brood behavior after fledging. Females made 60% and 78% of total observed feedings of nestlings and fledglings, respectively. Parental provisioning rates increased with nestling age, and per‐nestling provisioning rates increased with brood size. Parental provisioning rates declined just before fledging, then increased just after fledging. Fledging times of individuals in broods were asynchronous and concentrated during the late afternoon and early evening. Males stopped caring for fledglings before females even though this species is single‐brooded, with some late‐season broods being abandoned by males. Broods spent the first three weeks after fledging within 400 m of nests, after which they began to disperse. Most aspects of the breeding biology of Cordilleran Flycatchers in our study, including the duration of nestling and fledging periods, female‐dominated provisioning, and movement patterns of fledglings, were similar to those of other Empidonax species. However, the times when young fledged were not concentrated in the morning as reported in most other songbirds, and this result warrants additional study of the timing of fledging in ecologically and taxonomically similar species. The increased per‐nestling provisioning rate with increasing brood size was unexpected, and additional study is needed to determine if this increase results from a trade‐off between adult annual survival and productivity favoring increased provisioning of young in larger broods, or from the existence of high‐quality individuals where larger clutches and higher provisioning rates are linked.     

Facultative adjustment of pre-fledging mass loss by nestling swifts preparing for flight

Nestling birds often maintain nutritional reserves to ensure continual growth during interruptions in parental provisioning. However, mass-dependent flight costs require the loss of excess mass before fledging. Here we test whether individual variable mass loss prior to fledging is controlled through facultative adjustments by nestlings, or whether it reflects physiologically inflexible developmental schedules. We show that in the face of natural and experimental variation in nestling body mass and wing length, swifts always achieve very similar wing loadings (body mass per wing area) prior to fledging, presumably because this represents the optimum for flight. Experimental weights (approx. 5% body mass) temporarily attached to nestlings caused additional reductions in mass, such that final wing loadings still matched those of control siblings. Experimental reductions in nestling wing length (approx. 5% trimmed from feather tips) resulted in similar additional mass reductions, allowing wing loadings at fledging to approach control levels. We suggest that nestlings may assess their body mass relative to wing area via wing flapping and special 'push-ups' (on the tips of extended wings) performed in the nest. Thus, by facultatively adjusting body mass, but not wing growth, nestling swifts are always able to fledge with aerodynamically appropriate wing loadings.     

The influence of sex and body size on nestling survival and recruitment in the house sparrow

Differences in the survival rates of males and females over the period from hatching to recruitment can have important impacts on individual fitness and population demographics. However, whilst the influence of an individual's sex on nestling growth and survival has been well studied, less is known about sex‐specific survival over the period between fledging and recruitment. Here, we analyse nestling survival and recruitment in an isolated, island population of house sparrows (Passer domesticus), using data collected over a 4‐year period. Nestlings that had a greater mass at 1 day old were more likely to fledge. Recruitment was also positively associated with day 11 mass. The positive influence of nestling mass on survival to fledging also increased as brood size increased. There was no difference in the survival of male and female individuals prior to fledging. In contrast, over the period from fledging to recruitment, females had significantly less mortality than males. Recruitment was also positively associated with 11‐day‐old mass. Neither the nestling sex ratio nor the fledging sex ratio deviated from 0.5, but the sex ratio amongst recruits was female biased. Our study shows that sex can influence juvenile survival, but also shows that its effect varies between different life‐history stages; therefore, these stages should be considered separately if we want to understand at what point sex‐specific differences in juvenile survival occur. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 680–688.     

