Vanina D. Fiorini

Brood parasite eggs enhance egg survivorship in a multiply parasitized host

Despite the costs to avian parents of rearing brood parasitic offspring, many species do not reject foreign eggs from their nests. We show that where multiple parasitism occurs, rejection itself can be costly, by increasing the risk of host egg loss during subsequent parasite attacks. Chalk-browed mockingbirds (Mimus saturninus) are heavily parasitized by shiny cowbirds (Molothrus bonariensis), which also puncture eggs in host nests. Mockingbirds struggle to prevent cowbirds puncturing and laying, but seldom remove cowbird eggs once laid. We filmed cowbird visits to nests with manipulated clutch compositions and found that mockingbird eggs were more likely to escape puncture the more cowbird eggs accompanied them in the clutch. A Monte Carlo simulation of this ‘dilution effect’, comparing virtual hosts that systematically either reject or accept parasite eggs, shows that acceptors enjoy higher egg survivorship than rejecters in host populations where multiple parasitism occurs. For mockingbirds or other hosts in which host nestlings fare well in parasitized broods, this benefit might be sufficient to offset the fitness cost of rearing parasite chicks, making egg acceptance evolutionarily stable. Thus, counterintuitively, high intensities of parasitism might decrease or even reverse selection pressure for host defence via egg rejection.

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.

Host behaviour and nest-site characteristics affect the likelihood of brood parasitism by shiny cowbirds on chalk-browed mockingbirds

We investigated the association between brood parasitism by shiny cowbirds ( Molothrus bonariensis ), and behaviour and nest-site characteristics of chalk-browed mockingbirds ( Mimus saturninus ). This host builds nests on trees, it is aggressive against intruders and it is larger than shiny cowbirds. We conducted focal observations of mockingbird nests, and registered mockingbird activity and attentiveness around the nest. To characterize nest sites, we measured nest cover, nest height, and distance from the nest to the closest perch, and included host laying date and year as additional predictor variables. We also evaluated experimentally host agonistic behaviours directed towards a female cowbird and a control model, and the association between aggressive behaviour and parasitism. Nest attentiveness, nest cover and laying date were associated with parasitism. These results contradict the host-activity hypothesis, because more attentive pairs were less parasitized, and the nest-exposure hypothesis, because more concealed nests were more parasitized. Experiments showed that unparasitized pairs were more aggressive against cowbird models than were parasitized ones. Our findings indicate that shiny cowbirds prefer to parasitize more concealed nests, where they could lay undetected by the host, and that mockingbird nest attentiveness and aggression towards cowbirds are effective first lines of defence against brood parasitism.

Do shiny cowbird females adjust egg pecking behavior according to the level of competition their chicks face in host nests

Interspecific brood parasites, like the shiny cowbird (Molothrus bonariensis), lay eggs in nests of other species. Shiny cowbird females peck and puncture eggs when they parasitize host nests. This behavior increases the survival of cowbird chicks when they have to compete for food with larger nestmates. However, cowbird chicks may benefit from smaller nestmates as they increase food provisioning by parents and the cowbird chicks secure most extra provisioning. We investigated whether egg-pecking behavior by female shiny cowbirds might be adjusted to the competition that their chicks face in host nests. We found that more host eggs are destroyed per cowbird egg laid in a larger-bodied host (chalk-browed mockingbird, Mimus saturninus, 70-75 g) than a smaller-bodied host (house wrens, Troglodytes aedon, 12-13 g). We also tested egg-pecking preferences in choice experiments with female cowbirds in captivity and found cowbirds presented with eggs in artificial nests pecked first and more frequently, and punctured more frequently the larger egg when this was a host egg, but not when this was a cowbird egg. Our results are partially consistent with the hypothesis that shiny cowbird females adaptively adjust their egg pecking behavior according to the competition that their chicks face in host nests.

Strategic egg destruction by brood-parasitic cowbirds?

