WHY DO SOME SIBLINGS ATTACK EACH OTHER? COMPARATIVE ANALYSIS OF AGGRESSION IN AVIAN BROODS

In many parentally fed species, siblings compete for food not only by begging and scrambling, but also by violently attacking each other. This aggressive competition has mostly been studied in birds, where it is often combined with dominance subordination, aggressive intimidation, and siblicide. Previous experimental and theoretical studies proposed several life-history, morphological, and behavioral variables that may facilitate the evolution of broodmate aggression, and explain its taxonomic distribution. Here we apply phylogenetic comparative analyses for the first time to test the influence of five hypothesized facilitators of the evolution of broodmate aggression, analyzing 69 species in seven avian families using two quantitative measures of aggression: incidence and intensity. We show that incidence and intensity of aggression increase with long nestling periods and indirect feeding, and small brood size is associated with intense aggression. Large food parcels were not correlated with either the incidence or intensity of aggression. Our study suggests that indirect feeding, long nestling periods, and small broods, possibly in combination with other factors, have tended to favor the evolution of aggressive broodmate competition.     

Breeding behaviour and time-activity budgets of Bonelli’s Eagles Aquila fasciata: marked sexual differences in parental activities

ABSTRACT

Capsule: Differences in parental investment between sexes and stage of the breeding period were found in Bonelli’s Eagles Aquila fasciata.

Aims: To describe the sexual differences in parental behaviour of Bonelli’s Eagles and to assess the sex-specific pattern of variation in parental investment in relation to the breeding period.

Methods: Between 2006 and 2016, we monitored the parental behaviour of 11 pairs of Bonelli’s Eagles during the incubation and chick-rearing periods in southeastern Spain. Observations were made using 20–60× telescopes from points overlooking the territory at a distance of about 500–800?m from the nest.

Results: Our results reveal a marked division in parental duties in Bonelli’s Eagles. Females invested more effort in incubation, nest attendance, chick feeding and nest-building, while males contributed more to food provisioning. Nest attendance and feeding by females decreased with time, and both parents adjusted their provisioning effort in relation to nestling age. Most changeovers took place during the middle of the day, when male provisioning rates and temperatures reach their maximum.

Conclusion: Intersexual differences are discussed in the context of the prey capture difficulty hypothesis, which proposes that intra-pair prey differences, due to large sexual size dimorphism, should be particularly advantageous among raptors that pursue agile prey.     

THE EFFECTS OF PREDATION ON THE AGE AND SIZE OF MATURITY OF PREY

The effects of nonselective predation on the optimal age and size of maturity of their prey are investigated using mathematical models of a simple life history with juvenile and adult stages. Fitness is measured by the product of survival to the adult stage and expected adult reproduction, which is usually an increasing function of size at maturity. Size is determined by both age at maturity and the value of costly traits that increase mean growth rate (growth effort). The analysis includes cases with fixed size but flexible time to maturity, fixed time but flexible size, and adaptively flexible values of both variables. In these analyses, growth effort is flexible. For comparison with previous theory, models with a fixed growth effort are analyzed. In each case, there may be indirect effects of predation on the prey's food supply. The effect of increased predation depends on (1) which variables are flexible; (2) whether increased growth effort requires increased exposure to predators; and (3) how increased predator density affects the abundance of food for juvenile prey. If there is no indirect effect of predators on prey food supply, size at maturity will generally decrease in response to increased predation. However, the indirect effect from increased food has the opposite effect, and the net result of predation is often increased size. Age at maturity may either increase or decrease, depending on functional forms and parameter values; this is true regardless of the presence of indirect effects. The results are compared with those of previous theoretical analyses. Observed shifts in life history in response to predation are reviewed, and the role of size-selective predation is reassessed.     

