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.     

Calling at a cost: elevated nestling calling attracts predators to active nests

Begging by nestling birds has been used to test evolutionary models of signalling but theory has outstripped evidence. Eavesdropping predators potentially impose a cost on begging that ensures signal honesty, yet little experimental evidence exists for such a cost at active nests because the use of artificial nests, long playback bouts and absence of parents may have exaggerated costs. We broadcast short periods (1 h) of either nestling vocalizations or background noise at active white-browed scrubwren, Sericornis frontalis, nests. Nestlings called naturally during both treatments, allowing us to test whether elevated calling increases risk, a key but rarely tested assumption of evolutionary models. Predators visited nests exclusively during periods of elevated calling. Furthermore, playbacks affected neither adult visits nor nestling activity, suggesting that calling alone attracted predators. Adults gave alarm calls and nestlings usually called less when predators approached nests. Predation risk to broods is, therefore, likely to fluctuate substantially over short periods of time, depending on nestling hunger and whether adults or young have detected predators. This study confirms a present-day cost of nestling begging, demonstrates that this cost can be incurred over short periods and supports the importance of parent-offspring antipredator strategies in reducing predation risk.     

Ambient noise increases missed detections in nestling birds

Ambient noise can mask acoustic cues, making their detection and discrimination difficult for receivers. This can result in two types of error: missed detections, when receivers fail to respond to the appropriate cues, and false alarms, when they respond to inappropriate cues. Nestling birds are error-prone, sometimes failing to beg when parents arrive with food (committing missed detections) or begging in response to stimuli other than a parent's arrival (committing false alarms). Here, we ask whether the frequency of these errors by nestling tree swallows (Tachycineta bicolor) increases in the presence of noise. We found that nestlings exposed to noise had more missed detections than their unexposed counterparts. We also found that false alarms remained low overall and did not differ significantly between noise and quiet treatments. Our results suggest that nestlings living in noisy environments may be less responsive to their parents than nestlings in quieter environments.     

Begging, food provisioning, and nestling competition in great tit broods infested with ectoparasites

Ectoparasites are a ubiquitous environmental component of breedingbirds, and it has repeatedly been shown that hematoph-agousectoparasites such as fleas and mites reduce the quality andnumber of offspring of bird hosts, thereby lowering the valueof a current brood. Selection acting on the hosts will favorphysiological and behavioral responses that will reduce theparasites' impact. However, the results of the few bird studiesthat addressed the question of whether parasitism leads to ahigher rate of food provisioning are equivocal, and the beggingresponse to infestation has rarely been quantified. A changein begging activity and parental rate of food provisioning couldbe predicted in either direction: parents could reduce theirinvestment in the brood in order to invest more in future broods,or they could increase their investment in order to compensatefor the parasites' effect on the current brood. Since the nestlingsare weakened by the ectoparasites they may beg less, but onthe other hand they may beg more in order to obtain more food.In this study we show experimentally that (1) hen fleas (Ceratophyllusgallinae) reduce the body mass and size of great tit (Parusmajor) nestlings, (2) nestlings of parasitized broods more thandouble their begging rate, (3) the male parents increase thefrequency of feeding trips by over 50%, (4) the females do notadjust feeding rate to the lowered nutritional state of nestlings,and (5) food competition among siblings of parasitized broodsis increased. Ultimately the difference in the parental feedingresponse may be understood as the result of a sex-related differencein the trade-off of i0vesting in current versus future broods.     

Song post exposure, song features, and predation risk

Male birds use song to attract mates and deter other males,but in doing so, they also attract the attention of predatorsand parasites. Such viability costs are inherent in reliablesignals, potentially causing females to prefer mates that displayfrom the most exposed sites. However, viability costs of sexualsignals may be ameliorated by affecting the choice of microhabitat,which in turn may affect the design of song features that aremost efficiently transmitted in this microhabitat. We estimatedthe exposure of song posts (microsites used by males when singing)used by passerine birds in relation to prey selection by thesparrowhawk Accipiter nisus, by calculating the proportion ofmales that sang from song posts that were at the maximum levelof the vegetation, in an attempt to quantify the costs of sexualselection. We quantified prey susceptibility to predation asthe difference between the log-transformed observed number ofprey minus the log-transformed expected number of prey in theenvironment. This prey susceptibility index increased with increasingsong post exposure similarly in sexually dichromatic and monochromaticspecies, although the prey susceptibility index was relatedto sexual dichromatism. Song post exposure was dependent onhabitat, but comparative models controlling for the potentiallyconfounding effects of habitat, sexual dichromatism, hole nesting,coloniality, body mass, cognitive capacities, and flying abilitiesindicated that the relationship between the prey susceptibilityindex and song post exposure is strong. Path analyses of therelationship between song post exposure, sexual dichromatism,and prey susceptibility index revealed that selection actingon sexual dichromatism and song post exposure has secondaryimpact on prey susceptibility index. The opposite causal mechanismsby which predation affects sexual traits are less likely. Thesemodels suggest that female preference for high song posts ordichromatic plumage increases predation risk on an evolutionarytime scale.     

