Small Nestlings Research Articles

An immunological cost of begging in house sparrow nestlings

Parent-offspring conflict predicts that offspring should demand a greater parental investment than is optimal for their parents to deliver. This would escalate the level of offspring demand ad infinitum, but most of the models on the evolution of parent-offspring communication predict that begging must be costly, such costs limiting the escalation and defining an optimal level of begging. However, empirical evidence on this issue is mixed. A potential begging cost that remains to be accurately explored is a decrease in immunocompetence for offspring begging fiercely. This study experimentally analyses this cost in house sparrow (Passer domesticus) nestlings. A group of nestlings was forced to beg fiercely for a prolonged time while a control group begged at low levels, both groups receiving the same quantity of food. At the same time, the nestling response to an antigen (phytohaemagglutinin) was measured. Nestlings forced to beg fiercely showed a reduction in immunocompetence with respect to control chicks, but the two groups showed no difference in growth rate. The largest and the smallest nestlings in each brood showed a similar response to the treatment. These results strongly suggest a trade-off between begging and immunocompetence in this species. This trade-off may be a consequence either of resources from the immune system being reallocated to begging behaviour, or of adaptive immunosuppression in order to avoid oxidative stress. Steroid hormones are proposed as mediators of such a trade-off.

PATHOLOGY AND VIRUS DETECTION IN TISSUES OF NESTLING HOUSE SPARROWS NATURALLY INFECTED WITH BUGGY CREEK VIRUS (TOGAVIRIDAE)

Alphaviruses (Togaviridae) infect wild birds, but clinical illness and death attributable to virus in naturally infected birds is rarely reported, particularly for small passerine species or nestlings. Buggy Creek virus is a unique alphavirus in the Western equine encephalomyelitis virus (WEEV) complex that is vectored by the cimicid swallow bug (Oeciacus vicarius), an ectoparasite of the colonially nesting Cliff Swallow (Petrochelidon pyrrhonota) and the introduced House Sparrow (Passer domesticus). While sampling birds for Buggy Creek virus (BCRV) during the summers of 2007 and 2008, we discovered large numbers of clinically ill or dead House Sparrow nestlings. Ill nestlings exhibited ataxia, torticollis, paresis, and lethargy. Histologic examination revealed that encephalitis was the most common finding, followed by myositis, myocarditis, and hepatic changes, but pathology was highly variable. We isolated BCRV from brain tissue in most of the ill or dead nestlings, and from blood, liver, kidney, spleen, lung, feather pulp, and skin in some birds. To our knowledge, this is the first report of clinical illness, gross pathology, and histopathology for a WEEV-complex alphavirus in a field-collected passerine species.

Parental Sex Differences in Food Allocation to Junior Brood Members as Mediated by Prey Size

AbstractIndividual offspring within a brood may receive different amounts of provisioning from the male and female parents. Some hypotheses suggest that this bias is the result of an active and adaptive choice by parents. An alternative hypothesis is that feeding biases arise as a result of a constraint of fitting large prey items into small gapes. In an experiment with pied flycatchers, Ficedula hypoleuca, we tested for sex‐biased allocation to junior nestlings in asynchronous broods and whether this could be explained by active parental choice or by passive allocation according to prey size and gape size. In both control broods and broods with experimentally increased degree of asynchrony, prey types did not differ between parents but females brought smaller prey than males at younger but not older nestling stages. At younger but not older nestling stages, the majority of feeds to junior nestlings were from females, and the smaller nestlings consumed smaller prey than older siblings. However, there was no evidence of active preference of small nestlings by females as parents did not differ in the tendency to bypass a begging senior nestling in order to feed a junior nestling. Provisioning rates by females were lower than those by males when nestlings were young and we suggest that foraging time constraints caused by the need to brood offspring result in females bringing smaller prey than males. In turn, the larger prey brought by males was more often transferred to larger offspring after the smaller ones failed to swallow it. In such cases, ‘preferential’ feeding of small nestlings by females may simply be a passive side effect of foraging constraints and gape‐size limitations.

