Young Nestlings Research Articles

Methylmercury accumulation in tissues and its effects on growth and appetite in captive great egrets.

To test the hypothesis that fledging wading birds would be more at risk from mercury toxicosis than younger nestlings, captive great egret nestlings were maintained as controls or were dosed from 1- to 14-wk-old with 0.5 or 5 mg methylmercury chloride/kg wet weight in fish. Birds dosed with 5 mg/kg suffered from subacute toxicosis at wk 10-12. Growing feather concentrations were the most closely correlated with cumulative mercury consumed per weight. Blood concentrations of mercury increased more rapidly after 9 wk in all groups when feathers stopped growing. Total mercury accumulated in tissues in concentrations in the following order: growing scapular feathers > powderdown > mature scapular feathers > liver > kidney > blood > muscle > pancreas > brain > bile > fat > eye. The proportion of total mercury that was methylated depended upon tissue type and dose group. Selenium accumulated in liver in direct proportion to liver mercury concentrations. After wk 9, appetite and weight index (weight/bill length) declined significantly in both dosed groups. At current exposure levels in the Everglades (Florida, USA) mercury deposited in rapidly growing feathers may protect nestlings from adverse effects on growth until feathers cease growing.

Cholinesterase inhibition in tree swallows (Tachycineta bicolor) and eastern bluebirds (Sialia sialis) exposed to organophosphorus insecticides in apple orchards in Ontario, Canada

Apple orchards occupy approximately 12,565 ha of land in Ontario, Canada, and are treated with the greatest number and applications of pesticides among all agricultural crops within the province. The potential for exposure to wildlife from frequent organophosphorus (OP) spray applications is therefore considerable. This study investigated the impact of agricultural spray ap- plications of azinphos-methyl, diazinon, phosalone, and phosmet in apple orchards on the cholinesterase (ChE) activity of wild birds nesting in orchards. These four OP pesticides ranked among the most widely used insecticides used on all fruit crops in Ontario in 1988. Tree swallows ( Tachycineta bicolor) and eastern bluebirds (Sialia sialis) were chosen as monitor species because they commonly breed and forage within apple orchards, utilize different feeding strategies, and are readily attracted to nest boxes. Brain and plasma ChE activities were determined for nestling bluebirds and swallows, and plasma ChE was measured in adult swallows for comparison with nestling data. In adult tree swallows, mean plasma ChE levels were significantly inhibited 41% compared to controls 12 h after a second application of azinphos-methyl. Mean plasma ChE activity in another group of adult tree swallows dropped by 21 and 19% following a multiple phosmet application. In nestlings, brain ChE activities obtained postspray often fell below predicted activities calculated from control siblings. This trend was especially apparent in the younger nestlings, less than 6 d old. However, for bluebirds the rates of increase of brain ChE with age in nestlings from treated sites was significantly lower than in nestlings from control sites. Brain ChE activity of two nestlings found dead following a phosalone application gave no indication that insecticide exposure was responsible. No other dead or moribund birds were discovered during the course of the study. Sequential plasma sampling of siblings, before and after OP application, was beneficial in reducing the required number of nests sampled by one half, but plasma ChE was less reliable overall than brain ChE in indicating exposure. Results of depressed ChE levels in tree swallows and eastern bluebirds inhabiting apple orchards are consistent with avian species in other orchard monitoring studies. No indication was found that OP exposure due to agricultural spraying in apple orchards adversely affected the survival of the birds we monitored.

Red-winged blackbird females fail to increase feeding in response to begging call playbacks

The begging behaviour of nestling birds appears to be the main signal telling parents how much food is needed by the brood. Not only may the accuracy of that information be reduced through exaggeration due to competition, but it may also vary with the age of the nestling. Younger nestlings may vary more in how frequently they call than older nestlings because they cannot discriminate stimuli as well. Thus, even if ‘honest’, begging from younger nestlings could be a less reliable hunger signal than that from older nestlings. We tested the prediction that parents should respond less to the increased frequency of begging from very young versus older nestlings by playing back begging calls at the nests of both younger (≤5 day) and older (≥6 days) red-winged blackbirds,Agelaius phoeniceus. In neither case did parents respond with consistent increases in feeding, nor did young gain more weight during playbacks. This lack of response is discussed in relation to two other recent studies of blackbirds that did demonstrate increased feeding with playbacks of begging calls.

