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Hauber, M. E., Pearson, H. E., Reh, A., & Merges, A. (2002). Discrimination between host songs by brood parasitic brown-headed cowbirds ( Molothrus ater). Anim. Cogn., 5(3), 129–137.
Abstract: Songbirds can learn both to produce and to discriminate between different classes of acoustic stimuli. Varying levels of auditory discrimination may improve the fitness of individuals in certain ecological and social contexts and, thus, selection is expected to mold the cognitive abilities of different species according to the potential benefits of acoustic processing. Although fine-scale auditory discrimination of conspecific songs and calls has been frequently reported for brood parasitic brown-headed cowbirds ( Molothrus ater), it remains unclear why and how they perceive differently the songs of their many host species. Using habituation-dishabituation paradigms and measuring behavioral and physiological (heart-rate) responses, we found that captive female cowbirds consistently discriminated between songs of two host species, the song sparrow ( Melospiza melodia) and the red-winged blackbird ( Agelaius phoeniceus). Playback experiments with stimuli composed of con-specific followed by heterospecific vocalizations in the field also demonstrated discrimination between these heterospecific songs even though cowbirds were not attracted to playbacks of either host species' songs alone. Our results do not directly support a nest-searching function of heterospecific song discrimination by cowbirds and are most consistent with a function of the parasites' avoidance of attacks by their aggressive hosts. These data demonstrate discrimination between heterospecific vocalizations by brown-headed cowbirds and add a novel dimension to the already expansive auditory perceptual abilities of brood parasitic species and other songbirds.
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Hauber, M. E., & Sherman, P. W. (2003). Designing and interpreting experimental tests of self-referent phenotype matching. Anim. Cogn., 6(1), 69–71.
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Hauber, M. E., Sherman, P. W., & Paprika, D. (2000). Self-referent phenotype matching in a brood parasite: the armpit effect in brown-headed cowbirds (Molothrus ater). Anim. Cogn., 3(2), 113–117.
Abstract: Most birds and mammals learn characteristics of conspecifics from their parents and siblings. In interspecific brood parasites, however, early social learning could lead to species recognition errors because young are reared among heterospecifics. Conceivably, juvenile parasites might inspect and memorize aspects of their own phenotype, and later match features of encountered individuals to that template. We tested for such self-referent phenotype matching by manipulating feather colors of hand-reared fledglings (n = 21) of the parasitic brown-headed cowbird (Molothrus ater). In simultaneous choice trials (n = 6 trials/subject) between dyed and normal-colored adult females, juvenile cowbirds (< 2 months old) approached more quickly and associated preferentially with individuals that were colored similar to themselves. These preferences remained even when differences between the associative behaviors of juvenile males and females were controlled statistically. Our data imply that cowbirds incorporate their own plumage color into their recognition template. This provides the first evidence of self-referent phenotype matching through experimental manipulation of a recognition cue.
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Moskat, C., & Hauber, M. E. (2007). Conflict between egg recognition and egg rejection decisions in common cuckoo (Cuculus canorus) hosts. Anim. Cogn., .
Abstract: Common cuckoos (Cuculus canorus) are obligate brood parasites, laying eggs into nests of small songbirds. The cuckoo hatchling evicts all eggs and young from a nest, eliminating hosts' breeding success. Despite the consistently high costs of parasitism by common cuckoos, great reed warbler (Acrocephalus arundinaceus) hosts sometime accept and other times reject parasitic eggs. To explore the cognitive basis of this seemingly maladaptive variation in host responses, we documented differences in egg rejection rates within 1-day periods just before and during the egg-laying cycle across host nests. Hosts rejected cuckoo eggs at 28% of nests during the pre-egg-laying stage, but when cuckoos exchanged the first host egg with the parasite egg, rejections increased to 75%. Even later, when several host eggs remained in a nest after parasitism, rejection rate fell to 37.5%. Experimental parasitism with conspecific eggs on the first and second day of host laying showed a similar directional change in relative rejection rates, dropping from 35 to 0%. Mistakes in egg discrimination (ejection error and ejection cost) were observed mostly in the latter part of the laying cycle, mainly when nests contained 5-6 eggs. These correlational and experimental patterns of egg rejection support a cognitive process of egg discrimination through several shifts in hosts' optimal acceptance thresholds of foreign eggs. The results are also consistent with the evolution of foreign egg rejection in the context of nest-sanitation (i.e. the removal of foreign objects). Our results suggest that common cuckoo hosts may recognize more eggs than they reject. This implies that the experience of the host with one or more of its own eggs in the clutch is a key factor in rejecting parasite eggs by allowing inspection and learning about their own clutch.
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