Factors Affecting Vocalization in Tengmalm’s Owl (Aegolius funereus) Fledglings during Post-Fledging Dependence Period: Scramble Competition or Honest Signalling of Need?

Begging behaviour of nestlings has been intensively studied for several decades as a key component of parent-offspring conflict. There are essentially two main theories to account for intensity of food solicitation among offspring: that intensity of begging is related to some form of scramble competition between nest mates or that it offers honest signalling of need to parents. The vast majority of studies which have addressed begging behaviour have been based on observations of, and experiments on, nestlings and have not considered begging behaviour, during the post-fledging period. Begging vocalizations in this post-fledging phase of dependence have rarely been studied, despite the importance of vocalizations as a communication method between offspring and parents, particularly for nocturnal species. We radiotracked 39 fledglings of the Tengmalm’s owl (Aegolius funereus) in two years with different availability of prey: 2010 (n = 29 fledglings) and 2011 (n = 10 fledglings) and made 1320 nightly localizations in which we recorded presence or absence of begging calls. Within years, the most important measures related to the probability of vocalization were body condition at fledging, time of night, number of surviving siblings, age and weather conditions. Begging intensity increased with age in both years; however, in the year with low prey availability fledglings vocalized significantly more often. The main factor causing these differences between years was probably the different availability of prey, affecting breeding success, post-fledging behaviour, and thus also both short- and long-term needs of offspring. We believe that our results suggest honest signalling of their fledgling’s need.     

Post-fledging interactions between the Common Cuckoo Cuculus canorus and its cavity-nesting Common Redstart Phoenicurus phoenicurus host

Brood parasite–host interactions during the incubation and nestling stages have been well studied, but the post-fledging period remains virtually unknown. Using radiotracking, we provide the first detailed data on post-fledging interactions between the Common Cuckoo Cuculus canorus and its only regular cavity-nesting host, the Common Redstart Phoenicurus phoenicurus. Cuckoos raised alone (‘solitary’) fledged at higher mass, with higher wing and tarsus length and started to fly at a younger age than Cuckoos raised alongside young Redstarts (‘mixed’). However, a further 23 fledging and post-fledging parameters measured at five pre-determined times (fledging, first-flight, predation, starvation, independence) did not differ between solitary and mixed Cuckoos. In addition, none of the parameters measured during the post-fledging period (growth, dispersal distances, number of flights) differed between solitary and mixed Cuckoos. Redstart fledglings from non-parasitized broods (‘solitary’) showed generally similar fledging and post-fledging parameters to fledglings reared alongside a Cuckoo (‘mixed’). Surprisingly, there were no significant differences in post-fledging predation rate, starvation or overall survival rates between mixed and solitary Cuckoos or mixed and solitary Redstarts. Thus, during the post-fledging period, mixed Cuckoo fledglings successfully compensated for the poorer performance experienced during the nestling stage whereas mixed and solitary Redstarts did not differ in any measured parameters. This suggests that the regular occurrence of mixed broods in this host–parasite system – which is unique among the many Cuckoo hosts – is evolutionarily stable for both hosts and parasites.     

Differential Movement Patterns of Juvenile Tengmalms Owls (Aegolius funereus) during the Post-Fledging Dependence Period in Two Years with Contrasting Prey Abundance

Fledgling behaviour and movement patterns throughout the post-fledging dependence period (PFDP), especially in relation to changing environmental conditions, have been rarely studied, despite the fact that this period is recognized as of crucial significance in terms of high mortality of juveniles. The PFDP can extend over quite a protracted period, particularly in birds of prey, and a knowledge of the movement patterns of individuals is fundamental for understanding mechanisms underlying survival, habitat use and dispersion. We radiotracked 39 fledglings of the Tengmalm’s owl (Aegolius funereus) in two years with different availability of prey: 2010 (n = 29) and 2011 (n = 10) and obtained 1455 daily locations. Fledglings reached independence on average in 45 days after fledging in 2010 (n = 22) and 57 days in 2011 (n = 6). Within years, the most important measures influencing the distance moved from the nest box were age of fledglings and number of surviving siblings present. Individual home range size and duration of PFDP in particular were dependent on maximal number of siblings seen outside the nest box. In the season with low prey availability fledglings were observed at greater distances from the nest box than in the year with higher prey availability (mean distance: 350 m in 2010 and 650 m in 2011) and occupied larger home ranges (mean: 30.3 ha in 2010 and 57.7 ha in 2011). The main factor causing these differences between years was probably the different availability of prey in these two years, affecting breeding success and post-fledging survivorship of the Tengmalm’s owls.     