Adaptive host-abandonment of ectoparasites before fledging? Within-brood distribution of nest mites in house sparrow broods

We studied the within-brood distribution of a haematophagous mite Pellonyssus reedi living on nestling house sparrows (Passer domesticus) near the time of fledging. We measured the natural level of infestation of individual nestlings, and determined the feeding efficiency of mites, by scoring their feeding status. Within-brood distribution of mite loads was unrelated to nestling body mass, tarsus length, or immunocompetence. These results did not support parasite preference for large or susceptible hosts. Mite feeding-efficiency was also unrelated to these nestling characteristics, confirming that large nestlings or nestlings with less-developed immunocompetence did not provide superior feeding conditions for mites. Therefore, our results do not support the hypothesis that within-brood distribution of avian ectoparasites is explained by the parasites' preferences for characteristics, such as large body size or low immunocompetence, that make nestlings suitable hosts. On the other hand, we found that mite loads were negatively correlated with nestling age and feather length, suggesting that nestlings closer to fledging harbored fewer mites then their less-developed nestmates. Furthermore, feather length had a stronger relationship with parasite distribution than did nestling age. We presume, therefore, that feather characteristics, i.e., length, may serve as a signal for mites to perceive the ready-to-fledge state of nestlings, inducing abandonment behavior. These results support another, largely neglected hypothesis, i.e., that the avoidance or abandonment of those nestlings that are close to fledging may also explain the parasites' distribution in a brood. This hypothesis is based on the argument that many nest-dwelling ectoparasites breed in the nest material and emerge only periodically to feed on nestlings. In such parasites, the ability to recognize and avoid mature fledglings can be adaptive because this may help the parasites to avoid their removal from the nest so they can continue to reproduce by feeding on unfledged chicks of the current or later broods. Our results suggest that adaptive host-abandonment by nest-dwelling ectoparasites can influence within-brood parasite distributions around the time of fledging.     

The evolution of fledging age in songbirds

In birds with altricial young an important stage in the life history is the age at fledging. In this paper we use an approach proven successful in the prediction of the optimal age at maturity in fish and reptiles to predict the optimal age of fledging in passerines. Integrating the effects of growth on future fecundity and survival leads to the prediction that the optimal age at fledging is given by a function that comprises survival to maturity, the exponent of the fecundity-body size relationship and nestling growth. Growth is described by the logistic equation with parameters, A, K and t(i). Assuming that the transitional mortality curve can be approximated by the nestling mortality, M(n), the optimal fledging age, t(f), is given by a simple formula involving the three growth parameters, nestling mortality (M(n)) and the exponent (d) of the fecundity-body size relationship. Predictions of this equation underestimate the true values by 11-16%, which is expected as a consequence of the transitional mortality function approximation. A transitional mortality function in which mortality is approximately 0.3-0.4 of nesting mortality (i.e. mortality declines rapidly after fledging) produces predictions which, on average, equal the observed values. Data are presented showing that mortality does indeed decline rapidly upon fledging.     

Variation in growth in Sandwich Tern chicks Sterna sandvicensis and the consequences for pre- and post-fledging mortality

Fitness consequences of variation in body mass growth and body condition were studied in a Sandwich Tern Sterna sandvicensis colony on Griend, Dutch Wadden Sea, during 1990–2000. Body mass increment during the linear growth phase predicted nestling survival probabilities accurately. Chicks growing less than 8 g per day had low survival probabilities until fledging, but within a range of 8–11 g per day growth only small effects on chick survival were observed. Effects of slow growth on survival became obvious after about 10 days after hatching. Slow growing chicks reached a much lower fledging mass, whereas slow growth had only small effects on structural size at fledging. Body condition of the chicks was highly variable and had strong effects on survival until fledging. However, body condition during the nestling stage did not influence post-fledging survival. Body condition at fledging had no effects on post-fledging survival and did not affect final mass or body size. It is argued that low fledging mass can be overcome soon after fledging, as parents take their fledglings closer to the foraging areas, thereby avoiding high rates of kleptoparasitism by Black-headed Gulls Larus ridibundus .     

