Parental care of nestlings by male red-winged blackbirds

Summary Nestling feeding by males is less common among birds with polygynous mating systems than in monogamous species, because of the pronounced trade-off between parental behavior and the attraction of additional mates. In this study, however, we found that male red-winged blackbirds (Agelaius phoeniceus) commonly assisted females in feeding nestlings in several Ontario marshes. Male parental care was additional to that provided by females, and it significantly enhanced the fledging success of nests (Table 2). Male redwings did not help to feed all nests on their territories: primary and secondary nests were much more likely to receive male parental care than tertiary and later nests. Contrary to expectation, male parental care was not restricted to the nests of primary females: a greater proportion of secondary than primary nests were assisted (Tables 4a and b). The presence of new females settling on the territory at the same time that a resident female had nestlings, resulted in males deferring parental care until a later brood. This suggests that males trade off the recruitment of females against parental care to an existing brood. Although the number of nestlings fledging from a male's territory was strongly influenced by the number of females in the harem, males could additionally increase their reproductive success by feeding nestlings in one or more nests on their territories (Fig. 2). The reproductive success of females was significantly enhanced by male assistance in feeding nestlings (Table 3). However, those females not receiving male assistance on territories of feeding males did not suffer a significantly reduced reproductive success in comparison to females on territories of non-feeding males (Table 2). Males varied considerably in the quality of brood care given. We therefore suggest that the quality of male parental care may be a factor considered by females in choosing a breeding situation.     

An experimental study of paternal behavior in red-winged blackbirds

Summary The effect of brood size and female nesting status on male parental behavior was investigated in red-winged blackbirds Agelaius phoeniceus using brood size manipulation experiments. Male redwings allocated parental effort on the basis of brood size and nestling age. Males began assisting females only at nests with at least three offspring older than three days. Female nesting status had no singificant influence on male parental care. When females were unable to meet a brood's demand for food, males assisted females with nestling feeding. Females did not reduce the amount of food delivered to nestlings when males assisted. The amount of food brought to nestlings by the male was additional to the amount of food provided by the female. Male assistance increased fledgling success. When female provisioning was sufficient to meet a brood's demand for food males did not assist. The value of male parental care varied inversely with the ability of the female to meet nestling food demands. The ability of unassisted females to provide sufficient food and to raise a brood of nestlings successfully appeared to be influenced by resource abundance.     

Begging and provisioning in broods of asynchronously-hatched yellow-headed blackbird nestlings

Studies of begging have found a positive relationship between begging level and provisioning level. Studies of unequal nestlings, however, have found that small nestlings generally beg more but are fed less than their larger siblings. We manipulated the begging levels of yellow-headed blackbird (Xanthocephalus xanthocephalus) chicks to investigate how begging benefits individuals in broods of unequal siblings. Food-deprived chicks begged more and were fed more; satiated chicks begged less and were fed less. When we deprived each chick of a brood in turn, large and small chicks generally increased begging and received more provisioning. Small chicks, however, rarely received more food than their larger siblings even when they behged relatively more. Parent yellow-headed blackbirds increase provisioning to hungry begging chicks, but also allocate food based on relative offspring size.     

Male characteristics,parental quality and the study of mate choice in the red-winged blackbird (Agelaius phoeniceus)

Summary A demonstration of adaptive mate choice by females in resource-defence mating systems requires clear predictions as to how females should rank breeding situations (defined by the quality of both the resident male and the territory he defends) so as to maximize their fitness. Since male quality is only weakly correlated with territory quality in red-winged blackbirds (Agelaius phoeniceus), ranking breeding situations in this species will require a consideration of both those parameters independently of each other as long as both vary among males, are predictable before mating, and affect female fitness. Data from a two year study of an eastern Ontario population of this species suggested that two components of male parental quality, nest defence effort and provisioning of nestlings with food, both varied among males and were somewhat predictable. Two measures of nest defence effort were correlated with an index of epaulet size (a reliable predictor of captive dominance rank in this species) (Table 5), and provisioning appeared to be predictable on the basis of both courtship behavior and breeding experience. Our data also suggest that these two components of male parental quality do not covary. Since male provisioning tends to be restricted to the nestlings of the primary and secondary mates in this species, breeding situations must be ranked not only with respect to their manifold quality but also with respect to the mating status of individual females.     

