Brood parasitism disproportionately increases nest provisioning and helper recruitment in a cooperatively breeding bird

Obligate avian brood parasites lay their eggs in nests of other species (hosts), which raise parasitic young. Parasitic nestlings are likely to influence host’s parental behaviours as they typically beg for food more vigorously than young host for a given hunger level. However, few studies have tested this idea, with conflicting results. These prior studies were largely limited to biparental hosts, but little is known about the effect of brood parasitism on parental behaviours in hosts that breed cooperatively. We followed a multimodel approach to examine the effect of brood parasitism on nest provisioning and helper recruitment in the baywing (Agelaioides badius), a cooperative breeder parasitised by screaming (Molothrus rufoaxillaris) and shiny (Molothrus bonariensis) cowbirds. Multimodel inference results indicated that feeding visits increased with nestling age, cooperative group size and number of cowbird nestlings in the brood. Brood size had little influence on feeding visits, which further suggests that baywings adjusted their provisioning effort in response to cowbird parasitism. In addition, nests parasitised artificially with shiny cowbird eggs or hatchlings recruited more helpers than unmanipulated nests having only host or screaming cowbird young. Our results provide novel evidence that brood parasitism and cooperative breeding interact in determining the levels of nest provisioning.     

Spectral tuning and perceptual differences do not explain the rejection of brood parasitic eggs by American robins (Turdus migratorius)

By laying their eggs in the nests of other birds, avian brood parasites impose the cost of rearing young upon their hosts. The recognition and rejection of foreign eggs are primary host defenses against costly brood parasitism. Hosts of parasitic brown-headed cowbirds (Molothrus ater) challenge coevolutionary theory because most cowbird hosts accept parasitic eggs despite their drastically different appearance from the hosts’ own eggs. American robins (Turdus migratorius) are one of only 10 % of the over 200 potential cowbird host species to robustly reject parasitic eggs, but the mechanisms driving the sensory bases of foreign egg rejection remain elusive. Our research combined avian visual perceptual modeling and behavioral experimentation to investigate chromatic cues eliciting parasitic egg rejection in American robins. We assessed the effects of perceivable background color differences between real host and model parasite eggs, across all four avian photoreceptors, on rates of rejection of model eggs spanning in color across the entire avian spectral sensitivity range, and including immaculate model eggs matching the natural colors of robin and cowbird eggs. The results suggest that egg rejection in robins is driven by the overall perceivable difference in color between own and artificial eggs, and input from all four single-cone avian photoreceptors affects the rejection decision. The results, however, also reveal that when viewed by the avian eye, natural cowbird eggs appear more similar in background color to robin eggs than predicted by the high rejection rate of these parasitic eggs. This suggests that robins respond specifically to parasitism by cowbirds, despite an apparent lack of sensory tuning toward the detection of the background color of cowbird eggs.     

Tactics of parasitic American coots: host choice and the pattern of egg dispersion among host nests

Summary I examined the tactics adopted by a conspecific brood parasite, the American coot (Fulica americana), and the degree to which these tactics reflect sources of mortality for parasitic eggs. Only 8% of parasitic eggs produced independent offspring, compared to a 35% success rate for non-parasitic eggs, and most mortality was due to egg-rejection by hosts or the consequences of laying eggs too late in the host's nesting cycle. Parasites usually laid parasitically before initiating their own nests and usually parasitized immediate neighbours. Parasites did not remove host eggs before laying their own egg, and egg disappearance in general was not more common at parasitized nests. I found no evidence for non-random host choice, either on the basis of stage of the host's nesting cycle or the host's brood size. The absence of adaptive host choice is likely a consequence of the fact that, due to host limitation, only a small proportion of parasites had meaningful variation among potential hosts to choose from. The pattern of egg dispersion among host nests by individual parasites appears to be a compromise between constraints imposed by host limitation and the increased success obtained from spreading eggs among nests. Most females laying fewer than five parasitic eggs laid them in a single host nest while females laying five or more eggs normally parasitized two or more hosts. An examination of egg rejection and survival rates showed that parasites would maximize success by laying a single egg per host nest, and the pattern of laying several eggs per host nest is likely a consequence of host limitation. However, no egg that was the fifth laid, or later, parasitic egg in a host nest was ever successful and this probably explains why most females laying five or more eggs parasitized more than one host.     