Obligate avian brood parasites do not provide direct care to their young but can indirectly increase their offsprings success in host nests. One way in which parasitic cowbirds (Molothrus sp.) could achieve this is through egg puncturing, whereby, prior to laying in a nest, females puncture the eggs that are already present in the nest to reduce the competition that their offspring will later face for food. In this study we investigated whether cowbirds strategically increase their puncturing effort with increasing competitiveness of the future brood. We filmed egg-puncturing behaviour by shiny cowbirds, Molothrus bonariensis, at nests of chalk-browed mockingbirds, Mimus saturninus, a large host whose nests often receive multiple cowbird eggs. We presented cowbirds with large (4 eggs) or small (1 egg) clutches of either mockingbird or cowbird eggs, where large clutch sizes predict greater intrabrood competition than small clutch sizes, and mockingbird eggs (which are larger) predict greater competition than other cowbird eggs. Cowbirds delivered more pecks and punctured more eggs per visit to larger clutches, and mockingbird eggs were broken more often than cowbird eggs, but pecked less per visit. The higher number of pecks aimed at cowbird eggs, despite these producing less competitive nestmates, could reflect responses to eggshell strength rather than egg size, as cowbird eggs are harder to break because of their thicker shells and so require more effort to puncture. Our results show that cowbird puncturing behaviour is not rigid and varies with nest contents. We suggest this variation is consistent with females increasing their offsprings chance of survival.

Dense canopy cover over House Wren (Troglodytes aedon) nests increases latency of brood parasitism by Shiny Cowbirds (Molothrus bonariensis)

Abstract Obligate brood parasites must find host nests in which to lay their eggs. The search for a suitable host nest is predicted to be more difficult if the host nest is well-concealed by vegetation (nest-concealment hypothesis) and brood parasitism of better concealed nests should be less common than parasitism of less well-concealed nests. We experimentally tested this hypothesis by placing nest-boxes used by House Wrens (Troglodytes aedon) in woodland sites with sparse and dense canopy cover and measuring the frequency (proportion of nests parasitised), latency (days elapsed since laying of the first host egg and parasitism), and intensity (number of parasite eggs in parasitised nests) of parasitism by Shiny Cowbirds (Molothrus bonariensis). The frequency and intensity of parasitism did not differ between sites with sparse or dense canopy cover, but the latency of parasitism was shorter in sites with sparse cover than in sites with denser cover. Brood parasites that find host nests more quickly have higher reproductive success, whereas House Wrens have higher reproductive success when parasites lay their eggs later during the incubation period, owing to reduced mortality of host nestlings. Because woodland degradation is associated with sparser canopy cover, host species nesting in degraded woodlands may suffer more from parasitism than those nesting in protected woodlands.

Planning host exploitation through prospecting visits by parasitic cowbirds

We studied visits to potential host nests by two avian brood parasites, the host generalist shiny cowbird, Molothrus bonariensis, and the host specialist screaming cowbird, Molothrus rufoaxillaris, in the periods preceding and overlapping the laying period of their hosts. Our goal was to examine the hypothesis that during prelaying visits, cowbird females form a dynamic memory library of laying opportunities, which they deploy to target suitable nests at a later predawn period. We recorded presence of radio-tagged females within a fixed area around nests of chalked-browed mockingbirds, Mimus saturninus (a common host of shiny cowbirds), and baywings, Agelaioides badius (the main host of screaming cowbirds), using proximity data loggers placed at nests during prelaying, laying and early incubation. Our data confirmed that females of both species visit potential host nests prior to laying and that parasitic events occurred before dawn, earlier in screaming than shiny cowbirds but with little chance of host nests having been discovered on the laying day. There were interesting species differences: visits were less frequent in shiny than screaming cowbirds and the former rarely returned after laying, while screaming cowbirds visited nests repeatedly after laying and occasionally showed repeat parasitism. The higher frequency of revisiting by screaming cowbirds to baywing nests is consistent with the uncommonly long and variable baywing prelaying period, and the incidence of repeat parasitism may reflect low availability of baywing nests and greater flexibility of its parental care potential thanks to it being a social breeder.Significance statementAvian brood parasites synchronize their laying with that of their hosts, as this reduces egg rejection and optimizes hatching time. They also avoid parasitizing nests repeatedly, thus preventing harm to their own previously laid eggs and competition among their offspring. Further, they lay at dawn, so that location of target nests must be known from previous days’ exploration. It has been argued that these adaptations must depend on memory for the location and status of host nests within their home range, a memory feat known as ‘bookkeeping’. We study nest prospecting in a host specialist and a host generalist parasitic cowbird, using a combination of proximity radio tracking and video recordings. Our results confirm the prospecting hypothesis, report previously unknown interspecies differences and illustrate how cognitive adaptations can be studied in the context of field behavioural ecology.