Intraspecific brood-parasitism and dispersal in fledgling Sparrowhawks Accipiter nisus

The progeny of early-nesting Sparrowhawks Accipiter nisus have a higher survival rate than those of late nesters. To ascertain the reasons for greater survival in early-season Sparrowhawk fledglings, I studied post-fledging dispersal behaviour in this species by direct observations and radio-tracking during 3 years in Rockingham Forest, Northamptonshire, U.K.
Post-fledging brood-parasitism was found among early-dispersed young of both sexes.
Early-dispersed young spent up to 6 days being fed by the parents of other fledged, but still dependent, broods, as far as 6 km from their own nests.
Three broods were provided with supplementary food for 4 weeks, starting 1 week before expected dispersal. These young dispersed when significantly older than young from control broods. In both groups, males dispersed, on average, 3–4 days earlier than females. The ultimate dispersal age of young in control broods was negatively correlated with their rate of mass gain during the nestling period. Unlike the young of the control broods, fledglings in broods with augmented food were usually silent.
These findings offer an explanation for why Sparrowhawk young that disperse early in the season survive better than those which disperse late.     

Family break-up in Black and Red Kites Milvus migrans and M. milvus: is time of independence an offspring decision?

I observed the natural process of family break-up in 13 families of Black Kites Milvus migrans and five families of Red Kites M. milvus in which fledglings had been individually marked. In other broods, I performed experiments which modified the parental investment fledglings received by supplementing nests of both species with food or transferring Black Kite chicks to nests with younger or older chicks of the same species.
The time of family break-up in the Black Kite is mainly an offspring decision which is not affected by an artificial increase of parental investment. The duration of the post-fledging period was not increased in Black Kites that were given supplementary food. Chicks transferred to nests with a younger chick did not extend the post-fledging period, nor did chicks transferred to nests with an older chick shorten the post-fledging period. In Red Kites, parental investment seemed to have more influence on the timing of the family break-up. Red Kite adults invested less as parents than did Black Kites during the post-fledging dependence period, and in nests where supplementary food was given, Red Kite fledglings stayed attached for a longer period.     

波兰东南部乌灰鹞在雏鸟出羽后期的同窝雏间冲突

通过对波兰东南部的13巢乌灰鹞(Circus pygargus) ,37只雏鸟观察,对同窝雏鸟间的竞争进行了研究。无论是在食物的数量或者生物量上,早孵出的雏鸟占有空中喂养与巢内喂养的食物源的绝对优势。空中学习捕食的食物量占喂养食物量的比例随着雏鸟年龄的增长而增加。雏鸟在空中喂养中的攻击行为通过对空中食物传递进行研究。在370次的食物传递中,由于雏间竞争而引起的落地食物的比例57次(7·8 %)。由于雏间竞争而不能获得亲鸟喂养的幼鸟有时捕食周围的雀形目小鸟。亲鸟很少喂养较小的雏鸟。窝雏数越大,雏鸟间对食物的争斗持续时间越长。空中食物传递中捕食的成功率与雏鸟的空间分布相关。这种建立在雏鸟早期的等级制一直持续到雏鸟的出羽后期,从而保证早出雏具备开始迁徙的良好身体条件[动物学报51 (5) : 790 -796 ,2005]。     