The Effects of Experimentally Elevated Testosterone and Food Deprivation on Food Consumption and Prey Size Preferences in Male Dark-Eyed Juncos (Junco hyemalis, Emberizidae: Passeriformes)

Numerous studies have shown that the experimental elevation of circulating levels of testosterone reduces parental behaviour in male birds, particularly the provisioning of young. The mechanisms responsible for this change in behaviour are not fully understood. In this study, we examine the effects of elevated testosterone on food consumption and prey selection, both of which have potential consequences for nestling provisioning behaviour. We manipulated testosterone and performed two experiments on a captive, non-breeding population of male dark-eyed juncos ( Junco hyemalis ) on long day-lengths. In the first experiment, we subjected juncos to 3 h of food deprivation and compared food consumption and prey size selection by males with elevated testosterone (testosterone males) to that of control males. Testosterone males consumed more food than control males and showed a preference for larger prey. In a second experiment in which small prey were more abundant than large prey, food consumption and prey size preferences did not differ between testosterone and control males. We also manipulated the duration of food deprivation in the second experiment. Males of both treatments consumed more small prey under conditions of mild (1 h) or moderate (5 h) food deprivation and consumed more large prey under conditions of intermediate (3 h) food deprivation. We discuss our results and the effects that testosterone has on self-maintenance behaviour and male parental effort.     

Macroevolutionary evidence suggests trait‐dependent coevolution between behavior and life‐history

Species with fast life‐histories typically prioritize current over future reproductive events, compared to species with slow life‐histories. These species therefore require greater energetic input into reproduction, and also likely have less time to realize their reproductive potential. Hence, behaviors that increase access to both resources and mating opportunities, at a cost of increased mortality risk, could coevolve with the pace of life‐history. However, whether this prediction holds across species, remains untested under standardized conditions. Here, we test how risky behaviors, which facilitate access to resources and mating opportunities (i.e., activity, boldness, and aggression), along with metabolic rate, coevolve with the pace of life‐history across 20 species of killifish that present remarkable divergences in the pace of life‐history. We found a positive association between the pace of life‐history and aggression, but interestingly not with other behavioral traits or metabolic rate. Aggression is linked to interference competition, and in killifishes is often employed to secure mates, while activity and boldness are more relevant for exploiting energetic resources. Our results suggest that the trade‐off between current and future reproduction plays a more prominent role in shaping mating behavior, while behaviors related to energy acquisition may be influenced by ecological factors.     

Diet of Starling Sturnus vulgaris nestlings on farmland: the importance of Tipulidae larvae

Capsule?Tipulidae larvae are a key resource for adult Starlings provisioning their young on lowland farmland.

Aims?(i) To describe Starling nestling diet on lowland farmland. (ii) To examine the effects of brood size variation and nestling age on Starling parental food provisioning.

Methods?Over 4 years, we observed parental food provisioning at 42 nests across 4 sites on lowland farmland in Oxfordshire, UK using nestbox video recorders.

Results?Tipulidae larvae were the most frequent prey item recorded in provisioning loads (52%), the next most frequent were winged insects (11%), then Coleoptera larvae (6%), Lepidoptera larvae (5%) and Lumbricidae (5%), with 21% of prey unidentified. Estimates of prey lengths, dry mass and total energy content also confirmed that Tipulidae larvae were the key prey source. Generalized linear mixed-effects models showed that larger broods received fewer Tipulidae larvae per nestling per day and that the proportion of Tipulidae larvae in the diet did not vary with brood size or nestling age.

Conclusion?Our results support the idea that Tipulidae larvae are the primary dietary item for nestling Starlings on lowland farmland.     

Sibling competition in a brood-tending leech

Conflict among siblings over parental investment, particularly over parental feeding, is a feature of family life in many kinds of animals. In some bird species, the size of prey items provided to juveniles has been implicated as a cause of aggressive competition among sibling chicks, because prey size determines whether dominance allows monopolization of parental offerings. Our experiment was meant to test the generality of this factor in creating intrafamilial conflict. We investigated sibling competition in relation to prey size using the carnivorous, brood-tending leech Helobdella papillornata. We equalized the total amount of food available to H. papillornata broods, but varied the size of individual prey items. Competition, measured by disparity in body size at independence, was more intense in broods provisioned with small items than in broods receiving large items, but similar between broods receiving large items and broods fed ad libitum. These patterns suggest that the intensity of conflict did not depend only on the total food amount, but was enhanced by small prey size. Our results indicate that conflict over the provision of parental resources to offspring can have a similar basis across very dissimilar organisms.     