A parasite‐induced maternal effect can reduce survival times of fleas feeding on great tit nestlings

Parents can increase their reproductive success by assisting their neonate offspring in parasite defence. In birds, parental tactics include post‐hatching parental responses such as increased parental care and pre‐hatching maternal effects such as the transfer of maternal antibodies via the egg. These parasite‐induced parental responses are known to reduce the effects of parasites on offspring, but their costs for the parasite are largely unknown. In two separate experiments on great tits Parus major we assessed these costs for hen fleas Ceratophyllus gallinae. Half of the parents where exposed to fleas during egg‐laying to induce the parental response, while control nests were left flea‐free. In experiment 1 parents raised their own young and we measured the effect of combined pre‐ and post‐hatching parental effects, while in experiment 2 a cross‐foster design allowed us to assess the effects of pre‐hatching maternal effects alone. In both experiments we let fleas take a blood meal on nestlings from either flea‐exposed or unexposed parents. We then measured flea‐feeding duration, the quantity of extracted blood, and the fleas’ subsequent survival time. We found in both experiments that on the largest nestlings of a brood flea survival was significantly reduced by the parental effects, whereas on the smaller nestlings it was independent of parental effects. The pre‐ and post‐hatching parental responses did neither affect duration nor size of a flea blood meal. These results suggest first that the pre‐hatching maternal effects, i.e. the substances transferred to the nestling via the egg, have the potential to harm fleas without reducing flea feeding capacity, and second that the strength of the maternal response varies between the nestlings, either because maternal products are unequally distributed among eggs within a clutch, or because large nestlings can build up a response that enhances the effect of the maternal products.

Seasonal Variation in the Body Size-Body Mass Relationship in the Southern Grey ShrikeLanius meridionalis

The variation of male (n = 105) and female (n = 107) Southern Grey Shrike body mass has been analyzed in northern Spain during 2000–2002. The annual cycle was divided into autumn (October and November), winter (December to February) and breeding season (March to July), the latter subdivided into egg incubation, small nestlings and large nestlings periods. In order to analyze body mass, the residual index (RI) was used because it corrects body mass related to body size. The mean RI value for males was negative during the whole breeding season and positive during autumn and winter and it did not vary greatly between the periods. In females, the mean RI was definitely greater during egg incubation compared to other periods. The RI value of females was higher than that of males during the egg incubation and large nestlings periods, showing no significant differences in other periods. Female Southern Grey Shrikes (but not males) fitted to the programmed anorexia hypothesis during the breeding season whereas in winter they did not increase their body mass, in clear contrast to what has been put forward in some theoretical models.

Differences in parental food allocation rules: evidence for sexual conflict in the blue tit?

Evolutionary conflicts of interest between family members are expected to influence patterns of parental investment. In altricial birds, despite providing the same kind of parental care, patterns of investment in different offspring can differ between parents, a situation termed parentally biased favoritism. Previous explanations for parentally biased favoritism have received mixed theoretical and empirical support. Here, we test the prediction that in blue tits, Cyanistes caeruleus, females bias their food allocation rules to favor the smallest offspring during the nestling stage. By doing so, females could increase the subsequent amount of paternal care supplied by their partner during the fledging period, as a previous study showed that males feed the largest fledglings. When size differences within the brood are less pronounced, all offspring will require similar amounts of postfledging care, and thus, the male parent will lose the advantage of caring for the largest offspring that are closest to independence. In this study, we controlled the hunger of the smallest and largest nestlings in the brood and compared the food allocation rules of the 2 parents. We found that the male parent had a stronger preference than the female to feed the closest nestlings and made no distinction between nestlings based on size, whereas the female provisioned small hungry nestlings more when they were at intermediate distances from her. These differences in parental food allocation rules are consistent with predictions based on sexual conflict over postfledging parental investment.