Cues used by shiny cowbirds (Molothrus bonariensis) to locate and parasitise chalk-browed mockingbird (Mimus saturninus) nests

Unlike other birds, shiny cowbirds (Molothrus bonariensis) must locate host nests where to lay their eggs and then decide whether to parasitise them. They should also synchronise their laying with that of the host to increase the survival of parasite egg and young. Shiny cowbirds can discover nests using host behaviour as a cue, or by searching the habitat without need for the presence of a host. Besides, they can synchronise parasitism with host laying by monitoring nests during building and laying, or directly by assessing the degree of development of embryos through the puncture of host eggs. Alternatively, synchronization can arise by lower nest attentiveness during host laying. We determined the extent of synchronization between laying of shiny cowbirds and chalk-browed mockingbirds (Mimus saturninus) and estimated if parasitism was negatively associated with host nest attentiveness. We also conducted an experiment to test if host activity was necessary to locate nests, and if puncture of host eggs was a cue for deciding parasitism. Shiny cowbirds synchronised parasitism with host laying in 75% of the cases and synchronization was not explained by lower host nest attentiveness during laying. Shiny cowbirds located nests without need for presence of a host, but the decision of parasitising the nest depended on host activity at the nest. The information that shiny cowbirds could obtain through egg punctures was not necessary for deciding parasitism. Our results indicate that shiny cowbirds rely on the precise timing of their eggs and avoid laying in unsuitable nests.     

Mechanism of egg recognition in defenses against conspecific brood parasitism: American coots (<Emphasis Type="Italic">Fulica americana</Emphasis>) know their own eggs

Hosts of avian brood parasites use a variety of defenses based on egg recognition to reduce the costs of parasitism; the most important of which is rejecting the parasitic eggs. Two basic recognition mechanisms are possible: “true recognition”, whereby hosts recognize their own eggs irrespective of their relative frequency in the clutch, and minority recognition (or “recognition by discordancy”), whereby hosts respond to the minority egg type. The mechanism of recognition has been experimentally studied in a handful of species parasitized by interspecific brood parasites, but the mechanism used in defenses against conspecific brood parasitism is unknown. I experimentally determined the mechanism of egg recognition in American coots (Fulica americana), a species with high levels of conspecific brood parasitism, egg recognition, and rejection. I swapped eggs between pairs of nests to alter frequencies of host and “parasite” eggs and then used two criteria for recognition: egg rejection and nonrandom incubation positions in the clutch. Eight of 12 nests (66%) given equal frequencies of host and parasite eggs showed evidence of true recognition. In contrast, only one of eight (12.5%) nests where host eggs were in the minority showed evidence of recognition by discordancy. The nonrandom incubation positions of parasitic eggs indicates that birds sometimes recognize parasitic eggs without rejecting them and provides a means of assessing recognition on a per nest basis in species with large clutches. Adaptive recognition without rejection may also be an important evolutionary stepping stone to the evolution of egg rejection in some taxa.     

Egg removal and intraspecific brood parasitism in the European starling (Sturnus vulgaris)

Summary From 1983 to 1986 we monitored 284 European starling (Sturnus vulgaris) nests in New Jersey for evidence of intraspecific brood parasitism and egg removal during the laying period. Egg removal occurred significantly more often at nests where intraspecific brood parasitism was detected (12 of 35 nests, 34%) than at unparasitized nests (23 of 249 nests, 9%). Brood parasitism (92% of parasitized nests) and egg removal (74% of nests with egg removal) were most common at nests where egg laying began in April of each year (i.e., early nests). Egg removal occurred at 26 (19%) and brood parasitism at 32 (23%) of 138 early nests. Both brood parasitism and egg removal were concentrated during the first four days in the laying period when brood parasitism is most likely to be successful and when host nests are most vulnerable to parasitism (Romagnano 1987). Both parasitism and removal usually involved a single egg at each nest. We detected brood parasitism and egg removal on the same day at five of 12 nests (42%) where both were observed. Because starlings do not remove foreign eggs from their nests once they begin laying (Stouffer et al. 1987) we hypothesize that parasite females sometimes removed host eggs while parasitizing nests.     