The dynamics of parental care in Choughs(Pyrrhocorax pyrrhocorax)

The dynamics of parental investment throughout the nestling stage and the factors affecting it were studied in the Chough(Pyrrhocorax pyrrhocorax), a species whose patterns of apportioning parental care are largely unknown. The occurrence of important trade-offs between the sexes, among the different activities of parental care and between parents' survival and current offspring survival were estimated. The parental contributions of both sexes were assessed mainly in terms of food provisioning rate and nest attendance time. Only the female brooded young nestlings while the two sexes contributed equally in food deliveries and nest sanitation. Nestling age greatly affected nest attendance time. The female spent a long time brooding in the first 10 days after hatching. Both sexes increased attendance towards the end of the nestling stage. Conversely, feeding rate and feeding rate per nestling remained approximately constant throughout the nestling period. Nestlings in smaller broods received more feeding visits than those in larger broods. The shape of the per-nestling feeding rate curve was ‚concave-up’, supporting Nur's (1984) ‚trade-offs model’ rather than the Lack-Gibb hypothesis. Maintaining a high feeding frequency in broods already above the modal value might be disadvantageous, implying few benefits and large energy costs (i.e. the reduction of the parents' residual reproductive value). Female brooding time in relation to brood size showed the same decreasing ‚concave-up’ trend line. Short-term trade-offs proved to be important determinants of the dynamics of parental care. Specifically, the distance from the feeding areas greatly affected the delivery rate: pairs spent a disproportionately longer time foraging in more distant patches than in closer ones. Diurnal variations and changes owing to weather conditions were also examined.

The energetic cost of begging behaviour in nestling house wrens

This study presents data relevant to the hypothesis that the energy expenditure associated with begging influences the signalling of need by nestling birds. We used open-circuit respirometry to measure the energy costs of resting, begging and non-begging activities in nestling house wrens,Troglodytes aedon, ranging in age from 1 to 11 days post-hatching. Across all ages, begging caused a 27% increase in metabolism above resting rates. The metabolic rate during begging was not related to begging vigour. However, more vigorous begs were longer and so required a greater total energy expenditure. We analysed videotapes of broods for nestling behavioural time budgets which were combined with the metabolic data and data on growth at different ages to generate daily energy budgets. Over a 24-h period, the cumulative energy allocated to begging was slight, ranging from 0.02% of the energy budget in younger nestlings (3 days old) to 0.22% in older ones (10 days old). In contrast, non-begging movements accounted for 2 and 9% of the daily energy budget of younger and older nestlings, respectively. Relative to daily growth, the energy allocated to begging was equivalent to 0.05% (younger nestlings) and 2.3% (older nestlings) of the energy sequestered in new tissue, whereas the values for non-begging activities were 5.1 and 96.8%, respectively. These results suggest that the energetic cost of begging is not likely to have a substantial role in influencing communication of need by nestlings.

Morphology of the feeding apparatus in nestlings of Merops

The morphological comparison of the facial skeleton and tongue between young nestlings and subadults has shown that Merops desmognatism is due to the ossification of the processus prenasalis and solum nasi. The maxillo‐palatines are not fused along the midline and the pterigoid is fused to the vomer. The heterochrony in the development of the skull of Merops was suggested to be due to protracted parental care so that only when the bird captures its prey, do the stresses produced in the killing actions induce ossification of the solum nasi and of the processus prenasalis.

Is there an energetic cost to begging in nestling tree swallows (Tachycineta bicolor) ?

Most honest signalling theory predicts that signals must be costly to prevent the signaller from conveying false information to the receiver. However, there have been few direct measures of the costs associated with signalling. We measured oxygen consumption during begging in 5- and 10-day-old tree swallow ( Tachycineta bicolor ) nestlings to determine if there was an energetic cost associated with this signal, and if so, whether it increased with increasing begging intensity. Our results indicated that both 5- and 10-day-old nestlings used significantly more oxygen when begging than when resting. Although both the active metabolic rate and the resting metabolic rate were significantly higher in the older nestlings than in the younger nestlings, the scope (ratio of active metabolic rate to resting metabolic rate) of begging was the same in both age groups. Our results indicated that in general begging required 28% more energy than resting in tree swallow nestlings. Energy expenditure was not related to begging intensity in 5-day-old nestlings; however, these values were significantly related in the older nestlings. Although the energetic cost of begging seems relatively low, it may be important when the total energy budget of the nestlings is considered.