Hatching asynchrony and early nestling mortality: the feeding constraint hypothesis

Several benefits may be gained from staggered hatching in birds but asynchrony sometimes appears to cause unnecessary starvation of the youngest nestlings. We present a solution to this paradox by suggesting that foraging parents are sometimes constrained by complex interactions between prey size, energy demands and foraging efficiency (the feeding constraint hypothesis). The requirement to provide smaller nestlings with small and soft food items while at the same time satisfying the greater food demands of larger nestlings may cause a conflict that results in the starvation of junior siblings. We examined the hypothesis in a field experiment where pied flycatchers, Ficedula hypoleuca, were video filmed while provisioning synchronous and asynchronous broods. Prey size and load size increased with nestling age suggesting that larger prey were more profitable to bring to the nest than smaller prey. Although small nestlings had difficulty swallowing large prey, parents of asynchronous broods brought large items suitable for the size of the older nestlings. Hence, junior nestlings suffered from reduced growth and increased mortality.

Carotenoid availability in diet and phenotype of blue and great tit nestlings

Carotenoids are biologically active pigments of crucial importance for the development of avian embryos and nestlings. Thus parental ability to provide nestlings with a carotenoid-rich diet may enhance offspring fitness. However, very little is known about the possible effects of carotenoid availability in the diet on growing nestlings in natural populations. We experimentally manipulated dietary intake of carotenoids by nestlings of two closely related passerine species, the great tit Parus major and the blue tit Parus caeruleus, and measured nestling antioxidants, body condition, immunity and plumage colour. There was no detectable increase in plasma carotenoids after treatment in carotenoid-fed nestlings of either species despite regular supply of dietary carotenoids. However, in carotenoid-fed blue tit nestlings, plasma vitamin E concentration increased with plasma carotenoid concentration, while that was not the case for control nestlings. In both species, there was no significant effect of carotenoid supply on immune function. Carotenoid supplementation enhanced yellow feather colour in great tit nestlings only. In both species a strong effect of carotenoid supply was found on body condition with an increase in body mass for small carotenoid-fed nestlings compared to similarly sized control nestlings. Dietary availability of carotenoids may thus have important fitness consequences for tits. We hypothesise that the difference in effect of dietary carotenoids on the two species is due to relatively larger clutch size and higher growth rates of blue tits compared to great tits, leading to blue tit nestlings being more in need of carotenoids for antioxidant function than great tit nestlings.

Tree swallow reproductive investment, stress, and parasites

We reduced or increased tree swallow, Tachycineta bicolor (Vieillot, 1808), clutch sizes by three eggs (50% of modal clutch size) to test experimentally for relationships between stress and parasite loads. In the first year of the study (1996), we enumerated two measures of stress (granulocyte to non-granulocyte ratios and heterophil to lymphocyte ratios), blood parasites, and ectoparasites living on birds (and not in nesting material). Stress indices increased for parents, but not for nestlings, associated with larger broods. Only one blood parasite (a trypanosome) was detected in blood smears from 221 different individuals. On individual birds, we found a median of 0 fleas, 0 lice, and 7 feather mites. In the second study (1998), we focused on parasites living in nesting material. Here, we found a median of 106 fleas and, of the parasites we observed, these probably had the greatest potential impact on the birds. Per capita flea loads were higher in nests that contained more young. However, despite higher stress to parents and higher per capita flea loads in enlarged broods, flea numbers were not associated with smaller nestlings or with reduced fledging success.

Exposure to ecotourism reduces survival and affects stress response in hoatzin chicks ( Opisthocomus hoazin)

Ecotourism helps to protect many habitats, but may also have negative impacts on wildlife. We investigated effects of ecotourists on reproductive success of hoatzins ( Opisthocomus hoazin) and on hormonal status of their chicks in Amazonian rainforest lakes by comparing birds from undisturbed and from tourist-exposed nests. Hatching success was similar in both groups but chick survival was much lower at tourist-exposed nests than at undisturbed nests. This effect was due to an increased mortality of juveniles prior to fledging whereas small nestlings seemed largely unaffected. Juveniles, but not nestlings, living at tourist-exposed sites had a lower body mass and showed a stronger hormonal response to experimental stress compared to individuals at undisturbed sites. These data suggest that juvenile hoatzins were susceptible to tourist-induced stress which in turn may be responsible for the lower survival. In contrast, adult hoatzins that were incubating had apparently habituated to tourist presence because their flush distances at tourist-exposed nests were 50% lower than at undisturbed sites. Our findings demonstrate that individuals in different life stages show different susceptibilities to tourism. We suggest that even just watching animals during breeding can threaten their survival, but a proper scientific management of off-limit zones and area-specific guidelines for wildlife observation could reduce harmful effects.