A comparison of life‐history and parental care in temperate and tropical wrens

Comparing closely related species that live in different environments is a powerful way to understand selective pressures that influence life‐history evolution. We examined a suite of life‐history traits and parental care in neotropical buff‐breasted wrens Cantorchilus leucotis and north‐temperate Carolina wrens Thryothorus ludovicianus (Family Troglodytidae), to test hypotheses about life‐history evolution. As expected, buff‐breasted wrens exhibited smaller clutch sizes and higher annual adult survival than Carolina wrens. We found minimal support for the nest predation hypothesis, as nest survival and age‐corrected provisioning rates to whole broods were similar between species, and number of breeding attempts and breeding season length were greater in temperate wrens. Critical predictions of the food limitation hypothesis were not supported; in particular age‐corrected provisioning rates per nestling were higher in the tropical than temperate species. The adult survival and offspring quality hypothesis garnered the most support, as buff‐breasted wrens exhibited greater age‐corrected provisioning rates per nestling, a longer nestling period, longer re‐nesting intervals following nest success, and lower annual fecundity than Carolina wrens. Despite similarly prolonged breeding seasons, reproductive strategies differ between species with buff‐breasted wrens investing considerably in single broods to optimize first‐year survival and Carolina wrens investing in multiple small broods to optimize annual fecundity.     

Kind to kin: weak interference competition among white stork Ciconia ciconia broodmates

Altricial nestlings in structured families show a diverse array of behavioural mechanisms to compete for food, ranging from signalling scrambles to aggressive interference. Rates of filial infanticide are moderately high in white storks. It has been hypothesized that this unusual behaviour is an adaptive parental response to the absence of efficient mechanisms of brood reduction (aggression or direct physical interference) by nestlings. To test this latter assumption, we analyzed video recordings of 41 complete feeding episodes at 32 broods during the first half of the nestling period, when nestlings complete 90% of growth and chick mortality and size asymmetries are highest. Parents delivered food to all nestlings simultaneously by regurgitating on the nest floor. No direct (bill to bill) feeding was recorded. Senior nestlings were never observed to limit their junior nestlings from eating food, either by aggression or physical interference. Experimental feeding tests revealed that heavier nestlings handled prey items more efficiently and ate food at a higher speed. The high degree of tolerance shown by senior nestlings is unusual among birds with similar ecological and phylogenetic affinities, such as herons. Tolerance by seniors cannot be easily explained by absence of parental favouritism or proximate factors known to affect the occurrence of sibling aggression in other species (rate of food transfer, brood size, hatching asynchrony or length of nestling period).     

A review of the ecological parameters and implications of subsociality in Parastrachia japonensis (Hemiptera: Cydnidae), a semelparous species that specializes on a poor resource

Subsocial behavior or postovipositional parental care in insects has evolved in response to a variety of environmental stresses and ranges from briefly guarding eggs after oviposition to elaborate nidification and provisioning behaviors. Investment in parental care bears various costs, and should not continue beyond the point at which the costs to future reproductive success exceed the benefits to current reproductive effort. Progressive provisioning is a rare form of subsociality in insects. Females of the subsocial shield bug Parastrachia japonensis progressively provision their nymph-containing nests with drupes of the single resource, Schoepfia jasminodora, and this provisioning drastically enhances offspring survival. A female rears only one brood throughout her lifetime and continues provisioning the brood until about the third larval stadium, when the female dies. Thus, the female's entire reproductive effort is expressed in the success of that one brood, which suggests a reproductive strategy with enormous costs and risks. Why has such an extreme life history evolved in this species? Here, to answer that question and to contribute to our understanding of the evolutionary implications of subsocial behavior, in particular, progressive provisioning, we review what we have discovered about the ecological parameters of subsociality in this species during a long-term field study. We also discuss these parameters in P. japonensis in reference to other subsocial insects and related species and conclude with a suggestion that semelparity and progressive provisioning in this species are extreme adaptations to evolving complete dependency on an unreliable resource.     