Brood size and begging intensity in nestling birds

Theoretical models suggest that sibling competition should selectfor conspicuous begging signals. If so, begging intensity mightbe expected to increase with the number of competitiors. Thepurpose of our study was to examine the relationship betweenbegging intensity and brood size using nestling tree swallows(Tachycineta bicolor) as our model. Over 2 years, we videotapedbegging behavior in unmanipulated broods of different sizes.We found that begging intensity increased with brood size. Theaverage weight of nestlings in each brood did not vary withbrood size, but feeding rate per nestling decreased with broodsize, suggesting that nestlings in larger broods begged moreintensively, possibly because they were hungrier. We also conductedan experiment to examine the effect of nest mates on beggingin different-sized broods. We found that nestlings with similarweights, previous competitive environments, and food deprivationbegged more intensively in large broods than in small broods.Overall, our study indicates that begging intensity increaseswith brood size in tree swallows. This relationship may resultfrom interactions among brood mates rather than from lower feeding rates to individual nestlings in larger broods.     

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.     

Comparative manipulation of predation risk in incubating birds reveals variability in the plasticity of responses

The evolution of different parental care strategies is thoughtto result from variation in trade-offs between the costs andbenefits associated with providing care. However, changingenvironmental conditions can alter such fitness trade-offsand favor plasticity in the type or amount of parental care provided. Avian incubation is a form of parental care whereparents face changing environmental conditions, including variationin the risk of nest predation. Because parental activity candraw attention to the location of the nest, a reduction innest visitation rates is a predicted response to an increased,immediate predation risk. Here, we experimentally increasedthe risk of nest predation using model presentations at nestsof five coexisting species that differ in their ambient levelsof nest predation. We examined whether individuals detect changesin nest predation risk and respond by reducing visitation tothe nest. We also tested whether this behavioral response differsamong species relative to differences in their ambient risk of nest predation. We found that males of all species detectedthe increased predation risk and reduced the rate at whichthey visited the nest to feed incubating females, and the magnitudeof this change was highly correlated with differences in therisk of nest predation across species. Hence, as the vulnerabilityto nest predation increases, males appear more willing to trade the cost of reduced food delivery to the female against thebenefit of reduced predation risk. Our results therefore suggestthat nest predators can have differential effects on parentalbehaviors across species. We discuss how the comparative natureof our results can also provide insight into the evolution of behavioral plasticity.     

Non-equivalence of growth arrest induced by predation risk or food limitation: context-dependent compensatory growth in anuran tadpoles

1. To gain insight into the evolution of compensatory growth, we studied the growth patterns of anuran (Rana temporaria) larvae following either a period of exogenous growth depression (food restriction) or a period of endogenous depression (exposure to predators). We also investigated the potential deferred costs that larval compensatory growth could impose on post-metamorphic individuals. 2. Food-deprived larvae exhibited full compensatory growth in response to reduced growth rates caused by food limitation, and the growth trajectories of low- and high-rations tadpoles converged before the onset of metamorphosis. 3. According to our predictions, individuals exposed to larval predators did not show growth compensation following predator removal despite undergoing a significant reduction in growth rate associated with low activity levels. 4. Jumping ability of individuals exposed to predators during only 20 days from the commencement of the larval phase was equivalent to that of non-exposed animals, and greater than the jumping capacity of those maintained with predators until the time of metamorphosis. This pattern was consistent with the pattern observed for variation in relative leg length. 5. These results support the suggestion that submaximum and compensatory growth could have evolved to minimize the overall growth/mortality costs in environments with high spatiotemporal variation in predation intensity.     