Determinants of parental care and offspring survival during the post-fledging period: males care more in a species with partially reversed sex roles

Sexual conflict is magnified during the post-fledging period of birds when the sexes face different trade-offs between continuing parental care or investing in self maintenance or other mating opportunities. Species with reversed sex roles provide a unique opportunity to study the relationship between mating systems and investment in parental care. Here, we provide the first detailed study of the length of care by males versus females (n = 24 pairs) during the post-fledging period, assessing factors that may promote care within and between the sexes. In the northern flicker Colaptes auratus, a species with partly reversed sex roles, males cared longer than females (average 16 versus 12 days, respectively). Overall, 36 % of females but no males deserted the brood prior to fledgling independence. Parents that provisioned nestlings at a high rate also spent more days feeding fledglings. Among males, age and nestling feeding rates were positively associated with the length of care. Among females, a low level of feather corticosterone (CORT_f) was associated with a longer length of care. About 45 % of fledglings died within the first week, but fledglings with intermediate body mass had the highest survival suggesting stabilizing selection on mass. Fledgling survival was also higher in individuals with larger broods and lower levels of CORT_f. We demonstrate that because females can be polyandrous they often desert the brood before males, and that the sexes respond to different cues relating to their energy balance when deciding the length of care given to their offspring.     

Play behaviour and active training of Montagu’s harrier (Circus pygargus) offspring in the post-fledging period

Play bouts and active training of juveniles by Montagus harrier (Circus pygargus) adults in the post-fledging period were observed. Fledglings often played with prey and with a variety of inanimate objects such as bits of moss, regurgitated pellets, sticks and a wad of hay. Inanimate objects selected for play were, in length, very similar to the common vole (Microtus arvalis), which is the most common prey of the species during their breeding period. Some recently fledged individuals were trained to capture invertebrate prey by adults demonstrating techniques for the fledglings and thus develop their hunting skills. Training sessions took place only in the foraging areas of the adult birds.     

Post-fledging brood and care division in the roseate tern (Sterna dougallii)

Extended post-fledging parental care is an important aspect of parental care in birds, although little studied due to logistic difficulties. Commonly, the brood is split physically (brood division) and/or preferential care is given to a subset of the brood by one parent or the other (care division). Among gulls and tern (Laridae), males and females generally share parental activities during the pre-fledging period, but the allocation of parental care after fledging is little documented. This study examined the behaviour of male and female roseate terns (Sterna dougallii) during the late chick-rearing and early post-fledging periods, and in particular the amount of feeds and the time spent in attendance given to individual chicks/fledglings. Pre-fledging parental care was biparental in all cases. Post-fledging parental care was dependent on the number of fledglings in the brood. Males and females continued biparental care in clutches with one surviving fledgling, while in two-fledgling clutches, males fed the A-fledgling while females fed the B-fledgling. Overall, there was no difference in attendance, only in feeds. This division of care may be influenced by the male only being certain of the paternity of the A-chick but not by chick sex.     

The function of aggressive chases by breeding Black and Red Kites Milvus migrans and M. milvus during the post-fledging dependence period

Black and Red Kites Milvus migrans and M. milvus chase other raptors approaching their nests. The study of this behaviour during the post-fledging period suggested that it reflects mainly, but not only, anti-predator behaviour. The frequency of vigilance and aggressive chases decreased through the post-fledging period as predicted by theoretical models of nest defence. Although predation risks were similar, Black Kites invested more time chasing intruders than did Red Kites. Black Kites, unlike Red Kites, chased away intruding juveniles, which may be interpreted as a behaviour to avoid investment in unrelated fledglings. Black Kites usually nest in loose colonies where the risk of, and selection pressures against, accidental investment in unrelated fledglings is likely to be greater than for Red Kites. Differences in aggressive chases by Black and Red Kites are better related to this than to different predation risks.     