The Insurance-Egg Hypothesis and Extra Reproductive Value of Last-Laid Eggs in Clutches of American Kestrels

Many species of birds lay more eggs than they normally rear to independence. This is illustrated well by species with obligate siblicide, such as eagles (Meyburg 1974) and boobies (Anderson 1990), where the youngest nestling in a brood is killed by its older sibling in a physical struggle soon after hatching. A common explanation for such behavior is that parents can raise only one offspring, but lay the extra egg for insurance against infertility (the insurance-egg hypothesis; Dorward 1962); the surplus offspring is eliminated if its older sibling is viable. Field experiments testing the insurance-egg hypothesis have been performed for a few species with obligate siblicide (e.g. Cash and Evans 1986, Anderson 1990). Until recently, the insurance value of offspring was not considered to be important for birds with facultative siblicide (i.e. where survival of youngest offspring is conditional on prevailing environmental conditions). Such offspring usually were thought to be an adaptation to an environment where food was unpredictable at the time of laying. Parents could rear the entire brood if food proved to be abundant, but if food was scarce, they could eliminate the extra offspring through brood reduction (Lack 1947, Ricklefs 1965; called by Forbes 1991). Mock and Parker (1986) emphasized that the youngest nestling in a brood may serve two simultaneous functions in species with facultative brood reduction: resource tracking and insurance. The costs and benefits of insurance offspring have been modelled in a general way by Forbes (1990, 1991); extra offspring may provide multiple benefits simultaneously (Mock and Forbes 1995). Here, I examine the value of the lastlaid egg in broods of American Kestrels (Falco sparverius), a species with facultative brood reduction. Methods.-From 1988 to 1992, coworkers and I studied a population of about 200 pairs of American Kestrels breeding in nest boxes at Besnard Lake, Saskatchewan (55?20'N, 106?W). Details about the study site and nest boxes are summarized in Bortolotti et al. (1991) and Bortolotti (1994). Each year, adults in the population were trapped, banded, and measured (see Wiebe and Bortolotti 1993); by visiting boxes regularly, we recorded the reproductive performance of most parents. In many nests, eggs were numbered according to laying sequence with nontoxic felt markers, and nestlings were similarly marked according to hatching order (Wiebe and Bortolotti 1995a, b). The

Brood Reduction and Siblicide in Black-Billed Magpies (Pica pica)

In many avian species, brood reduction is considered to be adaptive and may be attributed either to sibling competition (passive starvation, active sibling aggression) or parental effects (manipulation of hatching spread, active infanticide). However, nonadaptive factors such as environmental effects, may contribute substantially to nestling mortality. I determined mechanisms of brood reduction and survival probabilities of nestling Black-billed Magpies (Pica pica) in relation to intraclutch egg-size variation, brood size, nestling age and size, and weather. Median brood survival time was weakly, but positively, correlated with intraclutch egg-mass variation. Starvation accounted for most nestling mortality within the first 9 days posthatching, although 12 dead and moribund nestlings were found badly bruised around the head. Siblicide and sibling cannibalism were observed in two broods and impli- cated in the deaths of nestlings in three other broods. Siblicidal events occurred when nestlings were between 15 and 20 days old. Expected median survival times and probability of survival to fledging of nestlings was not linearly related to brood size at hatching, being highest for broods of five, lowest for broods of three, and intermediate for broods of four and six. Dead nestlings were smaller and lighter than surviving siblings at any age; however, asymptotic body mass and linear measurements were negatively correlated with brood size at fledging. The probability of mortality was affected by prevailing weather conditions; rain and low temperatures doubled the estimated risk of death for young nestlings, independent of brood size. These data suggest that factors influencing the occurrence and maintenance of brood-reduction strategies in a population may be more complex than previously thought. Brood-reduction mechanisms are affected by the interaction of biotic and abiotic factors, and may vary in response to factors outside of either parental or offspring control. Received 9 September 1994, accepted 25 April 1995.