Effects of provisioning on the social behaviour of Japanese and rhesus macaques: Implications for socioecology

Long-term studies have shown remarkable similarity in the social behaviour and relationships of Japanese and rhesus macaques living in free-ranging groups. The vast majority of these studies have been of provisioned groups and many key principles have been derived from them. Provisioning is known to influence various aspects of life history and demography, as well as quantitative aspects of social behaviour, such as the frequencies of grooming and aggression. It has been widely assumed, however, that the fundamental characteristics of social behaviour and relationships observed in provisioned populations are representative of those that would occur under natural conditions. This paper reviews findings from fieldwork on Japanese macaques living under natural conditions, and compares them with patterns of social behaviour reported by multiple studies of provisioned groups of both species. Differences are apparent in the nature of social relationships between adult females, between adult males, and between adult males and females. Some of these differences can be attributed to the increased levels of aggression associated with provisioning. Others appear to be related to demographic peculiarities of provisioned groups, such as large size and skewed sex ratio. These differences can be used to generate predictions concerning the influence of ecological variables on the dynamics of social relationships and social structure. Ways in which these predictions could be tested by further fieldwork on provisioned and natural populations are discussed.     

Messy eaters: Swabbing prey DNA from the exterior of inconspicuous predators when foraging cannot be observed

Complex coevolutionary relationships among competitors, predators, and prey have shaped taxa diversity, life history strategies, and even the avian migratory patterns we see today. Consequently, accurate documentation of prey selection is often critical for understanding these ecological and evolutionary processes. Conventional diet study methods lack the ability to document the diet of inconspicuous or difficult‐to‐study predators, such as those with large home ranges and those that move vast distances over short amounts of time, leaving gaps in our knowledge of trophic interactions in many systems. Migratory raptors represent one such group of predators where detailed diet studies have been logistically challenging. To address knowledge gaps in the foraging ecology of migrant raptors and provide a broadly applicable tool for the study of enigmatic predators, we developed a minimally invasive method to collect dietary information by swabbing beaks and talons of raptors to collect trace prey DNA. Using previously published COI primers, we were able to isolate and reference gene sequences in an open‐access barcode database to identify prey to species. This method creates a novel avenue to use trace molecular evidence to study prey selection of migrating raptors and will ultimately lead to a better understanding of raptor migration ecology. In addition, this technique has broad applicability and can be used with any wildlife species where even trace amounts of prey debris remain on the exterior of the predator after feeding.     

Effects of food supplementation on a tropical bird

Tropical birds typically exhibit a ‘slow pace of life’ relative to higher latitude species. This is often manifested as slow development, low fecundity, and high survival. Following from this, it is predicted that tropical birds may be more likely to trade current reproductive effort to favor self-maintenance, thus supporting survival and future reproduction. To test this idea, we conducted two food supplementation experiments on tropical rufous-collared sparrows (Zonotrichia capensis) in the eastern Andes of Ecuador. In the first experiment, we food-supplemented pairs during the non-breeding life-history stage, and in the second experiment, we food-supplemented pairs that were provisioning fledglings. In both experiments, a larger proportion of food-supplemented birds exhibited pre-basic molt (replacement of feathers) than in a control group. To our knowledge, this is the first study to experimentally demonstrate that a food-supplemented bird invests extra resources into molt, a form of self-maintenance, and contrasts with the majority of food supplementation studies in high latitude birds that show they typically advance the initiation of, or extend the period of, reproduction. Our results are consistent with the syndrome of the slow pace of life in the tropics and support the concept of fundamental differences between temperate-zone and tropical birds.     

