Effect of alarm calling by male Red‐winged Blackbirds on nestling begging and female provisioning behavior

ABSTRACT Nestling begging and parental provisioning can attract nest predators and reduce reproductive success, so parents and their offspring might be expected to respond adaptively by minimizing predator‐attracting cues when predators threaten nests. Male Red‐winged Blackbirds (Agelaius phoeniceus) are well known for their antipredator alarm calls that contain information about the approach of potential nest predators. We examined the begging behavior of nestlings and the provisioning behavior of females in response to antipredator alarm calls of males to test the adaptive response hypothesis. Playback experiments provided no evidence that alarm calls function to switch off vocal begging; nestlings were equally likely to beg vocally during playback and control periods. Video recordings showed that male alarm calling had no significant effect on inappropriate vocal begging (in the absence of an adult), but significantly reduced the incidence of spontaneous calling (in the absence of begging). Adult females responded to male antipredator alarm calls by delaying their provisioning visits. In addition, although having no significant effect on use of nest‐arriving calls by females, male alarm calling significantly reduced their use of nest‐leaving calls. We conclude that nestling and female Red‐winged Blackbirds respond to male alarm calls in ways that might reduce the risk of predation, but nestlings beg vocally when females arrive to feed them, regardless of male alarm calling, perhaps to avoid a competitive disadvantage with broodmates.     

Effects of age and experience on the responses of territorial and floater male Red‐winged Blackbirds to models of receptive females

The ability of sexually mature non‐territorial floaters to sire offspring affects the success of floating as a breeding strategy. Red‐winged Blackbirds (Agelaius phoeniceus) have second‐year (SY) and after‐second‐year (ASY) floater males, and genetic studies suggest that floaters may gain paternity. Despite these studies, we still know little about the fitness costs and benefits of floating in this species. By presenting taxidermic models of females in soliciting, precopulatory postures in territories of experienced (previously attracted at least one mate in the study area) and inexperienced (did not previously defend a territory in the study area) males, I was able to examine the copulation behavior and success of floater male Red‐winged Blackbirds as well as the effect of experience for territorial males. Floaters trespassed during 66.1% of presentations and 85.4% of trespassers were SY males. Experienced territorial males (92.5%) and neighbors (87.5%) were most successful in attempts to copulate with models, inexperienced territorial males (62.5%) and ASY floaters (50.0%) had intermediate success, and SY floaters (6.9%) were least successful. Experienced territorial males were more likely to approach models than inexperienced males, and floaters were more likely to approach models in territories of experienced than inexperienced males. These results provide further evidence that floaters trespass frequently, suggest that floaters sire offspring, and demonstrate that prior breeding experience affects the behavior and reproductive success of territorial male Red‐winged Blackbirds. Floating appears to be a conditional strategy for ASY male Red‐winged Blackbirds, but, because it is still not known if SY floaters sire offspring, these males may be trespassing to gain information or experience.     

Responses of territorial and floater male Red‐winged Blackbirds to models of receptive females

Red‐winged Blackbirds (RWBL; Agelaius phoeniceus) have a polygynous mating system and, because territorial males commonly have harems of two to five females, some second‐year (SY) and after‐second‐year (ASY) males do not establish nesting territories, but become floaters. Previous studies have revealed high rates of extra‐pair copulations in this species and that sexually mature male floaters and territory owners do not differ in size, testosterone levels, or reproductive capability, suggesting that floaters may occasionally gain paternity. During May and June 2008, we observed the behavioral responses of floater males to taxidermic mounts (models) of female RWBL placed in a precopulatory position. Floaters intruded into territories during 46% of model presentations, with 20% of intrusions by ASY floaters and 80% by SY floaters. During intrusions, ASY floaters attempted to copulate with models 93% of the time compared to 80% for SY floaters. Copulations were successful during 30% of attempts by ASY males and 25% of attempts by SY floaters. The frequency of intrusions by ASY and SY floaters, attempted copulations by SY floaters, and successful copulations by ASY floaters increased as territorial males spent more time off their territories. Responses of floater males toward models in our study suggest that floater male RWBL attempt to exploit available breeding opportunities. The lack of evidence for extrapair young (EPY) fathered by floater male RWBL in previous studies, combined with our results indicating that the presence of territorial males limits floater intrusions, copulation attempts, and successful copulations, suggests that the reproductive success of floater males is limited in part by the aggressive behavior of territorial males.     