Mortality by moonlight: predation risk and the snowshoe hare

Optimal behavior theory suggests that prey animals will reduceactivity during intermittent periods when elevated predationrisk outweighs the fitness benefits of activity. Specifically,the predation risk allocation hypothesis predicts that preyactivity should decrease dramatically at times of high predationrisk if there is high temporal variation in predation risk butshould remain relatively uniform when temporal variation inpredation risk is low. To test these predictions we examinedthe seasonably variable response of snowshoe hares to moonlightand predation risk. Unlike studies finding uniform avoidanceof moonlight in small mammals, we find that moonlight avoidanceis seasonal and corresponds to seasonal variation in moonlightintensity. We radio-collared 177 wild snowshoe hares to estimatepredation rates as a measure of risk and used movement distancesfrom a sample of those animals as a measure of activity. Inthe snowy season, 5-day periods around full moons had 2.5 timesmore predation than around new moons, but that ratio of theincreased predation rate was only 1.8 in the snow-free season.There was no significant increase in use of habitats with morehiding cover during full moons. Snowshoe hares' nightly movementdistances decreased during high-risk full-moon periods in thesnowy season but did not change according to moon phase in thesnow-free season. These results are consistent with the predationrisk allocation hypothesis.     

Non-lethal effects of predation in birds

Predators can affect individual fitness and population and community processes through lethal effects (direct consumption or ‘density’ effects), where prey is consumed, or through non‐lethal effects (trait‐mediated effects or interactions), where behavioural compensation to predation risk occurs, such as animals avoiding areas of high predation risk. Studies of invertebrates, fish and amphibians have shown that non‐lethal effects may be larger than lethal effects in determining the behaviour, condition, density and distribution of animals over a range of trophic levels. Although non‐lethal effects have been well described in the behavioural ecology of birds (and also mammals) within the context of anti‐predation behaviour, their role relative to lethal effects is probably underestimated. Birds show many behavioural and physiological changes to reduce direct mortality from predation and these are likely to have negative effects on other aspects of their fitness and population dynamics, as well as affecting the ecology of their own prey and their predators. As a consequence, the effects of predation in birds are best measured by trade‐offs between maximizing instantaneous survival in the presence of predators and acquiring or maintaining resources for long‐term survival or reproduction. Because avoiding predation imposes foraging costs, and foraging behaviour is relatively easy to measure in birds, the foraging–predation risk trade‐off is probably an effective framework for understanding the importance of non‐lethal effects, and so the population and community effects of predation risk in birds and other animals. Using a trade‐off approach allows us to predict better how changes in predator density will impact on population and community dynamics, and how animals perceive and respond to predation risk, when non‐lethal effects decouple the relationship between predator density and direct mortality rate. The trade‐off approach also allows us to identify where predation risk is structuring communities because of avoidance of predators, even when this results in no observable direct mortality rate.     

Ultraviolet reflection and predation risk in diurnal and nocturnal Lepidoptera

According to our extensive data on Lepidoptera (883 species),UV wing patterns are almost three times more common in nocturnalthan in diurnal Lepidoptera. This might be due to predation,because the primary diurnal predators, birds, utilize UV lightin foraging and even prefer UV-reflecting prey. To test thishypothesis, we conducted a field experiment with tethered livingmoths whose wings were artificially manipulated to reflect (UV+,reflection at UV wavelength: 15%) or absorb (UV–) UV light,keeping longer wavelengths identical. Thus, any difference foundin survival rates would be the result of the difference in wingpatterns in UV spectrum. Significantly more UV+ moths than UV–ones were eaten in the daytime, but no difference in predationrates could be detected when moths were exposed to nocturnalpredators. The different survival rates indicate that UV reflectionincreased predation risk by visually orienting diurnal predators.The lack of difference at night arises from the lack of UV-sensitivepredators. UV wing patterns, even if they are important in intraspeciescommunication, seem to be costly to diurnal Lepidoptera by attractingpredators.     

Parental provisioning and predation risk in rhinoceros auklets (Cerorhinca monocerata): effects on nestling growth and fledging

We examined the effects of predation risk on the behavior ofrhinoceros auklets (Cerorhinca monocerata) breeding at PineIsland, British Columbia, in 1990. Provisioning parents in someareas of the colony risked predation by bald eagles (Haliacetusleucocephalus). Chicks in high and low predation risk areasof the colony hatched on approximately the same date, receivedsimilar amounts of food to 46 days of age, grew at the samerate, reached similar peak masses, and fledged at similar masses.However, chicks in high predation areas fledged at a youngerage than did chicks in low predation areas. These data are consistentwith the hypothesis that parents in high risk areas terminatedprovisioning several days before those in lower risk areas.Mass at fledging was inversely related to age at fledging inboth high and low risk areas. The regression line for the highrisk habitats lies below that from the low risk habitats, aspredicted by a model that examines optimal time of fledgingfrom the perspective of the parents. We conclude that risk ofpredation represents a significant cost of reproduction to somerhinoceros auklets and that individual auklets within the colonyvary their behavior according to predation risk.     