An experimental study of competition between fire ants and Argentine ants in their native range

An understanding of why introduced species achieve ecological success in novel environments often requires information about the factors that limit the abundance of these taxa in their native ranges. Although numerous recent studies have evaluated the importance of natural enemies in this context, relatively few have examined how ecological success may result from differences in the magnitude of interference competition between communities in the native and introduced ranges of nonnative species. Here we examine how native-range competitive environments may relate to invasion success for two important invasive species, the red imported fire ant (Solenopsis invicta) and the Argentine ant (Linepithema humile), in a region of native-range sympatry. At two study sites in northern Argentina, we used stable-isotope analysis, a variety of observational approaches, and two different reciprocal removal experiments to test (1) whether S. invicta competes asymmetrically with L. humile (as suggested by the 20th century pattern of replacement in the southeastern United States) and (2) the extent to which these two species achieve behavioral and numerical dominance. Stable-isotope analysis and activity surveys indicated that S. invicta and L. humile are both omnivores and forage during broadly overlapping portions of the diel cycle. Short-term removal experiments at baits revealed no competitive asymmetry between S. invicta and L. humile. Longer-term colony removal experiments illustrated that S. invicta and L. humile experience an approximately equal competitive release upon removal of the other. Our results indicate that neither S. invicta nor L. humile achieves the same degree of behavioral or ecological dominance where they co-occur in native populations as they do in areas where either is common in their introduced range. These results strongly suggest that interspecific competition is an important limiting factor for both S. invicta and L. humile in South America.     

Independent effects of male and female density on sexual harassment, female fitness, and male competition for mates in the western mosquitofish Gambusia affinis

Operational sex ratio (the ratio of sexually active males to fertilizable females) has a major influence on male competition for mates and male–female interactions. The contributions of male and female density per se to mating system dynamics, however, are rarely examined, and the fitness consequences are often inferred rather than quantified. Male mosquitofish (Gambusia affinis) compete aggressively and frequently harass females for copulations, a behavior thought to reduce female fitness. Female fitness can also be reduced by increases in female density, which may affect food availability, cannibalism rates, and chemical interactions between females. I manipulated male and female densities of G. affinis to measure their effects on male–male aggression, male harassment toward females, and female fitness. I found that males chased rivals more often and attempted fewer copulations when female density decreased, but surprisingly male density had no significant effect on the frequency of these male behaviors. In contrast, males’ agonistic displays toward other males increased with male density, but display behavior was unaffected by female density. These results suggest that male and female density do not always contribute equally or at all to the patterns of behavior we observe. Female fitness declined as female density increased, the opposite pattern expected if male harassment is costly to females. This suggests that a strong, negative effect of female density overwhelmed any potential costs of male harassment. Sources of female density dependence and the consequences of changes in male and female density to patterns of male behavior are discussed.     

Predator-induced female behavior in the absence of male incubation feeding: an experimental study

Parental care plasticity is critical to understanding the ecological and evolutionary influence of nest predation on life history strategies. In birds, incubation imposes a trade-off between the requirements of females (i.e., food) and egg requirements (i.e., heat and protection from predators). However, studies on this topic are rare and usually restricted to species where the male feeds the incubating female, relaxing her incubation costs. Males and females can reduce their activity at the nest to avoid detection by predators. However, females could follow two alternative antipredator strategies: to delay their return to the nest to avoid attracting attention from the potential predator or to return to the nest as soon as possible to enhance nest concealment. In this study, we manipulated the perceived risk of nest predation of incubating common blackbirds (Turdus merula), a species without incubation feeding, to study female behavioral changes induced by nest predation risk. We show experimentally that female blackbirds can reduce their nest visits in the situation with higher nest predation risk. In addition, we confirm that females significantly delay their return to the nest in the presence of a nest predator, contradicting the nest concealment hypothesis. However, our results could be interpreted as a passive antipredator response (to minimize clues given to predators) or as an active antipredator response (to search for predators to expel them from their territories).     

An experimental test of female choice relative to male structural coloration in eastern bluebirds

Several experimental studies have shown that female birds use ornamental melanin and carotenoid plumage coloration as criteria in mate choice. Whether females choose mates based on natural variation in structural coloration, however, has not been well established. Male eastern bluebirds (Sialia sialis) display brilliant ultraviolet (UV)-blue plumage coloration on their head, back, wings, and tail, which is positively correlated with condition, reproductive effort, and reproductive success. We experimentally tested the hypothesis that female eastern bluebirds prefer as mates males that display brighter structural coloration by presenting breeding-condition females with males of variable coloration. We conducted two types of mate-choice experiments. First, females chose between males whose coloration was manipulated within the natural range of variation in the population; feathers were either brightened with violet marker or dulled with black marker. Second, females chose between males with naturally dull or bright plumage coloration. In both manipulated and unmanipulated coloration trials, female choice did not differ significantly from random with respect to structural coloration. We found no support for the hypothesis that the UV–blue coloration of male eastern bluebirds functions as a criterion in female mate choice.     