Parental care in the Small Tree Finch Camarhynchus parvulus in relation to parasitism and environmental factors

The parental food compensation hypothesis suggests that parents may compensate for the negative effects of parasites on chicks by increased food provisioning. However, this ability differs widely among host species and may also depend on ecological factors such as adverse weather conditions and habitat quality. Although weed management can improve habitat quality, management measures can bring about a temporary decrease in food availability and thus may reduce parents’ ability to provide their nestlings with enough energy. In our study we investigated the interaction of parasitism and weed management, and the influence of climate on feeding rates in a Darwin’s tree finch species, which is negatively impacted by two invasive species. The larvae of the invasive parasitic fly Philornis downsi ingest the blood and body tissues of tree finch nestlings, and the invasive Blackberry Rubus niveus affects one of the main habitats of Darwin’s tree finches. We compared parental food provisioning of the Small Tree Finch Camarhynchus parvulus in parasitized and parasite‐free nests in three different areas, which differed in invasive weed management (no management, short‐term and long‐term management). In a parasite reduction experiment, we investigated whether the Small Tree Finch increases food provisioning rates to nestlings when parasitized and whether this ability depends on weed management conditions and precipitation. Our results provide no evidence that Small Tree Finches can compensate with additional food provisioning when parasitized with P. downsi. However, we found an increase in male effort in the short‐term management area, which might indicate that males compensate for lower food quality with increased provisioning effort. Furthermore, parental food provisioning was lower during rainfall, which provides an explanation for the negative influence of rain on breeding success found in earlier studies. Like other Darwin’s finches, the Small Tree Finch seems to lack the ability to compensate for the negative effects of P. downsi parasitism, which is one explanation for why this invasive parasite has such a devastating effect on this host species.     

Brood size manipulations in Great Horned Owls Bubo virginianus: are predators food limited at the peak of prey cycles?

The reproduction of raptors strongly depends on food resources. It is unclear whether predators experience superabundant food during cyclic peaks of prey populations. In order to test this hypothesis, four pairs of Great Horned Owls Bubo virginianus with two young were subjected to brood size manipulations during high densities of cyclic Snowshoe Hare Lepus americanus populations in southwestern Yukon, Canada. Broods older than 35 days were temporarily enlarged by one, and then by two, young. No effects were observed when one owlet was added, but the addition of two young resulted in significant weight losses in manipulated broods. Females with enlarged broods moved farther from their nest sites at night, presumably reflecting increased hunting effort, and also spent less time near the nest during the day. Food additions to enlarged broods returned the parental behaviour to normal. We conclude that these large predators did not experience superabundant food at this stage of the breeding season during a peak in cyclic prey.     

Song as an indicator of male parental effort in the sedge warbler

Repertoire size has been found to be a sexually selected trait in a number of bird species, although the advantages of mating with a male who possesses a complex song remain unclear. We studied the potential role of song as an indicator of male parental effort in the sedge warbler Acrocephalus schoenobaenus. The male provisioning rate was used as a measure of male parental effort and was found to increase with nestling age and brood size. When controlling for chick age, brood size and other variables, we found a highly significant positive correlation between a measure of song complexity (repertoire size) and male parental effort. Both male parental effort and repertoire size were found to be positively correlated with chick weight when controlling for chick age. We found no correlation between a measure of song output (amount of song flighting) or territory size and parental effort. Repertoire size is known to be the most important cue in female choice amongst sedge warblers and we discuss the possible reasons for this. We suggest that, in choosing a male with a large repertoire, a female obtains not only indirect benefits but also direct benefits in the form of increased parental effort.     

Costs of egg-laying and offspring provisioning: multifaceted parental investment in a digger wasp

Nest-building Hymenoptera have been a major testing ground for theories of parental investment and sex allocation. Investment has usually been estimated by the likely costs of offspring provisioning, ignoring other aspects of parental care. Using three experimental treatments, we estimated the costs of egg-laying and provisioning separately under field conditions in a digger wasp Ammophila pubescens. In one treatment, we increased the provisioning effort required per offspring by removing alternate prey items as they were brought to the nest. In two other treatments, we reduced parental effort by either preventing females from provisioning alternate nests or preventing them from both ovipositing and provisioning. Our results indicate that both egg-laying and provisioning represent significant costs of reproduction, expressed as differences in productivity but not survival. A trade-off-based model suggests that other components of parental care such as nest initiation may also represent significant costs. Costs of egg production and nest initiation are probably similar for male and female offspring, so that taking them into account leads to a less male-biased expected sex ratio. Mothers compensated only partially for prey removal in terms of the total provisions they gave to individual offspring.     

