Why do many hosts accept costly avian brood parasitism even when parasitic eggs and nestlings differ dramatically in appearance from their own? Scientists argue that evolutionary lag or equilibrium can explain this evolutionary enigma. Few, however, consider the potential of parasitic birds to enforce acceptance by destroying eggs or nestlings of hosts that eject parasitic eggs and thereby reject parasitism. This retaliatory “mafia” behavior has been reported in one species of parasitic cuckoo but never in parasitic cowbirds. Here we present experimental evidence of mafia behavior in the brown-headed cowbird (Molothrus ater), a widely distributed North American brood parasite. We manipulated ejection of cowbird eggs and cowbird access to predator-proof nests in a common host to test experimentally for mafia behavior. When cowbird access was allowed, 56% of “ejector” nests were depredated compared with only 6% of “accepter” nests. No nests were destroyed when cowbird access was always denied or when access was denied after we removed cowbird eggs, indicating that cowbirds were responsible. Nonparasitized nests were depredated at an intermediate rate (20%) when cowbirds were allowed access, suggesting that cowbirds may occasionally “farm” hosts to create additional opportunities for parasitism. Cowbirds parasitized most (85%) renests of the hosts whose nests were depredated. Ejector nests produced 60% fewer host offspring than accepter nests because of the predatory behavior attributed to cowbirds. Widespread predatory behaviors in cowbirds could slow the evolution of rejection behaviors and further threaten populations of some of the >100 species of regular cowbird hosts.

Hosts of avian brood parasites pay severe costs for rearing unrelated young (1–3). Cuckoo (Cuculidae) hosts typically eject parasitic eggs that do not mimic their own (4–6). Why then do most cowbird (Molothrus spp.) hosts accept parasitic eggs that differ dramatically in appearance from their own (2, 6)? At least three nonexclusive hypotheses have been suggested to resolve this paradox of nonrejection in the face of costly brood parasitism: (i) evolutionary lag [short time of coexistence (2, 7, 8)]; (ii) nonrandom association of parasitism status with individual hosts' repeated breeding attempts [i.e., limited horizontal transmission (9, 10)]; and (iii) evolutionary equilibrium [rejection costs and errors (11–13)] through cognitive and physiological constraints on detection and rejection (14–16).

Proponents of evolutionary equilibrium tend to focus on the limited abilities of hosts to recognize or reject parasitism. Few have explored the possibility that avian brood parasites could enforce acceptance by destroying eggs or nestlings of hosts that eject parasitic eggs (17). This “mafia-like” retaliatory behavior has been reported in one species of parasitic cuckoo (18), but there has never been an experimental test of whether parasites themselves are destroying nests of hosts that eject parasitic eggs. Mafia behavior has not been documented in parasitic cowbirds (2), but results from two studies suggest that brown-headed cowbirds (Molothrus ater) may occasionally depredate nonparasitized host nests, thereby creating opportunities to parasitize those hosts' renesting attempts [“farming” (19, 20)]. Here we present evidence of a mafia-like behavior in the brown-headed cowbird, the most abundant and widely distributed avian brood parasite in North America (21).

We studied the effects of cowbird parasitism on a cavity-nesting host, the prothonotary warbler (Protonotaria citrea), in the Cache River watershed in southern Illinois [37°18′N, 88°58′W (3, 22)]. During 1996–2002, we attempted to make some nests (n = 472) predator-proof by attaching nest-boxes to pieces of greased conduit instead of attaching them to trees (23), and we never removed brown-headed cowbird eggs from the parasitized warbler nests (n = 230). As a result, nearly all (>95%) nests in predator-proof nest-boxes were successful regardless of parasitism status (3, 23). In 2002, as part of a separate study, we removed cowbird eggs from some parasitized predator-proof nests (n = 50), and only 60% were successful, indicating that cowbirds may depredate nests in response to our rejection of their eggs (mafia-like retaliation). This finding led us to test experimentally for both mafia and farming behaviors in cowbirds. Specifically, we removed (ejected) or accepted cowbird eggs and controlled cowbird access to otherwise predator-proof nests of prothonotary warblers to determine whether cowbirds were retaliating (mafia behavior), farming, or having no predatory effect on the warbler nests in our study system.

The penalty to the warblers for ejecting parasitic eggs was the destruction of clutches, presumably by cowbirds. Mafia behavior in brood parasites can hold hosts in an evolutionary state of acceptance only if hosts that accept parasitic eggs have higher reproductive output relative to hosts that reject parasitism and suffer the penalty ( 17 ). In our experiment, accepters paid some costs ( 3 ) associated with being parasitized (category 3 versus category 5) ( Fig. 1 B), but the predatory tactics of cowbirds significantly reduced the mean number of warbler offspring produced per nest in ejectors (category 1) compared with accepters (category 3) ( Fig. 1 B).