Patterns of courtship behavior and ejaculate characteristics in male red-winged blackbirds

Sperm competition in birds is likely to have important effects on the behavior and physiology of reproduction in both sexes. For males, such competition should select for large sperm reserves and behavioral adjustment of copulation when reserves are low. We investigated both these possibilities in free-living red-winged blackbirds (Agelaius phoeniceus), a highly polygynous species with apparently strong sperm competition. We found that the recent copulatory behavior of males did not affect the propensity to copulate with a model female. Ejaculates collected from individual males at 1-h intervals showed no evidence of sperm depletion, yet repeated ejaculates collected less than 10 min apart did. Male ejaculate size was significantly larger if it was the first one of the day (i.e., after an overnight rest). The average ejaculate size was 12.5 (±12.5 SD) million sperm. Males captured during the breeding season had an average of 111.7 (±52.8) million sperm stored in their seminal glomera. Because males average a peak copulation rate of six per female per day, in one day a male might utilize all the sperm in his seminal glomera if more than two females on his territory are fertilizable. We hypothesize that polygyny and sperm competition in this species have combined to select for rapid replenishment of the seminal glomera throughout the day, in contrast to other species that have been studied. Testis size and sperm reserves of male red-winged blackbirds are intermediate between monogamous species and species with intense sperm competition. Several possible explanations for this are discussed. Received: 21 October 1997 / Accepted after revision: 15 February 1998     

Interspecific territoriality and competition for food between the reef fishes Forsterygion varium and Pseudolabrus celidotus

Long-term behavioural studies were made of the blennioid fish Forsterygion varium and the wrasse Pseudolabrus celidotus in the Cape Rodney-Okakari Point Marine Reserve in New Zealand. Both male and female F. varium defended small 1 to 2 m² territories from the wrasse all year round, althoughl aggressive encounters were most frequent from December to April. Only Large P. celidotus (≧180 mm standard length) were observed to dominate the blenny in interactions. Defence of nests, which were found only on the territories of males between May and October, did not explain the distribution of F. varium aggression between the sexes and seasons. An extensive overlap in the categories and size of prey eaten suggested that protection of food was a major function of F. varium territoriality. F. varium was most frequently aggressive toward P. celidotus of an intermediate size (110 to 160 mm SL) and exhibited greatest overlap in diet with wrasses in this size lange. This dnected the study toward examining the effect of F. varium on the feeding activity of P. celidotus. Within a shallow, Leterogeneous reef habitat, feeding pressure by P. celidotus (bites/are/time) was inversely related to the density of F. varium. Removal of F. varium from small 5x5 m quadrats resulted in large increases in wrasse feeding pressure, suggesting that the blenny influences the foraging of wrasses within their home ranges. When F. varium were removed from the entire home ranges of five 1+yr (110 to 130 mm SL) P. celidotus, these fish exhibited relative increases in both feeding rate and foraging time. The results suggest that P. celidotus grow through a period during which individuals compete with F. varium for food, adding support to the contention that territoriality in F. varium functions to preserve its food supply.     

How the mountain chickadee procures more food in less time for its nestlings

Summary Parent mountain chickadees (Parus gambeli) visit the same foraging location on consecutive flights more frequently than expected by chance. This pattern of repetitive return to a foraging location does not increase the frequency of capturing the same prey on consecutive flights. The quantity of prey captured on a visit also does not change significantly with repeated visits to a foraging location. Foraging time, however, is significantly shorter during repeated visits to a foraging location. Foraging time is therefore a major determinant of whether a run of consecutive flights to a location occurs. Decreased foraging time is also a benefit of selecting the same prey type on consecutive flights. Over the course of a day, the prey types returned on the greatest percentage of flights are returned multiply per flight. These prey are usually small and are probably present at high densities in the environment. Large prey size is the second best predictor of frequency of a prey type in the diet. Short foraging time to capture a prey is the third best predictor. Thus prey which can be readily obtained in large numbers, even if they are small prey, are preferentially returned to older nestlings. The mountain chickadee uses information on foraging time to decide when to move to a new foraging location or when to pursue a different type of prey. These decisions should increase the amount of time available for prey gathering.     