Intraspecific brood parasitism as a conditional reproductive tactic in the treehopper<Emphasis Type="Italic"> Publilia concava</Emphasis>

In species exhibiting egg guarding as well as communal egg laying, females may adopt the strategy of laying eggs in the nests of conspecifics and leaving without providing care (termed intraspecific brood parasitism). This study is the first to describe such a behavior in the insect Publilia concava (Hemiptera: Membracidae) through field studies that followed 849 marked females across 1,828 brood associations. While brood parasitism increased the total number of eggs in a host brood, it did not reduce the overall hatching success of host broods. Solitary females exhibited a range of guarding durations while parasitic females rarely remained to guard eggs. Females exhibiting the parasitic tactic increased their lifetime number of clutches without decreasing the number of solitary clutches that they were able to initiate. Estimates of egg number for these individual broods revealed that females adopting the parasitic tactic (in addition to solitary breeding) had higher lifetime fecundity relative to females that did not parasitize. In females that exhibited both tactics (solitary and parasitic), the parasitic tactic yielded a higher rate of oviposition. The major component of oviposition rate was the time to find hosts and this time decreased with increasing host availability across 222 replicate groups. Females exhibited a shift toward the parasitic tactic when host broods were more abundant (i.e. in larger groups and later in the season). However, the time to find hosts increased as the frequency of the parasitic tactic increased, suggesting that this tactic may be maintained through negative frequency dependence. The results of this study suggest that brood parasitism may be the preferred tactic, as part of a conditional strategy, when hosts are readily available with solitary breeding being the preferred tactic when hosts are in short supply.     

Eggshell spotting in brood parasitic shiny cowbirds (<Emphasis Type="Italic">Molothrus bonariensis</Emphasis>) is not linked to the female sex chromosome

For avian brood parasites in which individual females are host-specialists, the arms race between hosts and parasites has favored egg color polymorphism in the parasite, with female lineages laying mimetic eggs that resemble those of the host species they parasitize. Female sex-linked inheritance of egg color fosters evolutionary stability of egg polymorphism if female lineages show both consistent eggshell color and host use. This co-evolutionary relationship is unlikely to occur if individual brood parasites use different hosts or if egg color is not maternally inherited. The shiny cowbird (Molothrus bonariensis) is an extreme generalist brood parasite that shows a very high degree of egg polymorphism. We tested whether egg spotting in this species has female sex-linked inheritance. If genetic factors controlling the expression of egg spotting were present on the female-specific W chromosome, we expected co-segregation between spotting patterns and mtDNA haplotypes, as both W and mtDNA are maternally inherited. In contrast to the known maternal inheritance of spotting patterns in great tits, we found no associations between eggshell spotting and mtDNA haplotypes, which suggests that eggshell spotting is not maternally inherited in this cowbird species.     

Intraspecific brood parasitism in the moorhen: parentage and parasite-host relationships determined by DNA fingerprinting