Relatively high rates of parasitism would be predicted for the renesting attempts (renests) of those female warblers who lost their nest to suspected cowbird predation. We were able to document the parasitism status (yes or no) of renests of 20 such female warblers for comparison with other nesting attempts (n = 81) that occurred within the study area during the same period. Renests linked to cowbird predation were parasitized more frequently (85%) than the other nesting attempts (36%; χ 1 2 = 15.65; P < 0.001).

Effect of cowbirds on rates of nest predation and reproductive output in nests of prothonotary warblers. (A) Rates of nest predation were significantly different among the five categories of nests (χ 4 2 = 42.22; P < 0.001). Bars shown with the same letter above them were not statistically different (P > 0.05) from each other based on pairwise comparisons using χ 2 tests. (B) Mean + SE numbers of warbler offspring produced per nest were significantly different among the five categories of nests (Kruskal–Wallis test; H 4 = 40.22; P < 0.001). Bars shown with the same letter above them were not statistically different (P > 0.05) from each other based on pairwise comparisons using Mann–Whitney U tests. Sample sizes for nest categories are 46, 72, 32, 16, and 16 for categories 1–5, respectively. See Table 1 and Materials and Methods for a description of each category.

When cowbird access was allowed, 56% of “ejector” nests (category 1) were depredated compared with only 6% of “accepter” nests (category 3) ( Fig. 1 A). Nests that were not parasitized but still accessible to cowbirds (category 2) were depredated at an intermediate rate (20%) ( Fig. 1 A). No nests were depredated when cowbird access to ejector nests was denied after incubation commenced (category 4) or when cowbird access was never allowed (category 5), suggesting that cowbirds were responsible for nest-predation events. These differences in rates of nest predation provide evidence that cowbirds employ both mafia and farming behaviors in this system.

We assigned a total of 182 nests to one of five categories ( Table 1 ) depending on parasitism status (yes or no), ejection status (cowbird eggs removed by observers or accepted), and cowbird access (always allowed, denied after incubation commenced, or never allowed). We then monitored the fates of all nesting attempts and compared actual rates of nest predation with those predicted given different effects of cowbirds ( Table 1 ). Nest-predation events (n = 44) all occurred during the incubation period and involved the damage or destruction of most or all warbler eggs (n = 12), disappearance of most or all warbler eggs (n = 21), or both (n = 11).

Discussion

Implicating Cowbirds. We removed cowbird eggs from nests of a cowbird host that presently accepts brood parasitism (22), and by doing so, we were able to test experimentally for and demonstrate a significant increase in nest predation in response to the ejection of parasitic eggs. These nest-predation events during the incubation period fit well with how cowbirds typically damage or remove host eggs from nests (2, 6, 24). Can any other organism be responsible for the variation in rates of nest predation that we observed? House wrens (Troglodytes aedon), a species known to destroy nests of rival cavity-nesting species (25), do not occur in our study system. Carolina wrens (Thryothorus ludovicianus) occasionally use our nest-boxes (including those with openings that exclude cowbirds), but in 13 years of research, we have no evidence that they depredate or take over warbler nests. Some cowbird hosts may desert nests in response to egg loss [a reduction in clutch size (26)]. One could argue that in our study, the prothonotary warblers themselves removed or damaged their own eggs and deserted nests in response to our removal of cowbird eggs, giving the appearance that the nest had been depredated by a cowbird. However, this argument does not hold for two reasons. First, clutch size at the onset of incubation was not different between categories 1 and 4 (mean ± 1 SE = 4.04 ± 0.18 and 4.12 ± 0.18 eggs, respectively; t 60 = 0.257; P = 0.78). We removed cowbird eggs from nests in both categories, yet nests were depredated only when we continued to allow cowbird access (category 1) (Fig. 1A). Second, clutch size was not different between nests that were depredated in category 1 and those that were not (4.04 ± 0.26 and 4.05 ± 0.29 eggs, respectively; t 44 = 0.032; P = 0.97). One additional piece of evidence implicating cowbirds is that naturally nonparasitized warbler nests (n = 17), initiated late in the breeding season after cowbirds had stopped laying eggs in warbler nests, were never depredated even though cowbird access to nests was allowed. Rates of nest predation for the nests where we removed cowbird eggs and continued allowing cowbird access (category 1) were consistently high (54–66%) across 4 years of experimental manipulation. There is no logical alternative to retaliation by cowbirds that can explain why nests where cowbird eggs were accepted (category 3) remained safe, whereas the majority of nests where we removed cowbird eggs (category 1) were depredated. Brown-headed cowbirds make their living by finding host nests and monitoring them to synchronize their egg laying with that of the host (2, 6, 21). Cowbirds are adept egg predators, and they often remove a host egg before laying one of their own in a nest (2, 6, 24). A number of studies report personal observations (27) or video documentation (28–31) of nest predation by female cowbirds of host eggs and nestlings (rarely) in a variety of host species. Although these observations do not show cowbirds depredating a substantial number of nests, they do show that cowbirds occasionally destroy entire clutches or broods of hosts. Our results show that cowbirds often retaliated against the removal of their eggs by depredating the warbler nests from which cowbird eggs were ejected experimentally. To a lesser extent, additional opportunities for parasitism were created when some nonparasitized nests were also depredated. By manipulating cowbirds' access to host nest-boxes that were otherwise predator-proof, our results provide strong evidence for mafia-like retaliatory behavior in these brood parasitic cowbirds. The presence of mafia and farming behaviors in cowbirds suggests that the parasitic and predatory behaviors of this species are even more sophisticated than previously thought. How common or widespread these behaviors are is still unknown.