Rainfall during parental care reduces reproductive and survival components of fitness in a passerine bird

Adverse weather conditions during parental care may have direct consequences for offspring production, but longer‐term effects on juvenile and parental survival are less well known. We used long‐term data on reproductive output, recruitment, and parental survival in northern wheatears (Oenanthe oenanthe) to investigate the effects of rainfall during parental care on fledging success, recruitment success (juvenile survival), and parental survival, and how these effects related to nestling age, breeding time, habitat quality, and parental nest visitation rates. While accounting for effects of temperature, fledging success was negatively related to rainfall (days > 10 mm) in the second half of the nestling period, with the magnitude of this effect being greater for breeding attempts early in the season. Recruitment success was, however, more sensitive to the number of rain days in the first half of the nestling period. Rainfall effects on parental survival differed between the sexes; males were more sensitive to rain during the nestling period than females. We demonstrate a probable mechanism driving the rainfall effects on reproductive output: Parental nest visitation rates decline with increasing amounts of daily rainfall, with this effect becoming stronger after consecutive rain days. Our study shows that rain during the nestling stage not only relates to fledging success but also has longer‐term effects on recruitment and subsequent parental survival. Thus, if we want to understand or predict population responses to future climate change, we need to consider the potential impacts of changing rainfall patterns in addition to temperature, and how these will affect target species' vital rates.     

Survival to independence in relation to pre‐fledging development and latitude in songbirds across the globe

Species differ strongly in their life histories, including the probability of survival. Annual adult survival was investigated extensively in the past, whereas juvenile survival, and especially survival to independence, received much less attention. Yet, they are critical for our understanding of population demography and life‐history evolution. We investigated post‐fledging survival to independence (i.e. survival upon leaving the nest until nutritional independence) in 74 species of passerine birds worldwide based on 100 population level estimates extracted from published literature. Our comparative analyses revealed that survival to independence increased with the length of nestling period and relative fledging mass (ratio of fledging mass to adult body mass). At the same time, species with higher nest predation rates had shorter nestling periods and lower relative fledging mass. Thus, we identify an important trade‐off in life history strategies: staying longer in the nest may improve post‐fledging survival due to enhanced flight ability and sensory functions, but at the cost of a longer exposure to nest predators and increased mortality due to nest predation. Additionally, post‐fledging survival to independence did not differ between species from the northern temperate zone vs species from the tropics and southern hemisphere. However, analyses of post‐fledging survival curves suggest that 1) daily survival rates are not constant and improve quickly upon leaving the nest, and 2) species in the tropics and southern hemisphere have higher daily post‐fledging survival rates than northern temperate species. Nevertheless, due to the accumulation of mortality risk during their much longer periods of post‐fledging care, overall survival until independence is comparable across latitudes. Obtaining high‐quality demographic data across latitudes to evaluate the generality of these findings and mechanisms underlying them should be a research priority.     

Cooperative breeding shapes post‐fledging survival in an Afrotropical forest bird

For avian group living to be evolutionary stable, multiple fitness benefits are expected. Yet, the difficulty of tracking fledglings, and thus estimating their survival rates, limits our knowledge on how such benefits may manifest postfledging. We radio‐tagged breeding females of the Afrotropical cooperatively breeding Placid greenbul (Phyllastrephus placidus) during nesting. Tracking these females after fledging permitted us to locate juvenile birds, their parents, and any helpers present and to build individual fledgling resighting datasets without incurring mortality costs or causing premature fledging due to handling or transmitter effects. A Bayesian framework was used to infer age‐specific mortality rates in relation to group size, fledging date, maternal condition, and nestling condition. Postfledging survival was positively related to group size, with fledglings raised in groups with four helpers showing nearly 30% higher survival until independence compared with pair‐only offspring, independent of fledging date, maternal condition or nestling condition. Our results demonstrate the importance of studying the early dependency period just after fledging when assessing presumed benefits of cooperative breeding. While studying small, mobile organisms after they leave the nest remains highly challenging, we argue that the telemetric approach proposed here may be a broadly applicable method to obtain unbiased estimates of postfledging survival.     