Limits to sexual size dimorphism in red‐winged blackbirds: the cost of getting big?

A fundamental assumption of sexual selection theory is that the reproductive advantage of large size is balanced by a survival disadvantage. Previous studies of the sexually size-dimorphic red-winged blackbird ( Agelaius phoeniceus ) have indicated that the largest adult males have a survival advantage, suggesting that the limit to male size may be the cost of getting big rather than the cost of being big. If the cost of getting big limits male size, then starvation rates for male nestlings should exceed those of female nestlings. In addition, given high heritability of body size, larger parents should lose more nestlings, particularly males, to starvation. We tested these predictions for red-winged blackbirds using data on the sex of 1356 fledglings from 465 nests collected over 10 years. We found no disadvantage for male nestlings relative to females – 49% of fledglings were male and previous research had shown that 48% of hatchlings are male. We also found no disadvantage for male nestlings that would become large adults (i.e. those with larger parents) – partial brood loss and fledging sex ratios did not vary with mid-parent size. Given no apparent disadvantage to large size for males either as adults or as nestlings, this leaves only the period between fledging and adulthood during which natural selection might limit sexual size dimorphism, although other mechanisms might explain the failure to find a limit to male size.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 85 , 353–361.     

Begging and the risk of predation in nestling birds

Theoretical models of the evolution of begging in nestling passerinesassume that begging is costly, either energetically or in termsof predation. However, few empirical measures of these costsexist. We examined whether nestling begging calls could attractpredators to nests by comparing predation rates at artificialnests with and without playbacks of tree swallow begging calls.Nests were baited with quail eggs and placed in pairs on theground or in modified nest-boxes. Nests with playbacks of beggingcalls were depredated before control nests significantly moreoften in both the ground and nest-box trials, suggesting thatpredators may use begging calls to locate nests. These resultssuggest that the risk of nest predation may be increased becauseof calling by nestlings and provide further support for theassumption that conspicuous begging is costly in terms of predation     

Parental effort and response to nestling begging in the house sparrow: repeatability,heritability and parent–offspring co‐evolution

Parental effort has a direct impact on individual fitness. Theoretical models exploring how parental effort evolves to cope with offspring demand and sexual conflicts may differ in the assumptions they make in respect to the genetic heritability of parental behaviours. Only a few attempts, however, have been made to estimate the heritability of parental behaviours and their possible co‐evolution with offspring solicitation behaviour. Analysing parent and offspring behaviours in four generations of cross‐fostered broods of house sparrows, we found that parental effort (food delivery rate) was repeatable across consecutive broods and heritable across generations. In contrast, parental response to experimentally induced changes in nestling begging was neither repeatable across broods nor heritable across generations or correlated to nestling begging. Thus, the results give no indication for genetic covariance between begging intensity and parental response, but provide the first cross‐fostering‐based evidence for the heritability of parental investment levels across generations.     

Discrimination and ejection of eggs and nestlings by the fan-tailed gerygone from New Caledonia

Nestling rejection is a rare type of host defense against brood parasitism compared with egg rejection. Theoretically, host defenses at both egg and nestling stages could be based on similar underlying discrimination mechanisms but, due to the rarity of nestling rejector hosts, few studies have actually tested this hypothesis. We investigated egg and nestling discrimination by the fan-tailed gerygone Gerygone flavolateralis, a host that seemingly accepts nonmimetic eggs of its parasite, the shining bronze-cuckoo Chalcites lucidus, but ejects mimetic parasite nestlings. We introduced artificial eggs or nestlings and foreign gerygone nestlings in gerygone nests and compared begging calls of parasite and host nestlings. We found that the gerygone ejected artificial eggs only if their size was smaller than the parasite or host eggs. Ejection of artificial nestlings did not depend on whether their color matched that of the brood. The frequency of ejection increased during the course of the breeding season mirroring the increase in ejection frequency of parasite nestlings by the host. Cross-fostered gerygone nestlings were frequently ejected when lacking natal down and when introduced in the nest before hatching of the foster brood, but only occasionally when they did not match the color of the foster brood. Begging calls differed significantly between parasite and host nestlings throughout the nestling period. Our results suggest that the fan-tailed gerygone accepts eggs within the size range of gerygone and cuckoo eggs and that nestling discrimination is based on auditory and visual cues other than skin color. This highlights the importance of using a combined approach to study discrimination mechanisms of hosts.     