Hatching asynchrony, nestling competition, and the cost of interspecific brood parasitism

All parental hosts of heterospecific brood parasites must pay the cost of rearing non-kin. Previous research on nest parasitism by brown-headed cowbirds (Molothrus ater) concluded that competitive superiority of the typically more intensively begging and larger cowbird chick leads to preferential feeding by foster parents and causes a reduction in the hosts' own brood. The larger size of cowbird nestlings can be the result of at least two causes: (1) cowbirds preferentially parasitize species with smaller nestlings and lower growth rates; and/or (2) cowbirds hatch earlier than hosts. I estimated the cost of cowbird parasitism for each of 29 species by calculating the difference between hosts' published brood sizes in nonparasitized and parasitized nests and using clutch size to standardize values. In this analysis, greater incubation length and lower adult mass, surrogate measures of the hatching asynchrony and size difference between parasite and hosts, were both related to greater costs of cowbird parasitism without bias owing to phylogeny. To establish causality, I manipulated clutch contents of eastern phoebes (Sayornis phoebe) and examined whether earlier hatching by a single cowbird or phoebe egg reduces the size of the rest of the original host brood. As predicted, greater hatching asynchrony increased the proportion of the original phoebe brood that was lost. This measure of the cost of parasitism was partially owing to increased hatching failure of the original eggs in asynchronous broods but was not at all related to the size differences of older and younger conspecific nestmates. However, proportional brood loss owing to an earlier hatching conspecific was consistently smaller than brood loss owing to asynchronous cowbirds in both naturally and experimentally parasitized phoebe nests. These results imply that although hatching asynchrony is an important cause of the reduction of host broods in parasitized clutches, competitive features of cowbird nestlings remain necessary to explain the full extent of hosts' reproductive costs caused by interspecific brood parasitism. Copyright 2003.

Antipredation Value of Colonial Nesting in Yellow-Headed Blackbirds

AbstractYellow-headed Blackbirds (Xanthocephalus xanthocephalus) in Manitoba breed in dense colonies in cattail marshes. Their reproductive success is affected mainly by predation. The most important predator on blackbird nests is the Marsh Wren (Cistothorus palustris), which breaks blackbird eggs and kills small nestlings. We examined whether colonial nesting in Yellow-headed Blackbirds may represent an adaptation to reduce Marsh Wren predation. Marsh Wren predation may be reduced by (1) mutual nest defense by adult blackbirds, (2) predator satiation or dilution, or (3) selfish-herd effects. We tested these hypotheses using experimental nests and found that their safety increased with decreasing distance to the nearest blackbird nest and with increasing density of simultaneously active blackbird nests located nearby. Safety also was higher for nests placed inside a blackbird colony rather than outside. These findings support the nest-defense hypothesis. We also found that Marsh Wrens are capable of destroying a whole blackbird colony in a few days, and that colony size is not correlated with nest safety. These results suggest that the satiation or dilution benefits are negligible. Finally, we found that central nests are safer than peripheral nests in a blackbird colony, but not in an artificial colony, providing weak support for the selfish-herd hypothesis. We conclude that nest predation is reduced mainly by mutual nest defense of adult birds and may represent an important selective force favoring colonial nesting in this species.