Gapes of Sexually Dimorphic Blackbird Nestlings Do Not Show Sexually Dimorphic Growth

Within broods of many altricial birds, one or more nestlings is often predictably smaller than the rest. This occurs both because of hatching asynchrony within broods and also because one sex rapidly becomes larger than the other in sexually dimorphic species. If parents rely on competitive, size-based cues from nestlings in deciding which to feed, this size asymmetry presents a major predictable problem to younger or smaller nestlings. A recent study of sexually dimorphic Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) suggested that the gape, a prominent feature of the begging signal of hungry young, may grow more rapidly in female nestlings than males. In this paper, I report similar data on nestling gape growth in another sexually dimorphic species, the Red-winged Blackbird (Age- laius phoeniceus), and show that the apparent relatively faster gape growth of females in both Yellow-headed and Red-winged blackbirds is an artifact of the analytical method used in the comparison. Gapes of males and females grow in exactly the same relation to their mass and, to the extent that the mass of females is less at a given age, females will have smaller gapes. I further analyze gape growth in a group of undernourished nestling Red-winged Blackbirds to evaluate the related prediction that, when resources for growth are limited, nestlings should conserve the growth of those morphological characteristics such as gapes that directly influence feeding. The data yield little support for this hypothesis. The specialized and prominent nestling gapes, however, may have important functions beyond feeding com-

Food-dependent benefits of hatching asynchrony in American kestrels Falco sparverius

Food supply and hatching asynchrony were manipulated for 90 broods of American kestrels (Falco sparverius) during 1989–1991. We measured the growth and mortality of nestlings within four treatment groups (asynchronous, synchronous, food-supplemented, unsupplemented) to test the brood reduction hypothesis of Lack (1947, 1954). Fledging success did not differ between synchronous and asynchronous broods when food was poor but consistent with the brood reduction hypothesis, nestlings died at a younger age in asynchronous broods. When food was supplemented, mortality did not occur in the synchronous broods but youngest nestlings still died in asynchronous nests despite apparently adequate food for the brood. Oldest nestlings in asynchronous broods fledged with a greater mass than their younger siblings, also consistent with Lack's hypothesis. Average nestling quality in synchronous broods was very dependent on food levels. Synchronous young that were supplemented were, on average, the heaviest of any treatment group but young from unsupplemented synchronous broods were the lightest. Overall, patterns of mortality and growth for kestrels support the brood reduction hypothesis when food is limited, but not when it is abundant. This food-dependent benefit of asynchrony in the nestling period is a prerequisite for facultatively adjusted hatching spans during laying.

Natural and Experimental Infections of Eustrongylides ignotus: Effect on Growth and Survival of Nestling Wading Birds