Nest‐dwelling ectoparasites reduce begging effort in Pied Flycatcher Ficedula hypoleuca nestlings

Parasitized nestlings might be expected to increase begging effort to obtain additional resources to compensate for those sequestered by their parasites. However, begging is costly and chicks harbouring parasites may find it more difficult to attain high begging levels. Consequently, we predicted that, for the same level of nutritional need, nestlings that are parasitized will invest less in begging than those that are not parasitized. We tested this prediction by measuring begging in Pied Flycatcher Ficedula hypoleuca nestlings parasitized with haematophagous mites Dermanyssus gallinoides and Dermanyssus gallinae and blowfly larvae Protocalliphora azurea, and subjected to different levels of food deprivation in order to control for short‐term nutritional need. Nestlings from nests with ectoparasites spent less time begging than those from nests without parasites, especially when very hungry, although there was no association with latency to beg or begging intensity. Our results suggest that time invested in begging may indicate not only the level of need, but also nestling parasitism status.     

Great spotted cuckoos respond earlier to the arrival of feeding foster parents and perform less erroneous begging when hungry than their magpie host nest‐mates

In many bird species, parents usually feed the first nestling that starts to beg before its nest‐mates. The pressure to avoid missed feeds could trigger nestlings to perform in erroneous begging in absence of parents, which has the same costs as begging in the presence of parents but without any reward. So, nestlings should try to minimize both erroneous begging and missed feeds simultaneously. The threshold to start begging is predicted to be lower for hungry nestlings and for nestlings that are unrelated to their nest‐mates, because they suffer lower inclusive fitness costs when depriving nest‐mates of food. In line with this idea, we found that brood parasitic great spotted cuckoo nestlings responded sooner than their magpie nest‐mates when an adult arrived to the nest. Under laboratory conditions, nestlings of both species rarely incurred in erroneous begging when food was abundant, but under conditions of restricted food, magpie nestlings increased erroneous begging while cuckoo nestlings did not. Highly conspicuous begging in cuckoos results in an increased predation risk, which could have resulted in stronger selection pressures on cuckoos to avoid erroneous begging, probably resulting in better developed perceptual abilities, allowing cuckoos to perform better than their host nest‐mates.     

Cross‐fostering reveals that among‐brood differences in ornamental mouth coloration mostly reflect rearing conditions in nestling house sparrows

Dependent offspring use specialized traits to attract parental care. In birds, this includes morphological ornaments (e.g. colourful plumage or mouthparts) that are associated with nestling condition and shape the allocation of parental care. Ornament expression often differs among broods, even after differences in individual condition are accounted for statistically. Understanding how this variation arises is important for understanding the information content of these signals, their functional importance, and their evolution. The present study used a cross‐fostering experiment to assess the relative contributions of parental effects to among‐brood differences in the mouth coloration of nestling house sparrows, specifically the carotenoid‐richness, overall brightness, and ultraviolet (UV) coloration of rictal flanges. The expression of carotenoid‐based coloration was explained by synchronous breeding, nest‐of‐rearing and nest‐of‐origin. Brightness and relative UV intensity, however, were explained only by synchronous breeding, and there was substantial unexplained variation in all three colour parameters. Among‐brood variation in mouth coloration, then, may primarily contain information about the environment in which offspring are reared. At the individual level, ontogenetic changes in the carotenoid‐richness and brightness of flanges positively reflected mass gain (a proxy for food intake). Larger and yellower chicks gained more mass, consistent with parental preferences for these traits. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 169–179.     