Juvenile survival and recruitment of wood thrushes Hylocichla mustelina in a forest fragment

We tested if juvenile survival and recruitment (returning to breed) of wood thrushes Hylocichla mustelina into their natal population was dependent on several measures of nestling condition, growth, and site fidelity. Overall, nestling mass, wing chord, condition, and rank within the brood did not affect the probability of survival to the post-fledging stage, independence, or recruitment. We did not identify any morphometric differences among nestlings that remained in the study area after fledging and those that did not (or those that did not survive). Time of season and the number of days nestlings were present in the study area after hatching, or site fidelity, were the only variables measured that positively influenced juvenile survival and recruitment. Based on a restricted sample, fledglings that eventually recruited did not differ in size during post-fledging or post-independence from fledglings that did not return to breed. Independent, immigrant hatching-year (HY) birds are briefly described. Overall, immigrant HY birds had a higher probability of recruitment than all local fledglings. There was no difference in the probability of recruitment between immigrant HY birds and independent local fledglings (those present ≥35 days after hatching), however. Similar to local young, the number of days present in the study area increased the likelihood of recruitment for immigrant HY birds, although these effects could not be distinguished from those of emigration or survival.     

OBSERVATIONS ON SOME KENYA EAGLES

This paper describes the continuation of work on eagles in Embu district, Kenya, especially at Eagle Hill, which has now been under observation continuously since 1949. Observations in other parts of Kenya have been included. The ecological changes possibly affecting eagles on Eagle Hill are discussed. The population fell from a pair each of Circaetus cinereus, Aquila verreauxi, Hieraetus fasciatus spilogaster, H. dubius, Polemaetus bellicosus and Stephanoaetus coronatus in 1952 to a pair each of H. dubius, P. bellicosus and S. coronatus in 1965. Possible causes of the decline are discussed. The species of eagles are not normally aggressive to one another, in contrast to other resident species such as Falco peregrinus and Buteo rufofuscus. Although the eagles appear to be ecologically separated by food preferences and habitat this is apparently not the whole explanation for the unusual concentration of eagles on this hill. Additional breeding data are given for H.f. spilogaster, H. dubius, P. bellicosus and S. coronatus. These species rear respectively 0.56, 0.65, 0.42 and 0.44 young per pair per annum. S. coronatus breeds in alternate years and cannot breed every year because of a protracted post-fledging period in which the young is fed for up to 350 days. P. bellicosus, with about the same annual reproductive rate, does not have the same breeding rhythm. Data on reproductive rates combined with other data suggest possible life spans in the wild state of adults of H.f. spilogaster 10–11 years, H. dubius nine years, P. bellicosus 14 years, and S. coronatus 16 years. At nests of H. dubius and S. coronatus changes of mates have been recorded for 16 and 17 years respectively. In S. coronatus a change occurs about every six years and in H. dubius about every four years, indicating that S. coronatus may live about 1.5 times as long as H. dubius in the wild state. One female S. coronatus was known to live for 8.5 years as an adult. Other incomplete life spans are eight and eight years for two male S. coronatus, and eight for one female of this species. Two male H. dubius have each lived for at least eight years but no female of this species has lived for more than five years. Two proven cases of re-laying after a natural disaster are recorded, one each in H. dubius and S. coronatus. Other instances are suspected in H. dubius. The habit may be commoner than is supposed in large eagles. The history of four pairs of S. coronatus, each observed for four years or more, totalling 34 pair/ years is given. S. coronatus breeds regularly every second year unless some unusual occurrence, such as a change of mates or a failure during incubation, upsets the rhythm. S. coronatus females lay 1–2 eggs at dates varying from June–October in Kenya; breeding is not confined to the dry season. Laying dates of individual females may vary by two months between one year and another. Incubation takes 48–49 days, fledging 105–116 days. The elder of two young hatched invariably kills the younger so that no more than one young is reared. Female adults are dangerously aggressive, especially during days 30–60 of the fledging period. In 86% of cases where eggs are laid a young bird is reared. Since clutches of two in practice do not result in more than one young this represents a breeding success of 86% of the potential, a very high percentage. The sex ratio of young leaving the nest is about equal, seven males to five females, in known cases. The post-fledging period in S. coronatus is 330–350 days, and the total breeding cycle about 560 days, making it impossible for the eagles to breed every year, if they rear a young bird to independence. In the post-fledging period the young S. coronatus remains within half a mile of the nest, where it is fed by the parents, the female bringing most of the prey. The adults call to attract the young bird, which flies into the nest receiving the prey there, or rarely on a tree nearby. If the adult obtains no response from the young it may carry the prey away. Although regularly fed by its parents the young eagle kills some of its own food from at least day 61 of the period onwards, but most often in the last third of the period, being then apparently stimulated by unusual periods of privation. Almost 100% of young eagles that leave the nest are reared to independence at about 15 months old. The possible biological advantages of this protracted adolescence in survival and economy of prey are discussed. The main prey of S. coronatus is antelopes, followed by hyrax. Monkeys are rarely taken. Killing methods, times, and relations with prey are discussed. The eagles usually kill in early morning or evening, but also at other times. They may cache portions of large kills. Most prey is brought to the nest between hours 4–6 of daylight. The male S. coronatus feeds his incubating mate about once every 3–3 days. Once the young has hatched his killing rate rises to about one kill per 1.7 days. The killing rate falls slowly to one kill per two days later in the fledging period. At normal times the killing rate of adults is apparently controlled by their own appetites, and the increased killing rate of the male after hatching is an exception to this rule. During the post-fledging period the feeding rate varies from 1: 2.0 days to 1: 6.2 days, averaging 1: 3 days in 130 cases. Periods of privation may last from 5–13 days. Alternatively several kills may be brought in a day, possibly from cached portions of large kills in some cases. Long foodless periods may stimulate the young eagle to kill for itself, especially in the last third of the post-fledging period. Final independence of the young is not brought about by aggressive parental behaviour, but is probably due to increasing indifference of the young to food-bringing adults. This indifference may act as a release to the adults, breaking the rhythm of bringing food to the young, and so stimulate the onset of a new breeding cycle.     