Egg pecking and puncturing behaviors in shiny and screaming cowbirds: effects of eggshell strength and degree of clutch completion

Shiny cowbirds (Molothrus bonariensis) are generalist brood parasites that use hosts varying in body and egg size. On the contrary, screaming cowbirds (M. rufoaxillaris), which are larger than shiny cowbirds, are host specialist that use mainly one host of similar body and egg size. Both parasites peck and puncture eggs when visiting nests. Through puncturing eggs, cowbirds can reduce the competition for food their chicks face (reduction of competition hypothesis), but the same behavior could also be a mechanism to enforce host to renest when nests are found late in the nesting cycle (farming hypothesis). Eggshell strength increases the difficulty to puncture eggs and therefore may modulate egg-pecking behavior. To test these hypotheses, we studied the effect of the degree of clutch completion and egg size on egg-puncturing behavior. Moreover, we evaluated if morphological differences between cowbird species and eggshell strength affected egg-pecking behavior. We presented captive females a nest with complete (four eggs) or incomplete (one egg) clutches of house wren (small egg size, low eggshell strength), chalk-browed mockingbird (large egg size, intermediate eggshell strength), or shiny cowbird (medium egg size, high eggshell strength). The proportion of nests with punctured eggs was similar for complete and incomplete clutches. Cowbirds punctured more eggs in complete than in incomplete clutches, but in complete clutches, they did not destroy the entire clutch. There were no differences in the egg-pecking behavior between cowbird species, which pecked more frequently the eggs with the strongest eggshell. Our findings are consistent with the reduction of competition hypothesis.Significance statementBrood parasitic birds do not build nests and raise their chicks. Instead, they lay eggs in nests of other species (hosts), which carry out all parental care. Some brood parasites, like the cowbirds, peck and puncture eggs when they visit host nests. This behavior may help to reduce the competition for food that their chicks face in the nest (reduction of competition hypothesis) or may enforce hosts to renest (farming hypothesis). We experimentally studied egg-pecking and egg-puncturing behaviors in the host generalist shiny cowbird and the host specialist screaming cowbird. We found that the degree of clutch completion and egg size modulate egg-puncturing behavior and eggshell strength modulates egg-pecking behavior. Our results indicate that by puncturing eggs, cowbirds reduce nest competition.

Thick eggshells of brood parasitic cowbirds protect their eggs and damage host eggs during laying

Avian brood parasites have thick eggshells and numerous hypotheses have been proposed to account for the significance of this trait. We examined whether thick eggshells protect the eggs of the shiny and brown-headed cowbirds during laying events. We found that cowbird eggs were significantly less likely to be damaged compared to host eggs during laying events and, at the same time, these eggs also caused significantly more damage to host eggs.

Parasitism by Botflies Philornis Sp. on European Starlings Sturnus vulgaris, an Exotic Bird in Argentina

Summary. We studied the parasitism of the exotic European starling Sturnus vulgaris by native botflies Philornis spp. in Argentina. We installed thirty nest boxes in the northeastern Buenos Aires province in the 2010–2011 breeding season. In the first brood, subcutaneous larvae of Philornis (Muscidae) parasitised 34 nestlings (89.4%) of 11 clutches (91.6%) and only three nestlings fledged. In the second brood, Philornis parasitised 15 (48.3%) nestlings of seven clutches (70%) and all the nestlings died. Compared to the mortality of other Neotropical birds parasitised by Philornis, the mortality in European starling nests is the highest found in the region. Our results show that the studied population of European starling suffers a high level of parasitism by Philornis, although this factor was not directly associated with the high mortality of starling nestlings. The fact that most non-parasitised nestlings also died indicates that other factors are also affecting nestling survival. Experiments that allow us to isolate the effect of Philornis from other variables would be needed to evaluate the impact of botfly larvae on starling nestlings.