Differences in the nestling begging calls of hosts and host-races of the common cuckoo, Cuculus canorus

We compared nestling begging calls of four hosts (reed warbler, Acrocephalus scirpaceus; great reed warbler, A. arundinaceus; dunnock, Prunella modularis; and meadow pipit, Anthus pratensis) and the respective host-races of the common cuckoo. Note structure varied between host species, but not between cuckoo host-races, so cuckoos did not vary their call note structure to match that of their hosts' chicks. Call rate increased with age, but there were marked differences between both host species and cuckoo host-races. Dunnock-cuckoos called more rapidly than reed warbler-cuckoos despite growing at the same rate. We suggest this difference reflects how cuckoos tune into the way these host species respond to begging signals from their own young, because dunnock chicks called much more rapidly than reed warbler chicks. Great reed warbler-cuckoos called at a lower rate than reed warbler-cuckoos when young, but at a greater rate when older than 8 days. This could also result from the cuckoo chicks tuning into differences in the way these hosts respond to begging signals. However, great reed warbler-cuckoos grew at a faster rate than the other cuckoo host-races, so they may also call faster to demand higher provisioning rates from this larger host. To test these hypotheses critically, data are needed on how the different host species integrate visual and vocal begging signals from their own broods. We discuss how differences in cuckoo begging might develop, given that cuckoo host-races are restricted to female cuckoo lineages. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.      

Vocal matching and intensity of begging calls are associated with a forebrain song circuit in a generalist brood parasite

Vocalizations produced by developing young early in life have simple acoustic features and are thought to be innate. Complex forms of early vocal learning are less likely to evolve in young altricial songbirds because the forebrain vocal‐learning circuit is underdeveloped during the period when early vocalizations are produced. However, selective pressure experienced in early postnatal life may lead to early vocal learning that is likely controlled by a simpler brain circuit. We found the food begging calls produced by fledglings of the brown‐headed cowbird (Molothrus ater), a generalist avian brood parasite, induced the expression of several immediate early genes and early circuit innervation in a forebrain vocal‐motor pathway that is later used for vocal imitation. The forebrain neural activity was correlated with vocal intensity and variability of begging calls that appears to allow cowbirds to vocally match host nestmates. The begging‐induced forebrain circuits we observed in fledgling cowbirds were not detected in nonparasitic passerines, including species that are close relatives to the cowbird. The involvement of forebrain vocal circuits during fledgling begging and its association with vocal learning plasticity may be an adaptation that provides young generalist brood parasites with a flexible signaling strategy to procure food from a wide range of heterospecific host parents. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 615–625, 2016     

The ontogeny of acoustic individuality in the nasal calls of captive goitred gazelles, Gazella subgutturosa

Individualistic voices are important for establishing personalized relationships among individuals. In young animals, individual vocal identity is affected by permanent changes of the acoustics due to the growth of their vocal apparatus. Different acoustic variables change uncoordinatedly, so vocal individuality should be repeatedly upgraded along development. We compared classifying accuracy of individuals and sexes by nasal calls in fast-growing goitred gazelles Gazella subgutturosa at two ontogenetic stages, juvenile (3-6 weeks of age) and adolescent (23-26 weeks of age). Juvenile "spring" nasal calls and adolescent "fall" nasal calls were examined in the same 35 calves (18 males, 17 females), wild-born in May and then hand-raised. Discriminate function analysis based on four formants, fundamental frequency, duration and three power quartiles, revealed an equally high potential of spring and fall calls to encode sex. The individuality was very high in both ages but significantly higher in fall calls. Classifying calls to individuals was based on the same three acoustic variables (fundamental frequency and third and fourth formants) in both ages, although their actual values changed uncoordinatedly from spring to fall in most subjects. Our results suggest updating acoustic individuality in nasal calls of adolescent goitred gazelles accordingly to the newly emerged acoustic variation.     

Testing sources of variation in nestling‐stage nest success of Florida Scrub‐Jays in suburban and wildland habitats