Heterophil/lymphocyte ratios and heat-shock protein levels are related to growth in nestling birds

Growing altricial birds may experience nutritional stress in the nest due to sibling competition, food restriction, or parasites. Nutritional stress may be detected through its effect on nestling growth, although genetic and maternal effects may interfere with its expression. A more direct way of estimating nutritional stress may be through measurements of organismic stress in growing birds. Heterophil/lymphocyte ratios (H/L) have been proposed as reliable physiological indicators of the stress response. Heat-shock protein (HSP) synthesis is induced by animals in response to various stressors, making the concentration of HSPs a suitable measure of organismic stress. In a study of growing pied flycatchers (Ficedula hypoleuca), we assessed H/L from cell counts in blood smears and levels of two HSPs, HSP60 and HSP70, from the cell fraction of peripheral blood. H/L and the level of HSP60 were significantly positively associated in nestlings of 13 days of age, while the level of HSP70 was not related to the other two measures. Small nestlings with respect to tarsus length on day 7 had higher levels of HSP60 and higher H/L values. Brood means of nestling mass, tarsus length, and wing length shortly before fledging were negatively related to both mean HSP60 level and mean H/L ratio. Brood variances in body mass and wing length on day 13 were positively associated with mean HSP60 levels but not with mean H/L values. Up to 80% of variation in some traits was explained by stress indicators.

ANTIPREDATION VALUE OF COLONIAL NESTING IN YELLOW-HEADED BLACKBIRDS

Abstract Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) in Manitoba breed in dense colonies in cattail marshes. Their reproductive success is affected mainly by predation. The most important predator on blackbird nests is the Marsh Wren (Cistothorus palustris), which breaks blackbird eggs and kills small nestlings. We examined whether colonial nesting in Yellow-headed Blackbirds may represent an adaptation to reduce Marsh Wren predation. Marsh Wren predation may be reduced by (1) mutual nest defense by adult blackbirds, (2) predator satiation or dilution, or (3) selfish-herd effects. We tested these hypotheses using experimental nests and found that their safety increased with decreasing distance to the nearest blackbird nest and with increasing density of simultaneously active blackbird nests located nearby. Safety also was higher for nests placed inside a blackbird colony rather than outside. These findings support the nest-defense hypothesis. We also found that Marsh Wrens are capable of dest...

Sibling competition affects individual growth strategies in marsh tit, Parus palustris, nestlings

How do different levels of sibling competition affect nestling growth strategies? We investigated this through the effect of different levels of per capita feeding rate on the average growth rate of nestlings and on individual growth strategies within broods. To manipulate sibling competition, we transferred marsh tit nestlings between nests creating broods with fewer (reduced) or more (enlarged) nestlings than originally. Mean mass decreased as brood size, and thus sibling competition, increased. This was due to a large difference in mass between the smallest young in each brood. The mass of the largest young did not differ significantly between the broods. In reduced broods, the largest and smallest nestlings had equal mass, whereas in enlarged broods the difference was large. In contrast, wing length close to fledging did not differ between the experimental broods. We conclude that the growth of the wing has priority over general increase in size. This depends on the risk of starving for nestlings unable to leave the nest when their most dominant sibling leaves. Thus, low intake rates induce a trade-off in the growth of different structures, leading subdominant nestlings to divert proportionally more resources to wing length growth at the expense of general size. Subdominant nestlings are thus facing both a food constraint, depending on parental feeding rate and number of dominant siblings, and a time constraint, depending on the growth rate of the largest sibling. Hence, to fledge successfully, subdominant nestlings will leave the nest at suboptimal masses with a likely increase in postfledging mortality.

Ectoparasites, nestling growth, parental feeding rates, and begging intensity of tree swallows

Many studies fail to show relationships between ectoparasite loads and nestling growth rates. One explanation is that parent birds increase feeding rates to compensate for nestling energetic losses to ectoparasites. Nestling begging behaviours could signal need to parents. Accordingly, we tested whether higher flea and blow fly loads in tree swallow (Tachycineta bicolor) nests were associated with smaller nestlings, higher parental feeding rates, and increased nestling begging intensity. The study area was the Gaspereau Valley of Nova Scotia, Canada. When nestlings were 10 days old, parental feeding rates and nestling begging intensity were measured with tape recorders. At 13 days of age, nestlings were weighed and measured. Within 2 days of fledging, nest material was removed from nest boxes and enumerated for adult fleas and blow fly pupae. After including brood size and date of first egg as covariates in general linear models, no significant relationships were found between ectoparasite loads and nestling size, parental feeding rate, or nestling begging intensity. Our results suggest that nestling tree swallows were able to buffer the effects of naturally occurring ectoparasite loads without significant help from their parents. Low levels of virulence may have resulted from relatively benign weather during the study, low numbers of ectoparasites, selection on ectoparasites to avoid killing their hosts, and host defences.