Effects of predation,parasites, and phylogeny on the evolution of bright coloration in north american male passerines

Summary Numerous mechanisms have been proposed to account for the evolution of cryptic and bright coloration in passerine birds. The Hamilton-Zuk revealing handicap model holds that cyclic interactions between hosts and parasites maintain additive genetic variance in secondary sexual traits and adaptive mate choice of resistant genotypes ensues (Hamilton and Zuk, 1982). Here I report no support for this model using various within-taxa techniques to test the functional relationship between the prevalence of hematozoan parasites and male brightness in many species of North American passerines. I establish that phylogeny and predation risk are most strongly associated with variation in male coloration. Ground-nesting passerines are considerably more cryptic than off-ground nesters, and there is evidence that ground-nesting passerines are under greater predation risk. Predation risk may limit the role of sexual selection in the development of bright coloration.     

Too risky to settle: avian community structure changes in response to perceived predation risk on adults and offspring

Predation risk is widely hypothesized as an important force structuring communities, but this potential force is rarely tested experimentally, particularly in terrestrial vertebrate communities. How animals respond to predation risk is generally considered predictable from species life-history and natural-history traits, but rigorous tests of these predictions remain scarce. We report on a large-scale playback experiment with a forest bird community that addresses two questions: (i) does perceived predation risk shape the richness and composition of a breeding bird community? And (ii) can species life-history and natural-history traits predict prey community responses to different types of predation risk? On 9 ha plots, we manipulated cues of three avian predators that preferentially prey on either adult birds or offspring, or both, throughout the breeding season. We found that increased perception of predation risk led to generally negative responses in the abundance, occurrence and/or detection probability of most prey species, which in turn reduced the species richness and shifted the composition of the breeding bird community. Species-level responses were largely predicted from the key natural-history trait of body size, but we did not find support for the life-history theory prediction of the relationship between species'' slow/fast life-history strategy and their response to predation risk.     

Empty seeds reduce seed predation by birds in Juniperus osteosperma

Utah juniper (Juniperus osteosperma) is one of many plant species that produce large numbers of fruits containing parthenocarpic or otherwise empty or inviable seeds. We tested the hypothesis that production of empty fruits in this species results in reduced levels of predation on fertile seeds. In a population in west-central Utah, we estimated the proportion of fruits with filled seeds in trees suffering high levels of fruit destruction by the seed-eating bird Parus inornatus and in neighbouring trees similar in crown and fruit-crop size but suffering negligible predation. We found that the heavily attacked trees had higher proportions of filled seeds. Thus, juniper may benefit from producing fruits that contain no offspring. This is the first study to demonstrate that empty seeds may reduce predation by vertebrate seed eaters and the first to demonstrate discrimination based on seed filling at the level of whole plants.     

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.     

Begging and cowbirds: brood parasites make hosts scream louder

Avian brood parasites have evolved striking begging abilitythat often allows them to prevail over the host progeny in competitionfor parental resources. Host young are therefore selected bybrood parasites to evolve behavioral strategies that reducethe cost of parasitism. We tested the prediction that the intensityof nestling begging displays functioning to attract parentalcare increases across species with the frequency of parasitismby the brown-headed cowbird (Molothrus ater). This was expectedbecause host young should try to prevail over highly competitiveparasitic broodmates in scramble interactions, act more selfishlywhen frequency of parasitism is high because brood parasitesoften affect more severely host condition than conspecific broodmates,and discount the kin selection costs of subtracting resourcesto unrelated parasites. Across 31 North American host species,begging loudness positively covaried with parasitism rate inPasserines, and such effect was stronger in species with smallcompared with large clutches. Begging loudness increased withbrood parasitism and nest predation among the most suitablehost species. These results held after controlling for concomitantecological factors and for common ancestry effects. Our resultssupport the hypothesis that avian brood parasitism has playeda role in the evolution of begging behavior of host young.     

A test of the risk allocation hypothesis: tadpole responses to temporal change in predation risk

The risk allocation hypothesis predicts that temporal variationin predation risk can influence how animals allocate feedingbehavior among situations that differ in danger. We testedthe risk allocation model with tadpoles of the frog Rana lessonae,which satisfy the main assumptions of this model because theymust feed to reach metamorphosis within a single season, theirbehavioral defense against predators is costly, and they canrespond to changes in risk integrated over time. Our experiment switched tadpoles between artificial ponds with different numbersof caged dragonfly larvae and held them at high and low riskfor different portions of their lives. Tadpoles responded stronglyto predators, but they did not obey the risk allocation hypothesis:as the high-risk environment became more dangerous, there wasno tendency for tadpoles to allocate more feeding to the low-riskenvironment, and as tadpoles spent more time at risk, they didnot increase feeding in both environments. Our results suggestthat the model might be more applicable when the time spentunder high predation risk is large relative to the time requiredto collect resources.