Sexual imprinting misguides species recognition in a facultative interspecific brood parasite

Abstract

Sexual reproduction relies on the recognition of conspecifics for breeding. Most experiments in birds have implicated a critical role for early social learning in directing subsequent courtship behaviours and mating decisions. This classical view of avian sexual imprinting is challenged, however, by studies of megapodes and obligate brood parasites, species in which reliable recognition is achieved despite the lack of early experience with conspecifics. By rearing males with either conspecific or heterospecific brood mates, we experimentally tested the effect of early social experience on the association preferences and courtship behaviours of two sympatrically breeding ducks. We predicted that redheads (Aythya americana), which are facultative interspecific brood parasites, would show a diminished effect of early social environment on subsequent courtship preferences when compared with their host and congener, the canvasback (Aythya valisineria). Contrary to expectations, cross-fostered males of both species courted heterospecific females and preferred them in spatial association tests, whereas control males courted and associated with conspecific females. These results imply that ontogenetic constraints on species recognition may be a general impediment to the initial evolution of interspecific brood parasitism in birds. Under more natural conditions, a variety of mechanisms may mitigate or counteract the effects of early imprinting for redheads reared in canvasback broods.

Figure 1.

Comparison of head profiles of (a) female canvasback, (b) female redhead, (c) male canvasback, and (d) male redhead.

Figure 2.

(a) View from the observer's blind of the arena used for spatial proximity tests of male preferences for stimulus females. (b–d) Examples of courtship behaviours in experimental flocks. (b) Canvasback and redhead males simultaneously following and neck-stretching to a female redhead. (c) Mutual neck stretch by a canvasback pair. (d) Female redhead responding aggressively to a kink-necked call by a male canvasback.

Figure 3.

Outcome of spatial proximity trials (50% random expectation indicated by dashed line). Means (±s.e.) of raw values are presented for each treatment group (n = 8 males per group). Note that (% time with canvasbacks) = 100% − (% time spent with redheads). Lines with filled triangles, canvasbacks reared with redheads; lines with filled circles, redheads reared with redheads; lines with open triangles, canvasbacks reared with canvasbacks; lines with open circles, redheads reared with canvasbacks.

Figure 4.

Male courtship preferences as indicated by the proportion of all observed courtship displays directed towards redhead females during focal animal observations and flock watches (50% random expectation indicated by dashed line). Note that (% displays directed to canvasbacks) = 100% − (% displays directed to redheads). Filled triangles, canvasbacks reared with redheads; filled circles, redheads reared with redheads; open triangles, canvasbacks reared with canvasbacks; open circles, redheads reared with canvasbacks.