Rapid change in nest size of a bird related to change in a secondary sexual character

Among bird species in which males contribute to nest building,sexual selection has favored larger nests. I investigated determinantsof nest size in the barn swallow Hirundo rustica and how nestsize changed during the period 1977–2003, when tail length(a male secondary sexual character) increased by more than 1.2standard deviations. Males with short tails contributed moreto nest building than long-tailed males, signaling their futureinvestment in food provisioning of offspring. Pairs of barnswallows were consistent in nest size when build ing new neststhe same or different years, and level of phenotypic plasticityin nest size was small and could not account for temporal patternsin nest size. Offspring resembled their parents with respectto nest size, indicating a significant heritability of nestsize, independent of whether offspring were reared by theirparents or by foster parents, and there was a significant negativegenetic correlation between male tail length and outer nestvolume and amount of nest material. The temporal increase inmale tail length was associated with a decrease in nest size,with the amount of nest material in 2003 on average being lessthan a third of the amount used in 1977. Temporal change innest size could be accounted for by indirect selection on taillength causing change in nest size to match that predicted fromchange in tail length and the genetic correlation between maletail length and nest size.     

Avoidance,tolerance, and resistance to ectoparasites in nestling and adult tree swallows

We examined avoidance, tolerance, and resistance strategies of nestling and adult tree swallows Tachycineta bicolor in response to ectoparasitic blowflies Protocalliphora sialia. Tree swallows avoided settling in north‐facing nest boxes early in the breeding season. These boxes were more likely to be parasitized later in the season, suggesting that box selection may facilitate blowfly avoidance. After experimentally manipulating blowfly intensity, we found that nestlings were generally tolerant of parasitism. Parasites significantly reduced nestling blood hemoglobin but had no effect on nestling body mass, primary feather growth, age at fledging, or fledging success. Parents of parasitized nestlings did not increase their food provisioning rate to promote nestling tolerance. Adult female tree swallows demonstrated both tolerance and resistance: blowfly parasitism had no effect on adult hemoglobin and body mass, and those with higher P. sialia‐binding antibody levels had fewer blowfly larvae in their nests. Nestling antibodies were unrelated to blowfly intensity. Despite considerable variation among years, our results suggest that the costs of blowfly parasitism to nestling and adult tree swallows are modest, and limited to blood loss in nestlings. Future work should examine the effects of reduced blood hemoglobin on fledgling survival and the importance of parasite‐specific antibodies.     

An observational and brood-exchange sudy of paternal provisioning in the red-winged blackbird, Agelaius phoeniceus

Male red-winged blackbirds (Agelaius phoeniceus) provisioned50%(118/235) of available broods over an 8-year period. In mostcases, each male provisioned only the oldest brood on his territory,but in 21 cases, the male fed a younger brood after first feedingan older one. In 10 of these cases the switch to the youngerbrood followed predation of the older brood, but in the remaining11 cases the cause of the switch was unknown. To determine whethermale red-winged blackbirds adjust their provisioning when thedemands of their broods change, we exchanged broods betweenpairs of nests on 10 territories. Before each exchange, eachmale had been feeding nestlings of only one of the two broods.In response to the exchanges, male (and female) red-winged blackbirdsimmediately adjusted their rates of provisioning to the increasedor decreased demands of the broods. In addition, most malesswitched their provisioning within a day after the exchangeso that they fed the same broods as before the exchanges. Ourresults show that male red-winged blackbirds adjust their provisioningwhen the demands of their broods change and suggest that maleshave enough information about other nesting attempts on theirterritories to shift to a more profitable brood when the expectedbenefits of provisioning change. Male red-winged blackbirdsdo not appear to follow an investment strategy based on priorinvestments (i.e., they do not commit the "Concorde fallacy").     

Individual variation in parental tradeoffs between the number and size of offspring at the pre- and post-natal stages

Life-history theory predicts that parents refer to the resources they hold to determine their breeding strategy. In multi-brooded species, it is hypothesized that single-brooded parents produce larger clutches and raise offspring with a brood survival strategy, whereas multi-brooded parents only do this under good breeding conditions. Under poor conditions, they produce smaller clutches and raise offspring with a brood reduction strategy. We tested this hypothesis in the Brown-cheeked Laughing Thrush Trochalopteron henrici, which can breed twice a year on the Tibetan Plateau, by investigating the life-history traits and provisioning behaviours of single- and double-brooded parents. Single-brooded parents laid larger clutches of smaller eggs and produced more and larger fledglings than double-brooded parents in their first brood. Double-brooded parents produced smaller clutches of larger eggs but fledged larger nestlings in their first brood than in their second brood. As single-brooded parents only need to raise one brood a year, then producing and raising as many offspring as possible (i.e. the brood survival strategy in a large brood) can maximize their reproductive success. For double-brooded parents, producing and raising fewer offspring in the first brood (i.e. the brood survival strategy in a small brood) can ensure their nesting success during a short breeding cycle. Additionally, producing more offspring but raising larger nestlings in the second brood (i.e. the brood reduction strategy in a large brood) can select for offspring of higher quality within the brood. Our findings indicate that different tradeoffs between single- and double-brooded parents in egg-laying and nestling-raising may be an adaptation to the seasonal variation in environmental conditions.     