Parasitic female moorhens (Gallinula chloropus) lay from one to six eggs in the nests of conspecific neighbours. DNA fingerprinting was used to show that parasitic eggs could be correctly identified when they appeared in addition to or outside the host’s laying sequence. Moorhen hosts accept all parasitic eggs laid after the 2nd day of their laying period. To understand why moorhen hosts tolerate parasitic eggs, we tested two hypotheses. (1) The quasi-parasitism hypothesis: females lay their eggs in the evening when the host males are normally in attendance at the nest, so host males may allow parasitic females to lay in their nests in exchange for fertilizing their eggs. However, DNA fingerprinting showed that all the parasitic eggs were sired by the parasites’ mates. Parasitic moorhens frequently continue laying a clutch in their own nest, without a break in the laying sequence after a parasitic laying bout. The eggs laid by brood parasites in their own nests were also sired by their own mates. Therefore this hypothesis was rejected. (2) The kin selection hypothesis: if one or both members of the host pair are close relatives of the parasite, the costs of rearing parasitic chicks will be to some degree offset by inclusive fitness benefits. We examined the genetic relationships between parasites and their hosts using DNA fingerprinting and genealogical data. Natal philopatry by both sexes was relatively common in this population, and the probability that a neighbour of either sex was a first-order relative (parent-offspring) was calculated as 0.18. Although first-order relatives were not preferentially chosen as hosts over individuals that were not first-order relatives, even through random host selection there is almost a one-in-five chance that brood parasites in this population are closely related to their hosts. This may facilitate host tolerance of parasitic eggs. Other hypotheses are also discussed. Received: 3 February 1995/Accepted after revision: 27 August 1995     

Costs of brood parasitism and the lack of defenses on the yellow-winged blackbird - shiny cowbird system

The shiny cowbird (Molothrus bonariensis) is a generalist brood parasite that lays either white-immaculate or spotted egg morphs in eastern Argentina and Uruguay. Some hosts accept both morphs, others accept spotted eggs and reject the white morph, but no host has been found to accept white eggs and reject spotted ones. It has been suggested that the yellow-winged blackbird (Agelaius thilius) may be that type of host. The finding of a white acceptor-spotted rejector species would help to explain the occurrence and maintenance of the parasite egg polymorphism. We studied the incidence of shiny cowbird parasitism on this host, its costs for their reproductive success and the presence of antiparasitic defenses in the yellow-winged blackbird - shiny cowbird system. The parasite affected the reproductive success of the host in two ways. Cowbirds punctured host eggs causing a reduction in clutch size, and yellow-winged blackbirds deserted their nests whenever they suffered high egg loss. In addition, parasitized nests suffered higher predation during the nestling stage, but not during egg stages, indicating that the difference found was related to the presence of the cowbird chick, and not to higher exposure of parasitized nests to both parasites␣and predators. Despite the costs imposed by the parasite, yellow-winged blackbirds have not evolved antiparasitic defenses. This host did not reject any egg morph of the shiny cowbird nor desert parasitized nests unless it had suffered high egg loss. Current explanations for the host lack of defenses, the “time lag” and the “equilibrium” hypothesis, are discussed. Received: 29 August 1997 / Accepted after revision: 10 January 1998     

Cowbird removals unexpectedly increase productivity of a brood parasite and the songbird host

Generalist brood parasites reduce productivity and population growth of avian hosts and have been implicated in population declines of several songbirds of conservation concern. To estimate the demographic effects of brood parasitism on Bell's Vireos (Vireo bellii), we removed Brown-headed Cowbirds (Molothrus ater) in a replicated switchback experimental design. Cowbird removals decreased parasitism frequency from 77% and 85% at unmanipulated plots to 58% and 47% at removal plots in 2004 and 2005, respectively. Vireo productivity per pair was higher at cowbird removal plots when years were pooled (mean = 2.6 +/- 0.2 [SE] young per pair) compared to unmanipulated plots (1.2 +/- 0.1). Nest desertion frequency was lower at cowbird removal plots (35% of parasitized nests) compared to unmanipulated plots (69%) because removal of host eggs was the proximate cue for nest desertion, and vireos experienced lower rates of egg loss at cowbird removal plots. Nest success was higher among unparasitized than parasitized nests, and parasitized nests at cowbird removal plots had a higher probability of success than parasitized nests at unmanipulated plots. Unexpectedly, cowbird productivity from vireo pairs was higher at cowbird removal plots (mean = 0.3 +/- 0.06 young per pair) than at unmanipulated plots (0.1 +/- 0.03) because fewer parasitized nests were deserted and the probability of nest success was higher. Our study provides the first evidence that increases in cowbird productivity may be an unintended consequence of cowbird control programs, especially during the initial years of trapping when parasitism may only be moderately reduced. Thus, understanding the demographic impacts of cowbird removals requires an informed understanding of the behavioral ecology of host-parasite interactions.     