Hunger and food allocation among nestlings of facultatively siblicidal ospreys

Summary Previous studies of food allocation among nestlings of siblicidal birds have focussed on the proximate link between food and aggression. However, food sharing among sibs (and brood reduction) need not involve aggression. Here I examine the role of hunger (defined operationally in terms of the amount of food eaten recently) within the broader framework of passive and aggressive food-sharing among nestlings of facultatively siblicidal ospreys (Pandion haliaetus). Feeding among brood members was hierarchical; senior, dominant sibs ate more when hungry, leaving a smaller residual share of food for junior sibs. Senior sibs did not become more generous at large meals (i.e., the prospect of a large meal did not quell the selfish tendencies of senior sibs). That individuals ate more when hungry indicates a proximate link between food and the allocation of resources among siblings. Overall, aggression was infrequent and seemed to be used when junior sibs contested food allocation at meals. Aggression was diminished and food allocation was skewed toward the junior sib in broods with artificially exaggerated hatching intervals, suggesting that senior sibs were more generous when their dominant status was not threatened.Offprint requests to the present address     

Use of the song repertoire in intersexual and intrasexual contexts by male red-winged blackbirds

Summary Live and stuffed male and female red-winged blackbirds (Agelaius phoeniceus) were presented to territorial males in order to study the repertoire organization of males during intersexual and intrasexual encounters. Territorial male red-winged blackbirds switched song types more frequently and sang more song types in response to presentation of a female stimulus than during control periods, but decreased their switching frequency when a male stimulus was presented. Switching frequency in response to the female stimulus was three times that in response to the male stimulus. These results support the hypothesis that song repertoires of male red-winged blackbirds evolved primarily in response to intersexual rather than intrasexual selection. They also suggest that male red-winged blackbirds have been selected to de-emphasize their song repertoires during encounters with conspecific males as a result of some as yet unidentified cost. Offprint requests to: W.A. Searcy     

An experimental study of the relationship between below-ground structure and meiofaunal taxa

An experimental study was conducted in a South Carolina, USA salt marsh to examine relationships between meiofaunal-sized tube structure and densities of burrowing meiofauna. Abundance of the tube-building polychaete Manayunkia aestuarina Bourne was increased by erecting exciosure areas from June–August 1979. Non-manipulated areas of identical dimensions were staked out in adjacent locales. Densities of burrowing meiofauna and nematode trophic groups, and volumes of Spartina alterniflora root mat and tube-builders were monitored approximately every 2 wk from each experimental treatment. Volumes of tube-builders were significantly higher inside exclosure sites on all dates in July and August. Root-mat volume and total number of nematodes were not different between treatments at any time. The dominant copepod species, Stenhelia (D.) bifidia, and the nematode species, Spirinia sp., first increased in number inside manipulated sites and then decreased in abundance as tube volume increased. No general trend was apparent for nematode trophic groups when areas with increased volumes of tubebuilders were compared to unmanipulated sites. The fact that densities of S (D.) bifidia and Spirina sp. decreased only when tube-builders attained highest abundance suggests that negative interactions between tube-builders and these meiofaunal taxa may be effective only at elevated abundances.     

Large male advantage for access to females: evidence of male-male competition and female discrimination in a territorial salamander

Summary In the natural habitat visited in this study, adult male red-backed salamanders (Plethodon cinereus) found near females were significantly larger than adult males found alone; there was no evidence for size assortative associations. Previous research has indicated that females associate preferentially with males that occupy high quality territories and that larger individuals are more successful at obtaining higher quality areas. In laboratory experiments, when resource quality was held constant and males were restrained so that male-male interactions were prohibited, females discriminated behaviorally between large and small males by spending more time visually and chemically assessing larger males and more time apparently attempting to leave the chamber when near smaller males. Females were found near the larger males at the end of the trials significantly more often than predicted by chance. In a separate experiment in which only females were restrained, larger males spent significantly more time in aggressive postures when paired with smaller males than when alone with the female. Smaller males spent significantly more time in submissive postures when paired with larger males and more time near the female when alone. Therefore, large body size of males may positively affect both intra- and intersexual interactions and, ultimately, mating success of male P. cinereus.     

The scent of dominance: female field crickets use odour to predict the outcome of male competition

Pheromones may convey information about mate quality and social status. In the field cricket Gryllus integer, females mount the males for copulation, such that males cannot coerce females to mate. We examined whether virgin G. integer females preferred the scent of potentially dominant males to that of subordinate males. First, we collected pheromones by confining males on filter paper. Next, we offered filter paper from each of two size-matched males and control paper to females that had never been exposed to males, and measured the time spent by the female on each kind of paper. Finally, dominance status of the males in each size-matched pair was determined by pitting the two males against one another in agonistic contests. When offered filter paper from subsequently dominant versus subsequently subordinate males, females spent more time on the paper from the dominant male than the subordinate male, and much less time on control paper. Thus, pheromones may inform female G. integer about a male's potential to achieve dominant social status. Male pheromones were also associated with the female's tendency to mount a male. In contrast to cockroaches, where females prefer the scent of subordinate males (presumably to avoid risk of injury), female crickets prefer the scent of potentially dominant males and are more likely than males to wound their mating partners.     