Field study of the relationship between personality and reproductive strategy in the White-collared Blackbird Turdus albocinctus

The discovery of personality traits in animal populations may help to explain individual variation in breeding strategies. In the White-collared Blackbird Turdus albocinctus, females, but not males, exhibited different nest defence behaviours that can be used to classify them into bold and shy personalities. Bold females had higher nest success, lower fecundity and higher parental investment than shy females, and the social mates of bold females had lower parental investment than that of shy females. Our findings suggest that the reproductive strategy of birds may be closely related to their personality traits.     

Growth rate variation among passerine species in tropical and temperate sites: an antagonistic interaction between parental food provisioning and nest predation risk

Causes of interspecific variation in growth rates within and among geographic regions remain poorly understood. Passerine birds represent an intriguing case because differing theories yield the possibility of an antagonistic interaction between nest predation risk and food delivery rates on evolution of growth rates. We test this possibility among 64 Passerine species studied on three continents, including tropical and north and south temperate latitudes. Growth rates increased strongly with nestling predation rates within, but not between, sites. The importance of nest predation was further emphasized by revealing hidden allometric scaling effects. Nestling predation risk also was associated with reduced total feeding rates and per-nestling feeding rates within each site. Consequently, faster growth rates were associated with decreased per-nestling food delivery rates across species, both within and among regions. These relationships suggest that Passerines can evolve growth strategies in response to predation risk whereby food resources are not the primary limit on growth rate differences among species. In contrast, reaction norms of growth rate relative to brood size suggest that food may limit growth rates within species in temperate, but not tropical, regions. Results here provide new insight into evolution of growth strategies relative to predation risk and food within and among species.     

Construction of Nest Defensive Structure According to Offspring Value and Its Effect on Predator's Attack Decision in Paper Wasps

Various organisms are known to build nests with defensive structures to protect their offspring from predation, but our understanding of plasticity in the nest structure remains poor. In this study, we investigated whether a paper wasp, Polistes chinensis antennalis, adjusted the construction of nest defensive structure according to the value of their offspring, and we also analysed the effect of adjusting the construction of the structure on predator's decision to attack. P. chinensis antennalis foundresses start a colony and maintain her nest alone until the emergence of workers. During this stage, foundresses often construct a defensive structure on cocoon caps of pupae using nest materials (pulp), which prevents predation of pupae by conspecifics from other nests. The value of pupae to the foundress varies among those in a nest, where the value is higher in pupae that spun the cocoon (and initiated pupation) earlier than other pupae in the nest. From field observations, we found that foundresses constructed a larger pulp structure on the cocoons of pupae that cocooned earlier in the order of cocoon spinning, even after considering confounding factors. We also found that the probability of a pupa being attacked by conspecific intruders decreased with the size of pulp structure on the cocoon. This indicates that intruders avoid attacking cocoons with larger pulp structures. Our study indicates that foundresses adjust the construction of nest defensive structures according to their offspring value, and this allows them to protect the high‐value offspring efficiently and effectively.     

Sexual conflict over parental care promotes the evolution of sex differences in care and the ability to care

Strong asymmetries in parental care, with one sex providing more care than the other, are widespread across the animal kingdom. At present, two factors are thought to ultimately cause sex differences in care: certainty of parentage and sexual selection. By contrast, we here show that the coevolution of care and the ability to care can result in strong asymmetries in both the ability to care and the level of care, even in the absence of these factors. While the coevolution of care and the ability to care does not predict which sex evolves to care more than the other, once other factors give rise to even the slightest differences in the cost and benefits of care between the sexes (e.g. differences in certainty in parentage), a clear directionality emerges; the sex with the lower cost or higher benefit of care evolves both to be more able to care and to provide much higher levels of care than the other sex. Our findings suggest that the coevolution of levels of care and the ability to care may be a key factor underlying the evolution of sex differences in care.     

Energy expenditure, nestling age, and brood size: an experimental study of parental behavior in the great tit Parus major

A brood manipulation experiment on great tits Parus major was performedto study the effects of nestling age and brood size on parentalcare and offspring survival. Daily energy expenditure (DEE)of females feeding nestlings of 6 and 12 days of age was measuredusing the doubly-labeled water technique. Females adjusted theirbrooding behavior to the age of the young. The data are consistentwith the idea that brooding behavior was determined primarilyby the thermoregulatory requirements of the brood. Female DEEdid not differ with nestling age; when differences in body masswere controlled for, it was lower during the brooding periodthan later. In enlarged broods, both parents showed significantlyhigher rates of food provisioning to the brood. Female DEE wasaffected by brood size manipulation, and it did not level offwith brood size. There was no significant effect of nestlingage on the relation between DEE and manipulation. Birds wereable to raise a larger brood than the natural brood size, althoughlarger broods suffered from increased nestling mortality ratesduring the peak demand period of the nestlings. Offspring conditionat fledging was negatively affected by brood size manipulation,but recruitment rate per brood was positively related to broodsize, suggesting that the optimal brood size exceeds the naturalbrood size in this population.     