Post-fledging survival of Sparrowhawks Accipiter nisus in relation to mass, brood size and brood composition at fledging

Using ring recoveries, the post-fledging survival of Sparrowhawks was examined in relation to the growth rates and fledging masses of young and in relation to the size and sex composition of broods, which contained up to six young. On most aspects, no significant relationships were found: light young survived as well as heavy ones, both in the population as a whole and in individual broods; and young in large broods survived as well as young in small broods. This was attributed to relative food abundance in the post-fledging dependency period and to the fact that most food-related mortality among young occurs at an earlier stage in the breeding cycle. However, males in all-male broods and females in all-female broods survived slightly better after fledging than others of their sex in two-sex broods. The reason is unknown.     

Development of chicks and predispersal behaviour of young in the Eagle Owl Bubo bubo

Little quantitative information on the development and behaviour of chicks and young is available for many species, despite the crucial importance of such data and the sensitivity of this stage in a bird's life. For Eagle Owls Bubo bubo , despite the large amount of scientific literature on this species, much basic information is lacking. This study provides a photographic and morphometric guide for age estimation of nestlings and fledglings, as well as data on the call behaviour of young, and patterns of movements during the post-fledging dependence period. The most remarkable event in chick development is the rapid increase in mass, and size gain, during the first 30 and 40–45 days, respectively. Because after this time morphometric differences become less evident, young-feather development is more useful for ageing. Patterns of chick call behaviour showed that the time spent calling increased with age and, from 110 days of age, chick vocalizations were usually uniformly distributed through the whole night and most synchronized at sunset and sunrise (the maximum recorded number of vocalizations per chick and per night was 1106 calls). During the post-fledging dependence period, radiotagged Owls moved widely, up to 1500 m from the nest after the age of 80–90 days. During such movements, the mean distance among siblings increased with age, from 168 m on average for juveniles less than 100 days old, to 489 m for those older than 100 days. Definitive dispersal started when young were about 150–160 days old. Information on chick call behaviour and movements is crucial for unbiased census and nest checking, as well as for the definition of young post-fledging areas. Knowledge of the latter is very important in terms of conservation and management (especially for those species that move largely around their nest before dispersal) owing to the high mortality that can occur during this period.     

