Relationship Between Nest Predation Suffered by Hosts and Brown-Headed Cowbird Parasitism: A Comparative Study

Aggressive hosts are undeterred by a cuckoo's hawk mimicry, but probably make good foster parents

Parasites face a trade-off if the highest quality hosts are also most resistant to exploitation. For brood parasites, well-defended host nests may be both harder to parasitize and harder to predate, leading to better survival of parasitic chicks. This trade-off could be accentuated if brood-parasitic adaptations to reduce front-line defences of hosts, such as mimicry of hawks by Cuculus cuckoos, do not deter hosts which aggressively mob raptors. Here we investigate the costs and benefits to the African cuckoo (Cuculus gularis) of specializing on a highly aggressive host species, the fork-tailed drongo (Dicrurus adsimilis). Field experiments showed that drongos strongly attacked and mobbed both cuckoo and hawk models, implying that hawk mimicry does not deter front-line defences against African cuckoos. Attacks on cuckoo and hawk models generally declined after the egg stage but attacks on snake models sharply increased, suggesting drongos may treat hawks more like cuckoos than predators. We suggest that the cost to cuckoos of parasitizing an aggressive host may be alleviated by subsequent benefits to their offspring, since drongo nests survived better than nests of other species with similar nesting ecology. These results are indicative of a trade-off between host quality and susceptibility for a brood parasite.

Effect of Seasonality on Rates of Gregarine Infection in Workers of a Social Wasp Polybia Paulista (Hymenoptera, Vespidae)

Social insects face strong selection from parasites because the conditions of group living often favor the transmission of infection among nestmates. However, there is little detailed information on the effects of parasite infection in the host species. Workers of Polybia species, neotropical swarm-founding wasps, are commonly infected by gregarines, protozoans that are exclusively parasitic on invertebrates. Previous studies showed that high rates of gregarine infection in workers of Polybia occidentalis (Olivier) have negative effects on their colony performance. However, the effect of seasonality on infection rates throughout the year or between wet and dry seasons has not been examined. Host-parasite interactions cannot be understood without consideration of the overall population dynamic. We compared rates of gregarine infection in workers of Polybia paulista (Ihering) between wet and dry seasons and among months. The 35% rate was by far the highest of the four wet seasons sampled, but the rates declined in the mid-wet season and were very low during the dry season. Strong seasonal differences in infection rates were also observed between the dry and wet seasons. Several potential factors affecting the seasonal differences are discussed.

A generalist brood parasite modifies use of a host in response to reproductive success

Avian obligate brood parasites, which rely solely on hosts to raise their young, should choose the highest quality hosts to maximize reproductive output. Brown-headed cowbirds (Molothrus ater) are extreme host generalists, yet female cowbirds could use information based on past reproductive outcomes to make egg-laying decisions thus minimizing fitness costs associated with parasitizing low-quality hosts. We use a long-term (21 years) nest-box study of a single host, the prothonotary warbler (Protonotaria citrea), to show that local cowbird reproductive success, but not host reproductive success, was positively correlated with the probability of parasitism the following year. Experimental manipulations of cowbird success corroborated that female cowbirds make future decisions about which hosts to use based on information pertaining to past cowbird success, both within and between years. The within-year pattern, in particular, points to local cowbird females selecting hosts based on past reproductive outcomes. This, coupled with high site fidelity of female cowbirds between years, points to information use, rather than cowbird natal returns alone, increasing parasitism rates on highly productive sites between years.

Differential reproductive success favours strong host preference in a highly specialized brood parasite

Obligate avian brood parasites show dramatic variation in the degree to which they are host specialists or host generalists. The screaming cowbird Molothrus rufoaxillaris is one of the most specialized brood parasites, using a single host, the bay-winged cowbird (Agelaioides badius) over most of its range. Coevolutionary theory predicts increasing host specificity the longer the parasite interacts with a particular avian community, as hosts evolve defences that the parasite cannot counteract. According to this view, host specificity can be maintained if screaming cowbirds avoid parasitizing potentially suitable hosts that have developed effective defences against parasitic females or eggs. Specialization may also be favoured, even in the absence of host defences, if the parasite's reproductive success in alternative hosts is lower than that in the main host. We experimentally tested these hypotheses using as alternative hosts two suitable but unparasitized species: house wrens (Troglodytes aedon) and chalk-browed mockingbirds (Mimus saturninus). We assessed host defences against parasitic females and eggs, and reproductive success of the parasite in current and alternative hosts. Alternative hosts did not discriminate against screaming cowbird females or eggs. Egg survival and hatching success were similarly high in current and alternative hosts, but the survival of parasitic chicks was significantly lower in alternative hosts. Our results indicate that screaming cowbirds have the potential to colonize novel hosts, but higher reproductive success in the current host may favour host fidelity.

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.