-One of three Tricolored Heron (Egretta tricolor) nestlings experimentally infected with Eustrongylides ignotus larvae died two days postinfection. During eight days postinfection, the other infected birds consumed less food per meal, consumed less food in relation to body mass, regurgitated more frequently, and exhibited lower bill and mass growth rates than did three uninfected control birds. Prevalence of eustrongylidosis among free-ranging Great Egret (Casmerodius albus) nestlings from Lake Okeechobee ranged from less than 5% in 1989 and 1991 to 28.4% in 1990. During 1990, infected nestlings suffered 10.6 to 17.9% higher mortality than uninfected birds, with younger nestlings suffering proportionately greater mortality. Summarized across colonies and nestling ranks (i.e. hatching order), infected nestlings averaged 2.9% shorter bills, 4.1% shorter tarsi, and 6.9% shorter wings than noninfected birds. Among rank 1 (oldest) and 3 (youngest) nestlings, infected birds were lighter (6.3-9.0%), but among rank 2 nestlings from both colonies infected birds averaged heavier than their noninfected counterparts. These results for mass may be due to the interactive effects of sibling rivalry. Eustrongylidosis generally had the greatest impact on those nestlings otherwise disposed to lower growth rates, whether due to colony location or nestling rank. The results emphasize the importance of monitoring diseases whenever prefledging growth and survival are of interest. Received 5 May 1993, accepted 17 November 1993. EUSTRONGYLIDES IGNOTUS INFECTIONS in nestling ciconiiforms, especially those of the family Ardeidae, appear to be associated with anthropogenic habitat alteration (Spalding et al. 1993), and can have a devastating effect upon reproductive success (Weise et al. 1977, Roffe 1988, Spalding et al. 1993, Spalding and Forrester 1993). In this study, we examined the effect of this nematode parasite, which burrows through the stomach wall, on the growth and survival of nestling ardeids in the laboratory and in the field. We experimentally infected Tricolored Heron (Egretta tricolor) nestlings to gather details about the effects of parasitism on young nestlings. We also monitored nestlings at Lake Okeechobee, Florida from 1989 to 1991 to gain information about prevalence and to measure the effects that eustrongylidosis might have on the growth and survival of nestlings in the wild. We provide evidence concerning both the mechanisms and magnitude of the effect of eustrongylidosis on ardeid nestlings and show that the impact on growth and survival results can be significant even during periods of only moderate prevalence. METHODS Experimental infections. -Six Tricolored Heron nestlings were collected from Frank Key, Florida Bay (25?06.3'N, 80?54.7'W) on 21 May 1990. Each was the intermediate-sized nestling of three siblings and was estimated to be between two to four days of age based on bill length. Nestlings were marked and placed in individual containers within an incubator. They were fed ad libitum four times each day with thawed fish or with fresh fish that had been examined and found to be free of eustrongylid larvae. Nestlings could take fish directly from a dish of shallow water or, if necessary, were offered fish held with forceps. They were never force fed. On the evening of the second day, three randomly selected birds (no. 2, 3, and 6) were each given four eustrongylid larvae per os prior to feeding. An average of 2.7 larvae per bird (range 1-10) have been found in very young naturally exposed birds (Spalding and Forrester 1993). Larvae were obtained from naturally infected mosquitofish (Gambusia holbrooki) collected in a canal in Gainesville, Alachua County, Florida, and maintained alive in the laboratory. Larvae were extracted from fish, placed in a gelatin capsule, and fed to nestlings immediately. Uninfected control birds (no. 1, 4, and 5) were given empty capsules. The nest container was examined

Toxicity assays of ladybirds using natural predators

Nestling blue titsParus caeruleus L. were given diets containing homogenized ladybirds, to assess the effects of their chemical defences. The 2spot, 10spot and water ladybirds produced no apparent toxic effects when small numbers were given at regular intervals. The pine and kidney-spot ladybirds slightly inhibit growth and may be toxic to very young nestlings. The Water ladybird is extremely distasteful to this predator, despite its lack of toxicity. The results are discussed with reference to the proposed Mullerian and Batesian mimetic relationships between the ladybird species.

Pathogenicity of larval Protocalliphora (Diptera: Calliphoridae) parasitizing nestling birds

Controlled tests showed that feeding by larvae of several species of Protocalliphora lowered haematocrit and haemoglobin levels of nestling magpies and bank swallows. Subsequent studies showed that natural populations of nestling magpies and Bank Swallows infested by P. asiovora and P. chrysorrhoea, respectively, experienced significantly reduced blood levels at the P0.05 and P0.01 levels except in the youngest nestlings. Despite the presumed heavy blood loss, most nestlings are not killed by larvae of Protocalliphora. Studies of natural populations of Protocalliphora in 48 species of birds indicate that the larval populations are usually too small to kill or seriously injure most nestlings.

Flexible Growth Rates in Nestling White-Fronted Bee-Eaters: A Possible Adaptation to Short-Term Food Shortage

Nestling White-fronted Bee-eaters (Merops bullockoides) in Kenya show an ability to slow their development rate in response to food stress and, more importantly, to recover from stress with resumption of normal development. Successful fledging occurred at ages ranging from 22-42 days. The magnitude of developmental slowing was greatest: (1) among the youngest nestlings in large broods, (2) in seasons of overall low food availability, and (3) in nests provisioned by breeding pairs alone (i.e., nests without helpers). We speculate that this ability to slow growth reduces a nestling's daily energy requirements and thereby increases its likelihood of surviving through short periods of severe food shortage.

