Evol Ecol Res 16: 37-49 (2014)     Full PDF if your library subscribes.

Sexual size dimorphism and the relationship between timing of breeding and size-assortative mating in a monogamous, marsh-nesting bird

David A. Shealer

Department of Biology, Loras College, Dubuque, Iowa, USA

Correspondence: D.A. Shealer, Department of Biology, Loras College, 1450 Alta Vista, Dubuque, IA 52004, USA.
e-mail: david.shealer@ loras.edu


Question: Can the process of mutual mate choice be inferred from patterns of size-assortative pairing?

Hypothesis: Mutual mate choice should result in a strong pattern of assortative mating early in the breeding season, with assortative pairing becoming weaker as the season progresses.

Organism: Black tern (Chlidonias niger), a semi-colonial, monogamous, marsh-nesting bird with substantial male parental care.

Study sites and dates: Wetland breeding sites throughout Wisconsin (USA) from 2000 to 2009.

Methods: Mated pairs (n = 277) of adults were captured and measured during incubation. I compared measures of body size between males and females to test for within-pair sexual size dimorphism. I used canonical correlation to test for size-assortative mating, with the sample divided into early (≤ 1 June) and late (≥ 2 June) breeding pairs.

Results: Males were larger than females (1–5%) for all measures of body size. Evidence for size-assortative mating was found in mated pairs, but only among late breeders. Hatching success did not depend on timing of breeding, total clutch mass or degree of size-assortative pairing.

Conclusions: Early breeding is not an expression of individual or pair quality. Black terns may exhibit mutual mate choice, but body size does not appear to be an important criterion for it. The pattern of size-assortative mating among late breeders only is difficult to reconcile with existing theoretical models.

Keywords: assortative pairing, black tern, Chlidonias niger, mate choice, mate retention, morphometry, sexual dimorphism.

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