Human modification of habitats can reduce reproductive success by providing novel cues to which birds may respond with behaviors that are actually maladaptive in those environments. Ad libitum human‐provided foods may provide the perception that urban habitats are food‐rich even as natural food availability decreases. Similarly, human activity may increase the perception that predation risk is high even as natural predators may decrease in abundance. In response, birds may reduce parental care with a subsequent cost to successful reproduction. Florida Scrub‐Jays (Aphelocoma coerulescens) in suburban areas have lower nest success during the nestling period than do wildland jays, possibly the result of such maladaptive responses, but maybe because of ecological differences with wildlands. We manipulated adult perception of predation risk and the availability of nestling foods in suburban and wildland areas to determine if these factors influenced parental care and nestling begging, and if the behavioral responses of adults influence nest survival during the nestling stage. Experimentally increasing perception of predation risk reduced parental care by both suburban and wildland females, but did not influence care by males. Increasing food availability, but not predation risk, had little influence on parental care, but resulted in decreased nestling begging rates and an increase in the frequency (pitch) of begging calls in both habitats. However, neither parental care nor food availability influenced nest survival during the nestling stage. Instead, the presence of helpers was the most important variable in nest survival analyses, suggesting that habitat‐specific differences in nest survival during the nestling stage were not simply the result of maladaptive parental behavior or shortage of nestling food resources in the suburban habitat. The lack of helpers combined with ecological differences, such as the abundance of nest predators, may be why fewer nests of Florida Scrub‐Jays survive during this stage in suburban areas.     

Variation in chick‐a‐dee calls of bridled titmice (Baeolophus wollweberi): Frequent use of non‐combinatorial calls in a combinatorial calling system

Chick‐a‐dee calls of Poecile (chickadee) and Baeolophus (titmouse) species are complex in terms of the structural composition of note types and the diversity of messages. Studies so far have mainly focused on the calls of various chickadee and just one titmouse species—the tufted titmouse (B. bicolor). To begin to address this lack of titmouse data, our study investigated variation in note composition of calls of bridled titmice (B. wollweberi). We obtained calls from 26 flocks in the Chiricahua Mountains of Arizona in the overwintering flocking period. Bridled titmice produce proportionally more non‐combinatorial call variants than combinatorial call variants. The number of the single noted calls furthermore exceeded the number of multinote calls. In general, structural variation in the combinatorial calls appears to be comparable to calls of better‐studied chickadees and of tufted titmice, although bridled titmice appear to have a unique call length distribution. We also analyzed some behavioral associations with call variation and found that flight behavior and close interactions between individuals were associated with use of specific note types. Finally, we found microgeographic variation in note type use in these calls. We discuss some possible explanations for call complexity in this species.     

Begging in the absence of parents by nestling tree swallows

Begging by nestling passerine birds has become a model systemfor studies in animal communication. Although most beggingoccurs when parents arrive at the nest to feed (here called"primary begging"), it also occurs between feeding visits andimmediately after parents leave the nest. Begging in thesecontexts (here called "secondary begging") may have relativelylittle influence on the probability of receiving food, but could increase the overall cost of the signal and thus influence nestlingbegging strategies. The purpose of our study was to determinehow often tree swallow (Tachycineta bicolor) nestlings begin contexts other than to parents with food and to examinewhat factors influence the frequency of this begging. Secondarybegging ranged from 7% of measured begging responses at day2 to 30% by day 8 and was more frequent when the interval betweenparental feeding visits was relatively long and when the timeto respond to the arrival of parents with food was short. Increasesin both age and intervisit interval were associated with decreasesin nestling response times, suggesting that secondary beggingmay be related to the speed with which nestlings respond to stimuli. We discuss possible functions of secondary beggingand raise the possibility that it may, in fact, be an error.     

Begging in the absence of parents: a "quick on the trigger" strategy to minimize costly misses

Nestling begging in the absence of parents may reflect "falsealarms" due to cognitive constraints or signaling activity towardnest mates (sibling negotiation). According to signal detectiontheory, cognitive constraints should result in both false alarms(begging in the absence of parents or to inappropriate stimuli)and misses (failure to beg during parental visits). In our studyof house sparrows, nestling begging in the absence of parentscomprised up to 50% of the begging events at the nest and wasmore frequent at an early age and among hungrier (lower ranked)nestlings. In contrast, the probability of begging during parentalvisits was constantly high (80% or more), suggesting that therate of misses must have been low even at an early age. Theseresults have 2 main implications. First, the observation thatbegging in the absence of parents decreases with nestling agefavors the cognitive constraints hypothesis over functionalexplanations such as the sibling negotiation hypothesis. Second,the low proportion of "misses" among young nestlings suggeststhat nestling respond to their cognitive constraints by usinglow decision criteria (a "quick on the trigger" strategy) thatincreases the frequency of false alarms but minimizes costlymisses.