Differences in acoustic directionality among vocalizations of the male red-winged blackbird (<Emphasis Type="Italic">Agelaius pheoniceus</Emphasis>) are related to function in communication

Studies of animal acoustic communication have found that the frequency and temporal structure of acoustic signals can be shaped by selection for efficient communication. The directionality of acoustic radiation may also be adapted for communication, but we know relatively little about how directionality varies with signal function, sender morphology, and the environment in which the sound is transmitted. We tested the hypothesis that the directionality of a vocalization is adapted to its function in communication. This hypothesis predicts that vocalizations that are directed to multiple conspecifics (e.g., advertisements and alarms) will be relatively omnidirectional because this will maximize the number of neighbors and mates that receive the signal, and that vocalizations directed to particular individuals will be relatively directional because this will maximize detection of the signal by the targeted receiver and minimize eavesdropping. To test these predictions, we measured the directionality and amplitude of red-winged blackbird (Agelaius pheoniceus) vocalizations in the field by recording vocalizations simultaneously on eight calibrated microphones encircling the bird. We found significant variation in directionality among vocalizations. Supporting our predictions, we found that the most omnidirectional vocalizations were those used to alert conspecifics to danger, and the most directional vocalizations are those used during courtship and solicitation of copulation, when the costs of eavesdropping are likely to be high. These results suggest that the directionality of red-winged blackbird vocalizations is shaped by selection for effective communication. This study is the first to provide statistical support for the hypothesis that directionality is related to the function of a signal in communication.     

An experimental study of male adornment in the scarlet-tufted malachite sunbird: II. The role of the elongated tail in mate choice and experimental evidence for a handicap

Summary Scarlet-tufted malachite sunbirds (Neetarinia johnstoni) are endemic to the high altitude zones of East African mountains. On Mount Kenya the males are resident and territorial all year. The females arrive over an extended period at the beginning of the breeding season, after spending the non-breeding season in nomadic flocks at slightly lower elevations. Males are bright iridescent green with elongated central tail feathers, which are displayed during courtship. We examined the role of the tail in mate choice using natural variation and investigated the influence of tail length on male time budgets by experimental manipulation. Territorial males that paired had longer tails than territorial males which remained unpaired. Males with naturally long tails started breeding earlier, resulting in fledglings being produced earlier in the season when the weather was more clement. Birds which bred together in one year were paired together in the next year if they both survived. Thus the male's tail may only be involved as a cue in the formation of new pairs. Males which were already paired were subjected to one of three experimental treatments — having their tail elongated, shortened, or manipulated but kept at the same length. Males with experimentally shortened tails spent more time in flight and hawked for flying insects with a higher efficiency than control males. Both control and elongated tail males reduced the amount of time in flight and had a lower hawking efficiency after manipulation. These results suggest that the long tail of male scarlet-tufted malachite sunbirds is a handicap. The factors influencing the reproductive success of a pair are discussed. Offprint requests to: M.R. Evans     

Chemically mediated competition between microbes and animals: microbes as consumers in food webs

Microbes are known to affect ecosystems and communities as decomposers, pathogens, and mutualists. However, they also may function as classic consumers and competitors with animals if they chemically deter larger consumers from using rich food-falls such as carrion, fruits, and seeds that can represent critical windfalls to both microbes and animals. Microbes often use chemicals (i.e., antibiotics) to compete against other microbes. Thus using chemicals against larger competitors might be expected and could redirect significant energy subsidies from upper trophic levels to the detrital pathway. When we baited traps in a coastal marine ecosystem with fresh vs. microbe-laden fish carrion, fresh carrion attracted 2.6 times as many animals per trap as microbe-laden carrion. This resulted from fresh carrion being found more frequently and from attracting more animals when found. Microbe-laden carrion was four times more likely to be uncolonized by large consumers than was fresh carrion. In the lab, the most common animal found in our traps (the stone crab Menippe mercenaria) ate fresh carrion 2.4 times more frequently than microbe-laden carrion. Bacteria-removal experiments and feeding bioassays using organic extracts of microbe-laden carrion showed that bacteria produced noxious chemicals that deterred animal consumers. Thus bacteria compete with large animal scavengers by rendering carcasses chemically repugnant. Because food-fall resources such as carrion are major food subsidies in many ecosystems, chemically mediated competition between microbes and animals could be an important, common, but underappreciated interaction within many communities.