Evidence from Other Systems. If cowbirds more than occasionally depredate host nests, then one would predict a positive correlation between rates of nest predation and rates of cowbird parasitism in numerous study systems. However, this correlation is often not found (32), or, when rates are correlated, scientists suggest that it is the result of coincidental preferences by other nest predators and cowbirds for particular habitat features (33, 34). Cowbird control programs, where cowbird numbers are reduced in an effort to increase nesting success for particular hosts, provide opportunities to test the prediction that rates of nest predation decrease when cowbird numbers decrease. Results from these studies show no clear pattern, and many report no change in rates of nest predation (35, 36), whereas others do report lower rates of nest predation (37) when cowbird numbers are reduced. Farming by cowbirds has been put forward as a possible explanation only when nonparasitized nests are depredated more frequently than parasitized nests within a particular study system (19, 20, 38). Numerous other studies have found no evidence of farming in various cowbird hosts (32, 39, 40). However, a lack of association between parasitism status and rates of nest predation (3) does not necessarily indicate a lack of nest predation by cowbirds. Cowbird farming behavior could elevate rates of nest predation for unparasitized nests (19), whereas other nest predators could be attracted to parasitized nests [e.g., certain nest sites prone to discovery by cowbird and predator, loud begging of cowbird nestlings, increased provisioning of nestlings by host adults (32, 39)], resulting in rates of nest predation that appear to be unrelated to parasitism status. Predatory behaviors in cowbirds may be difficult to uncover in conventional studies of the nesting success of cowbird hosts. Even within our own study system, we did not observe an association between parasitism status and nest predation before manipulating nests to reduce the effects of other nest predators (3) and removing cowbird eggs from some nests and not others. Until now there has never been an experimental test for farming and mafia behaviors in cowbirds whereby the effects of other nest predators were eliminated and cowbird access to nests was controlled. Our results provide clear evidence that these predatory behaviors exist.

Farming. The results we present here strongly support two other studies that found compelling, albeit indirect evidence that cowbirds may farm two additional hosts (19, 20). These two study systems consisted of relatively simple bird communities [(i) an island population of song sparrows, Melospiza melodia (19) and (ii) red-winged blackbirds, Agelaius phoeniceus, nesting in grass and sedge meadows (20)] where the availability of relatively few alternative hosts may have permitted closer monitoring by cowbirds and increased the importance of farming behavior. In simple systems, farming behavior may force nesting asynchrony on a host population that is essential for a cowbird that can only lay one egg per day. Prothonotary warblers, on the other hand, nest in diverse bird communities with many alternative hosts (41) where we would predict cowbirds to focus less on one particular host. Prothonotary warblers are excellent hosts (3) and differ from nearly all other cowbird hosts in being secondary cavity nesters (42). Secondary cavity nests, especially the nest-boxes used in this study, may be much more predictable nest sites than the ephemeral, often-hidden open-cup-shaped nests of other hosts. As such, they could be much easier for cowbirds to locate and monitor. Therefore, prothonotary warblers may be more vulnerable than most hosts to farming by cowbirds. Nevertheless, the fact that farming has now been reported in multiple study systems suggests that many hosts may be susceptible to this behavior. The farming and mafia behaviors create opportunities for cowbirds to parasitize renesting attempts of hosts while simultaneously reducing host reproductive output via nest predation and subsequent parasitism of renests (19). Farming and mafia behaviors are adaptive for the individual cowbirds using these behaviors only if they parasitize the renesting attempts caused by their predatory behaviors. Renesting attempts of female warblers whose nests were lost to suspected cowbird nest predation were parasitized at a very high rate (85%), suggesting that cowbirds do benefit from their own predatory behaviors.