The general protected invasion theory: Sex biases in parental and alloparental care

The biases towards eusociality, female workers and maternal care in haplodiploid versus diploid insects may result from the relatively low probabilities that rare mutant, partially dominant alleles promoting these behaviours will be lost by genetic drift in haplodiploid populations (Reeve, 1993). A generalization of this 'protected invasion' theory also predicts that parental and alloparental care will tend to be associated with the homogametic sex in diploid populations if the Y chromosome of the heterogametic sex is absent or largely inert. Sex differences in (allo)parental care (i.e. either parental or alloparental care) should increase with increased asymmetry between the sexes in the fraction of behaviour-influencing loci occurring on their characteristic sex chromosomes. The theory explains the strong predisposition towards female (allo)parental care in mammals, a contrasting tendency towards male (allo)parental care in birds, the propensity for joint male and female (allo)parental care in termites, and biases towards female cooperation in social spiders. The theory also explains the apparent rarity or absence of alloparental care in marsupials, an intriguing consequence of preferential paternal X-chromosome inactivation in this taxon. Thus protected invasion theory possibly provides new insights into the relationship between social structure and the genetic system. The theory does not compete with ecological or kin-selective hypotheses for the advantages of (allo)parental care; indeed, such advantages must exist for protected-invasion biases to operate.     

Male and female contributions to provisioning rates of thin-billed prions, Pachyptila belcheri , in the South Atlantic

We recorded individual provisioning patterns of male and female thin-billed prions in the largest known colony on New Island (Falkland Islands, South Atlantic Ocean), to determine male and female contributions to provisioning rates and to compare them with data from their other main breeding site, on Kerguelen, in the Southern Indian Ocean. Both male and female birds from New Island undertook trips of 1–8 days’ duration (median 3 days), similar to those on Kerguelen, although we did not observe a clear distinction between long and short trips. Meal sizes increased with longer trips but did not differ between male birds and female birds. Thus, as on Kerguelen, male and female thin-billed prions on New Island contributed equally to chick provisioning. The proportion of short trips was comparable to those observed in a more favourable year, and was higher than those in a poor year, on Kerguelen. In order to avoid potential disturbance to the birds in the colony by frequent capture and handling, we used miniature radio-tags to record the presence of individuals in the colony and to evaluate the influence of transmitter load on the birds. Despite generally deteriorating conditions towards the end of the breeding season, chicks of tagged adults were fed normally, indicating that the method is adequate.     

Nestling begging intensity and parental effort in relation to prelaying carotenoid availability

Carotenoids are antioxidants playing major roles in physiologicalfunctions at various stages of an animal's life. Female birdsdeposit large amounts of carotenoids into their eggs. Carotenoidsare, however, a limiting resource, and females are expectedto balance carotenoid deposition into the eggs with their utilizationfor themselves. Carotenoid availability is thus likely to determineboth the levels of yolk carotenoids and maternal care duringrearing. Carotenoids have been shown to benefit the embryo andthe growing nestling, and it can be hypothesized that an increasein carotenoid availability during laying leads to higher nestlingcondition and competitive ability. We manipulated carotenoidavailability to great tit pairs prior to and during egg layingand later partially cross-fostered chicks at hatching. Duringthe rearing period, we measured how carotenoid availabilityaffected nestlings begging behavior and male and female feedingeffort. We also manipulated the ectoparasite load, predictingthat carotenoid supplementation would help adults and nestlingto cope with parasites. Nestlings hatched from eggs laid bycarotenoid-supplemented females and raised in small broods beggedmore intensely. Nestlings in small deparasitized broods alsobegged more actively. The feeding effort of control femalesincreased with brood size, whereas the feeding effort of carotenoid-supplementedfemales was high whatever the brood size. Male feeding effortwas unaffected by our treatment. Our results support the hypothesisthat maternally derived carotenoids increase nestling beggingbehavior and hence competitive ability. They further suggestthat carotenoid availability determines the level of parentalinvestment and can mediate trade-offs between life-history traits.     

Dynamic switching in predator attack and maternal defence of prey

Predator and prey relationships are dynamic and interrelated. Thus, any offensive behaviour will vary according to differing defensive behaviours, or vice versa, within each species in any predator–prey system. However, most studies are one‐sided as they focus on just one behaviour, that of either the predator or prey. Here, we examine both predatory behaviour of an oophagus katydid and antipredator behaviour by a frog with egg‐stage parental care. Katydid offensive behaviour and predation success was greater in females and increased with predator maturity and size. Frog defensive behaviour was sex specific, probably because only mothers provide parental care. Defensive behaviour could be active, such as charging predators, or passive, such as sheltering eggs, with greater active defence against larger predators; neither was influenced by offspring age. These results are contrary to existing theory, which argues parental investment ought to be negatively correlated with parental predation risks and affected by offspring age. This study highlights the use of antipredator behaviour to test predictions of parental investment theories in amphibians. In addition, it illustrates the need to consider factors that influence both species concurrently when examining the complex interaction between predators and parents.