Differential parasitism of least flycatchers and yellow warblers by the brown-headed cowbird

Summary Brown-headed cowbirds (Molothrus ater) parasitized yellow warblers (Dendroica petechia) six times more frequently than least flycatchers (Empidonax minimus) nesting in the same riparian habitat in southern Manitoba. This difference was surprising because least flycatchers were higher quality hosts; flycatchers always accepted cowbird eggs while warblers did so on only about half the occasions. Both hosts were equally good foster parents, at least until fledging; thus, once an egg was accepted there was probably no further advantage in parasitizing one species over the other. The probability that a nest was parasitized decreased with increasing nest height, perhaps as a consequence of the cowbird's habit of searching for nests close to the ground. Since least flycatchers nested higher in the canopy than yellow warblers, a greater proportion of flycatcher nests probably were not detected by cowbirds. Nevertheless, nest height alone could not account fully for the lower incidence of parasitism on flycatchers. Clutch initiation in flycatchers peaked 6 days after warbler clutch initiations and 5 days after that of cowbirds. Despite the later peak in initiations, flycatcher nests were always available as potential hosts over the entire cowbird laying season and it was not until new clutch initiations by warblers declined in late summer that flycatchers were actually used as hosts. Because least flycatchers responded more aggressively than yellow warblers to a model female cowbird at the nest, we concluded that greater nest defense by flycatchers may have also reduced the rate of brood parasitism in this host. Together, our results suggest the large difference in the frequency of parasitism between these two hosts was primarily a product of nest location but that differences in host nest-defense and breeding season asynchrony may have contributed to preferential host selection.Offprint requests to: J.V. Briskie     

Location of suitable nests by great spotted cuckoos: an empirical and experimental study

Brood parasites depend entirely on their host species to raise their nestlings until independence. Thus, brood-parasite females must discover and select nests that are at a suitable stage for parasitism, and thus, the location of each parasitic egg is crucial in determining the brood-parasite female’s fitness. In relation to host behaviour, one of the main hypotheses proposed to explain how brood-parasite females find and select a suitable nest posits that the most active hosts during nest building should undergo a higher risk of being parasitised (the “host-activity hypothesis”). Here, using the great spotted cuckoo, Clamator glandarius–magpie, Pica pica system, we found that not only cuckoo females parasitise magpie nests regardless of the location and characteristics of nests, but also that the parasite’s observation of host activity near the nest determines a cuckoo female’s decision of laying in a nest. Only one experimental nest (without host activity) was parasitised before the magpie started laying, while 34.14?% of natural active nests were parasitised before the magpie started laying. These observations support the host-activity hypothesis for nest location in great spotted cuckoos.     

Patterns of egg yolk antioxidant co-variation in an avian brood parasite–host system

Despite their importance to offspring fitness outcomes, there has been little previous work on egg-mediated maternal effects in avian brood parasites which lay their eggs in the nests of other species. Here, we examine patterns of egg yolk antioxidant deposition in an avian host (red-winged blackbird, Agelaius phoeniceus) and their brood parasite (brown-headed cowbird, Molothrus ater). Cowbird nestlings compete directly for food with host blackbird chicks and succeed, despite hatching from a smaller egg, by begging more intensely and growing as or more rapidly than their hosts. Dietary-derived antioxidants, such as carotenoids and vitamins E and A, deposited in egg yolk, may enhance growth rate and reduce oxidative stress, and therefore provide a potential mechanism by which female cowbirds could enhance the competitiveness of their young in host nests. However, provision of antioxidants to eggs is costly so we predicted that female cowbirds should adjust the amount of antioxidants in response to variation in host quality. We found that whilst red-winged blackbird clutches parasitised by cowbirds did not differ in the pattern of antioxidant deposition compared to non-parasitised clutches, levels of vitamin E deposited in cowbird eggs were closely matched to that of the individual host clutch in which cowbirds laid their egg. This supports the prediction that female cowbirds adjust the antioxidant concentration of their eggs to increase the likely competitiveness of their offspring in the host nest into which they are laid and may help explain the success of cowbirds in competing with larger host young.     