Breeding Ecology of the Western Long-billed Corella, Cacatua pastinator pastinator

The breeding ecology of a small population of the western long-billed corella, Cacatua p. pastinator, was investigated for 6 years in the central wheatbelt of Western Australia. Pair-bonds tended to be long-lasting; however, the divorce rate for birds breeding for the first time was 25% with an overall rate of 15%. Females formed stable pair-bonds by age 2, and bred for the first time between 3 and 5 years old; males started breeding when 5 years old. Clutches (range from one to four, mode three) were started in early August; 78% are started in the last 3 weeks of the month. Mean clutch size, brood size and number fledged were 2.7 , 1.9 and 1.6 respectively; there were no significant differences in these parameters among the six years of the study. Hatching was asynchronous and was followed by the death of the young nestlings when they were significantly smaller (77%) than their older siblings. Nestlings fledged when they were about 60 days old; neither their mass or degree of wing growth influenced fledging age. Mean mass of fledglings from broods of three was significantly greater than that of broods of two or one. Productivity was assessed by the numbers of young fledging, surviving the post-fledging period, and reaching independence. Neither mass nor condition at fledging influenced any of these measures; productivity in all three increased with clutch size. Rainfall and temperature had no influence on the numbers fledged, but the number reaching independence was significantly and positively correlated with the mean maximum summer temperature. The results are discussed in relation to the results from studies of other species of cockatoos living in the wheatbelt of Western Australia.

Effects of House Sparrow Age and Arbovirus Infection on Attraction of Mosquitoes

To test the hypothesis that arbovirus infection or advancing age increases the attractiveness of avian hosts to vector mosquitoes, we used an olfactometer that measures the response of mosquitoes to the passive, vertical diffusion of host-related cues. St. Louis encephalitis or western equine encephalomyelitis virus infection in house sparrows (Passer domesticus (L.], regardless of age, had no detectable effect on Culex quinquefasciatus Say or Cx. tarsalis Coquillett attraction, respectively, at close range. Sparrow age, however, was associated positively with increased Cx. quinquefasciatus attraction; i.e., nestlings were significantly less attractive than adults, and there was a trend for increased mosquito attraction as nestling age increased. When birds were examined in groups, attraction was not equal to the sum of the individual birds in the group. Nestlings, either alone or in a group, were less attractive than a single adult. One adult plus three older nestlings were more attractive than one adult with three young nestlings, but no more attractive than one adult alone. We speculate on the epidemiological significance of these laboratory results to arbovirus epizootic transmission.

Development of Early Vocalizations and the Chick‐a‐dee Call in the Black‐capped Chickadee, Parus atricapillus

AbstractThe chick‐a‐dee call of the black‐capped chickadee (Parus atricapillus) is composed of discrete elements, or notes, that are combined to form hundreds of different calls. To investigate the development of this complex call, 12 families of color‐marked chickadees were observed and recorded in the wild. Vocalizations were monitored for 18 d in the nest and 14–18 d postfledging. Most vocalizations of nestlings and fledglings were associated with feeding. At hatching, vocalizations consisted of a structurally simple note type that became more complex and variable with age. Around 9–12 d, the development of the call occurred, when single notes became organized into a multiple‐note unit. Notes within the call differentiated into higher frequency, rapidly modulated initial note types and a lower frequency, moderately modulated terminal note type, features also present in adult chick‐a‐dee calls. Several adult‐like calls including chick‐a‐dee calls, fee‐bee songs, and a subsong‐like vocalization developed prior to fledgling dispersal. Based on resemblances of note structure and general call structure, the chick‐a‐dee call appeared to develop from calls of nestlings and fledglings, although not necessarily in a chronologically linear progression. Some features of the chick‐a‐dee call closely resembled features of older nestling and fledgling calls, while other features more closely resembled the sounds of very young nestlings. Vocal development in the chickadee is compared with song and call development in other species, and the possible significance of acoustic resemblances between chick‐a‐dee calls and the food‐associated calls of nestlings and fledglings is discussed.