Experimental addition of greenery reduces flea loads in nests of a non-greenery using species, the tree swallow Tachycineta bicolor

Several bird species, including cavity-nesters such as European starlings Sturnus vulgaris , add to their nests green sprigs of plants such as yarrow Achillea millefolium that are rich in volatile compounds. In this field study on another cavity-nester, tree swallows Tachycineta bicolor , we tested whether yarrow reduced ectoparasite loads (the nest protection hypothesis), stimulated nestling immune systems (the drug hypothesis), or had other consequences for nestling growth or parental reproductive success (predicted by both preceding hypotheses). Tree swallows do not naturally add greenery to their nests, and thus offer several advantages in testing for effects of greenery independent of other potentially confounding explanations for the behaviour. We placed fresh yarrow in 23 swallow nests on the day the first egg was laid, replenishing every two days until clutch completion (=three times), and at 44 control nests, nesting material was simply touched. At 12 days of age, we measured nestling body size and mass, and took blood smears to do differential white blood cell counts. We subsequently determined the number and proportion of young fledging from nests and the number of fleas remaining after fledging. Higher humidity was associated with higher flea numbers whereas number of feathers in the nest was not. Our most significant finding was that an average of 773 fleas Ceratophyllus idius was found in control nests, versus 419 in yarrow nests. Possibly, parents compensate for blood that nestlings lose to ectoparasites by increasing food delivery, because we detected no differences between treatments in nestling mass, nestling leukocyte profiles, or proportion of young fledging, or relative to flea numbers. Our results provide no support for the drug hypothesis and strong support for the nest protection hypothesis.     

Fitness consequences of timing of breeding in birds: date effects in the course of a reproductive episode

In seasonal environments, avian reproductive performance almost generally declines in the course of the season. Quantifying the associated fitness consequences of timing of breeding, i.e. of date‐related factors, is important for understanding the evolution of temporal patterns in avian life‐histories and for predicting consequences of climate change. The seasonal decline can also be caused by an effect of parental quality: individuals with high phenotypic quality may breed early. The results of existing experimental studies investigating whether date or quality effects cause the seasonal decline are inconsistent, indicating that both mechanisms might be involved. However, it remains unclear to what extent the confounding effect of quality occurs and what the fitness consequences of timing per se over a whole breeding episode are. In a cross‐fostering experiment using the barn swallows’ second broods we evaluated the causes for the seasonal decline in reproductive performance for three distinct periods of a reproductive attempt, the early nestling period, the late nestling period and the post‐fledging period, and we assessed the overall fitness consequences of timing per se. A seasonal decline in juvenile feather growth rate was mainly due to date effects in the late nestling period, although we determined quality effects during early nestling development. Date effects on survival were present in the post‐fledging period, but not in the nestling period. The decline in feather length due to date effects in the nestling period accounted for 9% of the seasonal decline in post‐fledging survival, whereas date effects arising only in the post‐fledging period caused 91% of the decline. These results suggest that date effects increase in the course of a reproductive episode. Thus, the benefits of an early timing of breeding can be quantified only when considering also the post‐fledging period. We suggest that the timing of breeding evolved through a trade‐off between date‐related benefits and quality‐related costs of early breeding.     

The Process and Causes of Fledging in a Cavity-Nesting Passerine Bird, the House Wren (Troglodytes aedon)

Little is known about the process or causes of fledging or nest‐leaving in passerine birds because researchers can rarely predict when fledging will occur in a given nest. We used continuous videotaping of nests to both document the process of fledging in the house wren, Troglodytes aedon, a small, cavity‐nesting songbird, and test hypotheses as to what might cause fledging to begin. Fledging began any time from 14 to 19 d after hatching commenced. Slower‐developing broods fledged later than faster‐developing broods. Fledging typically began within 5 h of sunrise and over 80% of all nestlings fledged before noon. All nestlings fledged on the same day at 65% of nests and over two consecutive days in most other nests. We found no evidence that fledging was triggered by changes in parental behaviour. Parental rate of food delivery to nestlings did not decline during a 3‐h period leading up to the first fledging, nor was the rate of feeding just prior to the first fledging lower than the rate at the same time the day before. Moreover, parents did not slow the rate of food delivery to nests after part of the brood had fledged. Hatching is asynchronous in our study population which creates a marked age/size hierarchy within broods. At most nests, the first nestling to fledge was the most well‐developed nestling in the brood or nearly so (as measured by feather length). This suggests that fledging typically begins when the most well‐developed nestlings in the brood reach some threshold size. However, at about one‐fifth of nests, the first nestling to fledge was only moderate in size. At these nests, severe competition for food may have caused smaller, less competitive nestlings to fledge first to increase their access to food. We found no strong support for the suggestion that the oldest nestlings delay fledging until their least‐developed nestmate reaches some minimum size, although further experimental work on this question is warranted.     