Acoustic signalling of hunger and thermal state by nestling tree swallows

The begging displays used by altricial nestling birds to solicit care from parents include vigorous movements and loud calling. These begging signals have attracted considerable interest, mainly because their intensity seems excessive for the function of transmitting information about nestling need to parents. However, how information on need is encoded in the various components of the signal, especially its acoustic components, is poorly understood. We examined how begging calls of large and small nestling tree swallows, Tachycineta bicolor, changed during a short period of food deprivation and cooling, as a first step in determining the role that various call characteristics played in advertising nestling need. In contrast to previous studies, we examined several call variables, and related them not only to need for food but also need for warmth. When nestlings were deprived of food, their calls increased in rate and length. Large nestlings also increased the amplitude of their calls. When nestlings were cooled during food deprivation, they decreased the frequency of their calls and their call rate. The latter trend was especially evident in small nestlings. Our results suggest that begging calls carry information not only on the overall hunger level of broods, as emphasized in previous studies, but also on the size, hunger and thermal need of individual nestlings. Further tests are needed to determine whether parents use this information and whether begging calls are optimally designed to convey it.

Size matters: competition between male and female great tit offspring.

1. Studies of sex allocation in birds have traditionally centred on Fisher's (1930) theory of equal parental investment in male and female offspring. They concentrate particularly on sexually dimorphic species, where costs of rearing offspring are assumed to vary between male and female young because of body size differences. 2. Higher mortality of the larger sex (males), particularly in poorer conditions, is expected to result in female-biased sex ratios in the great tit, Parus major. Several studies have found the contrary, reporting male-biased fledging and recruiting sex ratios when conditions are poor. One reason why this may be the case is that males can gain more food resources than their sisters because of the competitive advantage afforded by their larger size. They may thus suffer less mortality in the nest or fledge in better condition, thereby enhancing their survival prospects. 3. This study investigates the importance of size in competitive interactions between nestlings of different sexes. A cross-fostering design was employed to create broods of mixed size through swapping half a brood of 2-day-old 'small' and half a brood of 4-day-old 'large' nestlings. Nestling morphometrics and mortality were measured during the nestling period. Nestlings were sexed by PCR amplification of sex-linked genes. To test for a male advantage in competitive environments, size and mortality measures were compared between 'small' males and females, and 'large' males and females (i.e. the interaction term 'size treatment' and 'sex'). 4. There was greater sexual dimorphism between small nestlings than large nestlings at fledging. This is interpreted as revealing enhanced competitive ability of male offspring under stressful conditions. Offspring from the 'large' group suffered lower mortality, but there was no difference in mortality according to sex, and no significant size*sex interaction. Similarly, no difference in recruitment was found, although this may be due to the small overall proportion of birds recruiting (3·1%). 5. The study suggests that male biased fledging/recruitment sex ratios in great tits may be explained by enhanced competitive ability of male nestlings in poor rearing environments.

Correlates of Reproductive Success in Cooperatively Breeding Western American Crows: If Helpers Help, It's Not by Much

Abstract I measured annual reproductive success for a resident population of cooperatively breeding Western American Crows (Corvus brachyrhynchos hesperis) over 6 years, and examined the relationship between it and several ecological and social variables. Most nests (57%) failed to fledge any young, due largely to predation. Success in fledging young was associated with three interrelated variables: (1) the presence of helpers, (2) early nesting, and (3) consistent differences between pairs. Assisted pairs began incubation earlier, some pairs consistently nested early, and the proportion of years that pairs were successful was related to the proportion of years in which they had help. Although both help and incubation date were related to nesting success independent of each other, the effect of help became nonsignificant after controlling for differences between pairs. The slight increase in fledging success possibly attributable to helpers may have been the result of consistently successful pairs succeeding, with help, in otherwise poor years. Postfledging survival was related in part to the size of individuals; larger nestlings tended to have a greater chance of fledging, and once fledged, to have a greater chance of surviving the following 2-week period. Larger nestlings survived to one year of age significantly more often than smaller nestlings.