Brood parasitism and cooperative nesting in the moorhen,Gallinula chloropus

Summary During a two year study of the moorhen, Gallinula chloropus, a quarter of the nests found had two or more females laying in them. This was the result of two features of the moorhens' breeding biology: (1) Cooperative nesting; two (or more) females were paired to the same male. Both laid in the same nest, the second laid a large clutch synchronously to the first, and both cooperated in parental care. (2) Intraspecific brood parasitism; some females (parasites or dumpers) laid a small number of eggs at random in the nests' of neighbours. Cooperatively nesting females were mother and daughter, while parasite and host were not. Parasitic females laid in their own nest as well, and generally did so after completing a dumping sequence. Parasitic eggs were laid at random during the host's laying and incubation period, and were about a quarter as likely to produce an independent chick as were non-parasitic eggs. Parasitic females would probably have reared more chicks by laying in their own nests straight away, instead of dumping any eggs at all, because there was a strong decline in reproductive success with season. Possible reasons why they did not do so are discussed. Parasitised pairs tended to rear fewer own chicks than non-parasitised pairs. There was no evidence that hosts selectively destroyed parasitic eggs. Four hypotheses that explain why hosts accepted dumped eggs are considered. (1) The host pair were unaware of the dump, (2) the host male had copulated with the dumping female, (3) the dumper was related to one of the host pair, and (4) there was a benefit to the host pair because they gained extra helpers. These hypotheses were not supported by the data, but further tests are needed. As the cost of desertion was greater than the cost of acceptance of parasitic eggs, hosts may have been forced to accept.     

Post-ejection nest-desertion of common cuckoo hosts: a second defense mechanism or avoiding reduced reproductive success?

Hosts of the common cuckoo (Cuculus canorus), an avian brood parasite, develop antiparasite defense mechanisms to increase their reproductive success. Ejection of the parasite egg and desertion of the parasitized nest are the most typical adaptations in response to brood parasitism, but nest desertion may also occur in response to partial clutch reduction, independently from parasitism. Some great reed warblers (Acrocephalus arundinaceus) showed both mechanisms in the same incidence of cuckoo parasitism: in 18% of successful ejections of the parasite eggs, they deserted their nests. We studied if such cases of post-ejection nest-desertion are caused by brood parasitism or reduced clutch value. We experimentally parasitized clutches consisting of five or three host eggs with two painted conspecific eggs to mimic parasitic eggs, as multiple parasitism is frequent in the area. Although hosts ejected these parasitic eggs in both clutch categories (100% and 67% for the larger and smaller inital clutch sizes, respectively), we found that after manipulation, post-ejection nest-desertion frequently occurred at small (3-egg) clutches (40%), but rarely at large (5-egg) clutches (17%). The same phenomenon also occurred when unparasitized 3-egg clutches were reduced by two eggs, but not when 5-egg clutches were reduced in the same way. A logistic regression model revealed that only initial clutch size affected nest desertion of parasitized nests in our experiments. Therefore, we conclude that post-ejection nest-desertion is not a second antiparasite mechanism, which might serve as a redundant antiparasite defense, but a reaction to typically small and further decreased clutch size.     

Does the great spotted cuckoo choose magpie hosts according to their parenting ability?

When brood parasites are about to lay an egg, they have to decide which nest to parasitize. The best nest in which to lay will depend on the parenting ability of the host. We have studied selection of magpie (Pica pica) hosts by great spotted cuckoos (Clamator glandarius). Great spotted cuckoos preferentially parasitize large host nests. Nest volume in magpies is a good indicator of territory quality, since there is a negative relationship between magpie nest size and breeding date, and timing of breeding in magpies is known to be positively related to territory quality. Moreover, magpies occupying high-quality territories have high breeding success. Therefore, nest size is positively related to the quality of magpies. Parasitized magpie nests were of greater volume than the nearest neighbouring nest not parasitized by the great spotted cuckoo. In order to test whether the great spotted cuckoos might select high-quality magpie hosts, we manipulated pairs of parasitized and non-parasitized nests with identical laying dates and habitats, introducing into each of the nests the same number of parasitic and non-parasitic eggs. The number of fledglings reared (magpie plus great spotted cuckoo chicks) in naturally parasitized nests was higher than in experimentally parasitized nests. Thus, the probability of survival of the parasite chicks increased if cuckoo eggs were laid in the nests of high-quality hosts originally chosen by the parasite.     