BODY SIZE, ITS HERITABILITY AND INFLUENCE ON JUVENILE SURVIVAL AMONG GREAT TITS, PARUS MAJOR

This paper reports on part of a three-year study into the biological significance of variation in body size of individuals from a single population of Great Tits in Wytham Woods near Oxford, from 1973 to 1976. It is a reasonable hypothesis that the body size of an individual is controlled partly by heredity and partly by environmental factors. Genetical factors by themselves are shown to account for nearly three-quarters of the total observed phenotypic variance of body size. Of the likely environmental factors considered, only the date of hatching (in 1973) and an annual difference in body size are shown to have significant effects.
Previous workers have shown that heavier fledglings have a better chance of surviving the immediate post-fledging period than lighter fledglings (e.g., Perrins 1965). It had been postulated that this differential survival rate might be due to variations in the fat reserves of fledglings. However, the data presented here suggest that this hypothesis is unlikely, and that the body size of fledglings has a more significant influence. The aggressive behaviour of fledgling Great Tits towards one another may be the mechanism responsible for this since it is shown that larger fledglings tend to dominate smaller fledglings.     

Do fledgling and pre‐breeding Common Swifts Apus apus take part in aerial roosting? An answer from a radiotracking experiment

This study has shown that fledgling Common Swifts Apus apus spend the first post-fledging night of their life on the wing and that pre-breeders also spend the full night on the wing. Even though this work was conducted during an unusually cold, wet period, the results show that fledglings do not return to their natal colony in the week after fledging. It also demonstrates that yearlings are only slightly more likely than fledglings to spend any time at a particular colony, but are more likely to move from colony to colony. Older pre-breeders are more likely to spend more time at and revisit a particular colony more often than yearlings. It is our observation that only right at the end of the breeding duties will the parents participate in an evening ascent, and even then many of them return to their nest for the evening. Breeding adults displayed the greatest devotion to a particular colony. But even among such adults we detected some that ceased caring for their young a few days prior to their fledging. Adults roost in their nestbox if they meet with bad weather.     

Yearly variation in factors associated with local recruitment of Tree Swallows

Identifying factors that influence the chances of fledglings becoming local recruits can inform us about the ecology of population dynamics and factors influencing parental fitness. We studied Tree Swallows (Tachycineta bicolor) nesting in boxes in western Michigan from 1994 to 2005 and found that 145 of 2405 (6.0%) nestlings that fledged from 840 nests became local recruits. Most local recruits returned to breed at the study site the year after they fledged. The sex ratio of local recruits (76 males and 69 females) did not differ from 1:1. Analysis of data from all years combined revealed that the likelihood of fledglings becoming local recruits was not significantly influenced by the year they fledged, clutch initiation date, clutch size, brood size, age class and physical characteristics of female parents, or the physical characteristics of putative male parents. However, analysis of data from each year revealed that some variables had a significant effect on the likelihood of fledglings becoming local recruits in some years, including female age class in two years, female parent right tail fork length in one year, female parent right wing length in one year, clutch initiation date in two years, clutch size in two years, and brood size in one year. The proportion of fledglings produced each year that became local recruits was not correlated with either mean air temperature or total rainfall during June and July, the immediate post-fledgling period. Taken together, these analyses suggest that chance or yearly variation in some other factor(s) with the potential to affect post-fledging survival that we did not measure (e.g., predation post-fledging or climatic conditions during migration or in migratory stopover sites and wintering areas) had an important effect on local recruitment of fledgling Tree Swallows in our study. We did find that local recruits were more likely to be from nests where egg-laying began earlier rather than later in the season, with 137 of 145 (94%) of local recruits coming from clutches initiated between 5 and 30 May. Because swallows that fledged earlier in the season had more time to prepare for migration than those that fledged later, we hypothesize that the “relative age effect” had an important effect on local recruitment of Tree Swallows at our study site.     