窝雏数处理对两种雀形目幼鸟生长的影响

基于 1 997～ 1 999年野外实验 ,对高寒草甸小云雀和黄嘴朱顶雀两种雀形目鸟的窝雏数进行增减处理。结果表明 ,对照组的幼鸟生长率和离巢体重都大于增加组 ,说明窝雏数增加后 ,幼鸟质量下降。随着窝雏数增加 ,这两种幼鸟生长率显著下降 (小云雀 :r =-0 965 ,P =0 0 3 5 <0 0 5 ;朱顶雀 :r =-0 82 8,P =0 0 2 2 <0 0 5 )。窝雏数改变对小云雀幼鸟出飞重影响不显著 (r =-0 41 8,P =0 5 2 8>0 0 5 ) ,而对黄嘴朱顶雀有显著的影响 (r=-0 90 1 ,P =0 0 1 4<0 0 5 )。     

Multiple responses to increasing spring temperatures in the breeding cycle of blue and great tits (Cyanistes caeruleus,Parus major)

Many bird species start laying their eggs earlier in response to increasing spring temperatures, but the causes of variation between and within species have not been fully explained. Moreover, synchronization of the nestling period with the food supply not only depends on first‐egg dates but also on additional reproductive parameters including laying interruptions, incubation time and nestling growth rate. We studied the breeding cycle of two sympatric and closely related species, the blue tit Cyanistes caeruleus and the great tit Parus major in a rich oak‐beech forest, and found that both advanced their mean first‐egg dates by 11–12 days over the last three decades. In addition, the time from first egg to fledging has shortened by 2–3 days, through a decrease in laying interruptions, incubation time (not statistically significant) and nestling development time. This decrease is correlated with a gradual increase of temperatures during laying, suggesting a major effect of the reduction in laying interruptions. In both species, the occurrence of second clutches has strongly decreased over time. As a consequence, the average time of fledging (all broods combined) has advanced by 15.4 and 18.6 days for blue and great tits, respectively, and variance in fledging dates has decreased by 70–75%. Indirect estimates of the food peak suggest that both species have maintained synchronization with the food supply. We found consistent selection for large clutch size, early laying and short nest time (laying to fledging), but no consistent changes in selection over time. Analyses of within‐individual variation show that most of the change can be explained by individual plasticity in laying date, fledging date and nest time. This study highlights the importance of studying all components of the reproductive cycle, including second clutches, in order to assess how natural populations respond to climate change.     

Optimal brood size and its limiting factors in the Rufous Turtle DoveStreptopelia orientalis

Optimal brood size and its limiting factors of the Rufous Turtle Dove,Streptopelia orientalis, were studied at the campus of the University of Tsukuba, Japan, during the breeding season in 1990–92. The dove usually lays two eggs in a nest. I made nests of a brood size of one and three by transferring a hatchling from one nest to the other, and compared their fledging success, factors of breeding failure, weight and tarsus length at fledging, growth rate and nestling period with those of a brood of two. The index of fitness (fledgling weight multiplied by average number of fledglings per nest) was almost the same in broods of two and three. However, the highest variation in fledging weight within the brood and the extension of nestling period were observed in broods of three, which caused the extension of inter-brood interval and consequently the smaller number of broods in the total breeding season. Therefore, broods of three would not have an advantage in producing more offspring than broods of two. Crop milk production had an effect on the growth of nestlings in the early phase of the nestling period, but the rapid growth in the granivorous phase compensated for the growth delay of the smallest nestling in broods of three. Small brood size and a large number of broods in a season would also be more effective under high predation pressure.