Juvenile Foraging Proficiency, Parental Effort, and Avian Reproductive Success

We measured the allocation of time and energy in a population of adult Yellow—eyed Juncos (Junco phaeonotus) and their young (nestlings, fledglings, independent juveniles) throughout the breeding season using concurrent time—activity budgets and doubly labeled water. We constructed energy budgets by extrapolating laboratory measurements of metabolic heat production to field conditions using a linear heat—transfer model and the operative temperature and wind speed experienced by the free—living bird. From our data we calculated daily energy expenditure (HD), the proportion of HD allocated to physical activity vs. maintenance metabolism (basal + thermostatic costs), and foraging efficiency. We examined diet selectivity among parents and their young, and we calculated prey capture rates based on the measured energy content of insect prey. We found that adult juncos feeding young are neither food limited nor working maximally. Adults fulfilled their own energy demands, and those of their four dependent young, while foraging for 75% or less of the daylight hours. In contrast, recently independent young, 4—7 wk after fledging, must forage for >90% of the daylight hours to meet their own energy demands. Juncos younger than this cannot attain energy balance in a 15—h day without supplemental feeding by their parents, even if they forage continuously. The HD of adult juncos (mean mass 19.5 g) remained fairly constant throughout the breeding season, averaging 73.8 kJ/d (n=51), which is 2.1 times their measured nighttime basal metabolic rate. In contrast, HD of fledging juncos increased steadily with age, from 59.7 kJ/d during the 1 st wk out of the nest to 73.8 kJ/d during week 3. This increase resulted partly from an increase in the proportion of time spent foraging. Once their parents quit supplying them with food, the young juncos' HD increased dramatically, peaking at 100 kJ/d in 10—12 wk—old juveniles. The increase in HD of independent young resulted initially from a dramatic increase in time spent foraging, which gradually declined only to be supplanted by an increase in aggressive interactions, especially flights within the juvenile flocks, and a marked increase in energy demand associated with the postjuvenile molt. For adult and young juncos, thermostatic costs represented respectively 20 and 23% of total daily energy expenditure, an amount of energy approaching that devoted to all physical activities. Thermostatic costs would have been even greater than this except that juncos are much better insulated than typical 19.5—g passerine birds (their coefficient of heat transfer is only 76% of that predicted from their mass). For these small birds, adaptations that affect body insulation can have more effect on daily energy budgets than changes in the time allocated to various physical activities. Our data reveal that energy constraints are a major selective force in Yellow—eyed Juncos, operating not through food limitation among adults but rather through the inefficient foraging of young juncos. The critical stage when selection operates most strongly on the efficient use of time and energy occurs during the first 3 mo of a junco's life. The lack of proficient foraging by young juncos explains observed patterns of parental time and energy expenditure and breeding season phenology.

Breeding of the Peregrine FalconFalco peregrinus: III. Weather, Nest Quality and Breeding Success

SummaryOlsen, P.D. & Olsen, J. (1989). Breeding of the Peregrine Falcon Falco peregrinus: III. Weather, nest quality and breeding success. Emu 89, 6–14.Peregine Falcons laid clutches at 75% of territories annually and fledged young from 58%. Sixty-seven per cent of pairs fledged young each year. Brood size at fledging was 2.16, equivalent to 1.44 young per pair, or 1.23 young per territory. For the population persistent rain and low temperatures in the three to four months between egg laying and to a week or so after hatching had an adverse effect on breeding success; in years when the number of raindays was high fewer pairs bred, more clutches failed to hatch and some very young nestlings may have died. July-September raindays accounted for 60% of the variation in young fledged per territory while mean maximum temperature between August and October accounted for 74% of the variation in the percentage of territories from which young fledged and raindays in those months accounted for 58%. On the other hand, brood size was larger in years when it was wet during the nestling period. Overall, severe drought enhanced the breeding success of the population because more pairs bred successfully. Almost all of the adverse effect of rain could be attributed to nest quality: (1) in cold, wet years Falcons with a choice of nest, or with a well- drained, well-sheltered nest, bred as successfully as they did in dry years, while those without such nests performed poorly; and (2) the average number of young fledged from territories with a choice of nest was about 50% greater than that from territories with only one nest. The advantage of holding a territory with a high quality nest(s) as oppposed to a poorer nest, has clear selective value in the Darwinian sense.