Host–parasite coevolution beyond the nestling stage? Mimicry of host fledglings by the specialist screaming cowbird

Egg mimicry by obligate avian brood parasites and host rejection of non-mimetic eggs are well-known textbook examples of host-parasite coevolution. By contrast, reciprocal adaptations and counteradaptations beyond the egg stage in brood parasites and their hosts have received less attention. The screaming cowbird (Molothrus rufoaxillaris) is a specialist obligate brood parasite whose fledglings look identical to those of its primary host, the baywing (Agelaioides badius). Such a resemblance has been proposed as an adaptation in response to host discrimination against odd-looking young, but evidence supporting this idea is scarce. Here, we examined this hypothesis by comparing the survival rates of young screaming cowbirds and non-mimetic shiny cowbirds (Molothrus bonariensis) cross-fostered to baywing nests and quantifying the similarity in plumage colour and begging calls between host and cowbird fledglings. Shiny cowbirds suffered higher post-fledging mortality rates (83%) than screaming cowbirds (0%) owing to host rejection. Visual modelling revealed that screaming cowbirds, but not shiny cowbirds, were indistinguishable from host young in plumage colour. Similarly, screaming cowbirds matched baywings' begging calls more closely than shiny cowbirds. Our results strongly support the occurrence of host fledgling mimicry in screaming cowbirds and suggest a role of visual and vocal cues in fledgling discrimination by baywings.     

Foraging ecology of the California gnatcatcher deduced from fecal samples

The California gnatcatcher is a threatened species essentially restricted to coastal sage scrub habitat in southern California. Its distribution and population dynamics have been studied intensely, but little is known about its diet. We identified arthropod fragments in 33 fecal samples of the California gnatcatcher to gain insight into its foraging ecology and diet. Fecal samples were collected from adult males, adult females, fledglings, and nestlings. Leaf- and planthoppers (Homoptera) and spiders (Araneae) predominated numerically in samples. Spider prey was most diverse, with eight families represented. True bugs (Hemiptera) and wasps, bees, and ants (Hymenoptera) were only minor components of the gnatcatcher diet. Gnatcatcher adults selected prey to feed their young that was larger than expected given the distribution of arthropod size available in their environment, and chicks were provisioned with larger prey items and significantly more grasshoppers and crickets (Orthoptera) and spiders than adults consumed themselves. Both adults and young consumed more sessile than active prey. Further studies are needed to determine whether arthropods sampled in coastal sage scrub that are common in fecal samples are good indicators of California gnatcatcher habitat. Received: 30 December 1998 / Accepted: 28 April 1999     

Extreme gender-based post-fledging brood division in the toc-toc

The possibility that parents of one sex may preferentially investin offspring of a certain sex raises profound evolutionary questionsabout the relative worth of sons and daughters to their mothersand fathers. Post-fledging brood division—in which eachparent feeds a different subset of offspring—has beenwell documented in birds. However, a lack of empirical evidencethat this may be based on offspring sex, combined with the theoreticaldifficulty of explaining such an interaction, has led researchersto consider a gender bias in post-fledging brood division highlyunlikely. Here we show that in the toc-toc, Foudia sechellarum,post-fledging brood division is extreme and determined by sex;where brood composition allows, male parents exclusively provisionmale fledglings, whereas female parents provision female fledglings.This is the first study to provide unambiguous evidence, basedon molecular sexing, that sex-biased post-fledging brood divisioncan occur in birds. Male and female parents provisioned at thesame rate and neither offspring nor parent survival appearedto be affected by the sex of the parent or offspring, respectively.The current hypotheses predicting advantages for brood divisionand preferential care for one specific type of offspring arediscussed in the light of our results.