Egg adoption can explain joint egg-laying in common eiders

Hypotheses regarding the evolution and maintenance of intraspecific nest parasitism were tested with data collected during a 3-year study of common eiders (Somateria mollissima) breeding near Churchill, Manitoba. The nest parasitism rate was highest (42.4% of nests) during the year with the highest nest density and the best environmental conditions, and lowest (20.2% of nests) in the year with the lowest nest density and the poorest environmental conditions. Over the nesting season, parasitic eggs were laid at the same time as normally laid eggs. Most parasitic eggs (>75%) were laid before the host female laid her third egg. The majority of the parasitic eggs were the first or second egg produced by the parasitic female. When a parasitic egg was laid before or on the same day as the host female initiated her clutch, the probability of her first egg being depredated before incubation was significantly lowered. First- and second-laid eggs suffered a high rate of predation probably because nesting females do not attend their clutch until their second or third egg is laid. Hypotheses that some females use intraspecific nest parasitism to parasitize the parental care of other females were inconsistent with these data. Egg adoption is a likely explanation for the prevalence of females incubating parasitic eggs in this population. Received: 30 September 1997 / Accepted after revision: 6 May 1998     

The multidimensionality of behavioural defences against brood parasites: evidence for a behavioural syndrome in magpies?

Studies of antiparasite defences against cuckoo parasites have largely neglected the possibility that behavioural components of host defence may correlate giving rise to a behavioural syndrome. Furthermore, the different contribution of the host’s sex in nest defence has traditionally been disregarded. Here, we studied magpie (Pica pica) mobbing behaviour towards dummies of great spotted cuckoo (Clamator glandarius) and non-harmful hoopoes (Upupa epops) and egg rejection of parasite eggs in a population of colour-banded magpies. We predicted a positive correlation between the intensity of nest defence and egg rejection within each sex and that females respond more intensely than males to the threat of brood parasitism as they undertake incubation. Magpie males, but not females, defended their nests more intensely in those nests in which cuckoo model eggs were rejected. Individual magpies did significantly differ in their baseline level of nest attentiveness; however, there were no individual differences once pair identity was considered. Males and females defended their nests more intensely when it was exposed to the presence of a great spotted cuckoo dummy. Males, but not females, were more prone to appear at their nests, and females, but not males, were more prone to defend more intensely when their nests were challenged by a parasite threat. Our results thus agree with the view that mobbing behaviour and egg rejection in magpies may actually constitute a pseudosyndrome and highlight the necessity to integrate interindividual variation and the sex of the host in studies of the evolution of host defences.     

Utilization of a new host in the screaming cowbird <Emphasis Type="Italic">Molothrus rufoaxillaris</Emphasis>, a host specialist brood parasite: host switch or host acquisition?

The screaming cowbird Molothrus rufoaxillaris has been long known as a host specialist brood parasite. However, in the past years, the utilization of two new hosts has been documented. We examined the variation in mitochondrial control region sequences from screaming cowbird chicks found in the nests of two hosts, the bay-winged cowbird (Agelaioides badius), which is its regular host, and the chopi blackbird (Gnorimopsar chopi), which is a new host, in Formosa Province, Argentina. If a group of females switched to this new host, we expected to find an association between host use and haplotype frequency distribution, indicating the presence of host-specific female lineages, whereas we expected no such association if the cowbird population incorporated this new host and females use both hosts simultaneously. Haplotype frequency distributions differed between cowbird chicks from the nests of both hosts. This indicates that nest choice by females of this brood parasite is not random and that they preferentially parasitize the nests of the same host species.