Wide Disparity in Age of Nestlings in a Say's Phoebe Nest from New Mexico

Say's phoebes (Sayornis saya) are common throughout open country of western North America (Schukman and Wolf, 1998), building nests of dry grasses, forbs, and animal hair on ledges in natural (cliffs) and man-made (buildings, bridges) sites (Ohlendorf, 1976; Harrison, 1979; Baicich and Harrison, 1997). Abundance of this species probably has increased since European settlement because man-made nest sites now occur in many areas where no natural nest sites exist (Ohlendorf, 1976; Schukman, 1993). Say's phoebes often attempt to raise two broods, beginning in April (Harrison, 1979; Baicich and Harrison, 1997; Schukman and Wolf, 1998). In Kansas incubation averaged 15 days and fledging occurred at 16-17 days (Schukman and Wolf, 1998). At 1430 h on 12 July 1996 I entered a small building at Stone Lake, Rio Arriba Co., New Mexico that is regularly used by nesting Say's phoebes. A nest was found that contained two large young, estimated to be within a few days of fledging. Body feather growth appeared complete, and flight feathers were near full length but still partially sheathed. Between these two large young was a naked nestling (likely <24 h old) and an unhatched egg. Neither the egg nor the small nestling were those of a brown-headed cowbird (Molothrus ater; Sogge et al., 1997); the egg's size and coloration were typical of a Say's phoebe (Harrison, 1979; Baicich and Harrison, 1997). I banded both large nestlings. I returned to the nest on 15 July at 1450 h. One large nestling had fledged and the other was out of the nest but on the same ledge. It was nearly capable of sustained flight but was easily recaptured. A nestling appearing no larger than the smaller one seen on 12 July was in the nest; it had light white down on its back, but no other feather growth and was assumed to have hatched from the egg observed three days before. I photographed the large and small nestlings (on file with New Mexico Game and Fish Department, Santa Fe) and returned them to their respective locations. On a final visit on 20 July at 0900 h, I found only the remaining large fledgling. Breeding biology of the Say's phoebe has not been as intensively studied as that of its congener, the eastern phoebe (S. phoebe). Assuming that Say's phoebe nestlings grow at the ame rate as eastern phoebes (Weeks, 1994), the two larger young found on 12 July were at least 10 and probably 12-14 days old. One had fledged 3 days later, though the other remained in the building another 8 days. The two smaller young were not seen together, but the one seen 15 July had no primary dots (23 days in eastern phoebe; Weeks, 1994) or visible size increase, suggesting the two eggs hatched asynchronously but that neither young survived more than a day or two. Once egg-laying is initiated, eastern phoebes generally lay eggs daily until the clutch is complete (Murphy, 1994); a similar laying pattern may occur in Say's phoebe. Therefore it is unlikely that a female Say's phoebe would have a gap of 10-12 days between laying dates for her second and third eggs. Further, incubation of second clutches in eastern phoebes is usually initiated with the penultimate egg (Weeks, 1994). Although all eggs in many eastern phoebe first clutches hatch within a 24-h period, hatching in many second clutches occurs over a 2-3 day period (Murphy, 1994; Weeks, 1994). Hatching asynchrony of less than 2 days, with the greatest asynchrony in second clutches, also is typical of the Say's phoebe (J. M. Schukman, pers. comm.) but asynchrony of more than 10 days is unusual. Two explanations of this mixed-age clutch seem feasible. Eastern and black (S. nigricans) phoebes occasionally are polygynous; in the former species five marked males concurrently fed young at two nests (Weeks, 1994) and in the latter species one verified and one suspected case of polygyny were documented (Wolf, 1997). Though common use of a nest by two female phoebes has not been docu-