Bill harnesses on nestling Tufted Puffins influence adult provisioning behavior

ABSTRACT.   For burrow-nesting seabirds, investigators have examined nestling diet by attaching harnesses to the bills of nestlings to intercept food delivered by the parent. To determine whether this method provides an unbiased estimate of nestling diet, we evaluated its effect on the provisioning behavior of Tufted Puffins ( Fratercula cirrhata ) nesting on Triangle Island, British Columbia. Adults delivering food to nestlings with bill harnesses always hesitated before entering a burrow with food, increasing their susceptibility to kleptoparasitism by gulls, and did not always leave the food intended for the nestling. These responses by adult puffins could lead to underestimates of energy intake rates of nestlings and unreliable comparisons with other species if prey left by adults in nest burrows were the only source of data. We also compared estimates of the species, number, and size of prey delivered by adult puffins as determined by direct observation from blinds to samples of prey collected directly from nest burrows and found that the two sampling techniques produced similar results. However, identifying rare prey species and gathering precise information about prey length, mass, and condition require collection of prey, and we recommend using a combination of techniques to obtain the most reliable estimates of nestling diet.     

Evaluation of Trail-Cameras for Analyzing the Diet of Nesting Raptors Using the Northern Goshawk as a Model

Diet studies present numerous methodological challenges. We evaluated the usefulness of commercially available trail-cameras for analyzing the diet of Northern Goshawks (Accipiter gentilis) as a model for nesting raptors during the period 2007–2011. We compared diet estimates obtained by direct camera monitoring of 80 nests with four indirect analyses of prey remains collected from the nests and surroundings (pellets, bones, feather-and-hair remains, and feather-hair-and-bone remains combined). In addition, we evaluated the performance of the trail-cameras and whether camera monitoring affected Goshawk behavior. The sensitivity of each diet-analysis method depended on prey size and taxonomic group, with no method providing unbiased estimates for all prey sizes and types. The cameras registered the greatest number of prey items and were probably the least biased method for estimating diet composition. Nevertheless this direct method yielded the largest proportion of prey unidentified to species level, and it underestimated small prey. Our trail-camera system was able to operate without maintenance for longer periods than what has been reported in previous studies with other types of cameras. Initially Goshawks showed distrust toward the cameras but they usually became habituated to its presence within 1–2 days. The habituation period was shorter for breeding pairs that had previous experience with cameras. Using trail-cameras to monitor prey provisioning to nests is an effective tool for studying the diet of nesting raptors. However, the technique is limited by technical failures and difficulties in identifying certain prey types. Our study also shows that cameras can alter adult Goshawk behavior, an aspect that must be controlled to minimize potential negative impacts.     

Are colorful males of great tits Parus major better parents? Parental investment is a matter of quality

Given the known influence of parental investment on breeding success of great tits Parus major, females should be expected to use male parental quality as an essential criterion in mate choice. Since parental quality cannot usually be observed directly at the time of pairing, it has been suggested that females rely on male ornaments as indicative of their ability to provide parental care. This hypothesis, called the good parent hypothesis, has been tested repeatedly assessing only parental effort as the number of feedings made by parents. However, in evaluating parental investment, the focus should also be on the quality of prey captured rather than only on its quantity. We analyzed feeding rates and the provisioning of different prey in relation to both male yellow carotenoid-based breast coloration and the size of the black melanin-based stripe in a Mediterranean great tit population. We predicted that more carotenoid ornamented individuals would feed nestlings with a diet consisting of a higher proportion of caterpillars. However, and contrary to predictions, we found that males with higher values of hue in the yellow breast feathers, fed their offspring with a lower proportion of caterpillars and a higher proportion of spiders. In addition, nestlings that received a higher proportion of spiders showed an improved body condition after controlling for tarsus length and other variables. Male feeding rates correlated positively with brood size and tended to correlate negatively with date, although we did not find any effect of male coloration. Our data therefore support the good parent hypothesis, insofar as parental investment is also a matter of quality, and that, at least in the Mediterranean area, caterpillars are not the only key food source.