Assortative mating by colored ornaments in blue tits: space and time matter

Abstract

Assortative mating is a potential outcome of sexual selection, and estimating its level is important to better understand local adaptation and underlying trait evolution. However, assortative mating studies frequently base their conclusions on small numbers of individuals sampled over short periods of time and limited spatial scales even though spatiotemporal variation is common. Here, we characterized assortative mating patterns over 10 years in four populations of the blue tit (Cyanistes caeruleus), a passerine bird. We focused on two plumage ornaments—the blue crown and the yellow breast patch. Based on data for 1,657 pairs of birds, we found large interannual variation: assortative mating varied from positive to negative. To determine whether there was nonetheless a general trend in the data, we ran a within‐study meta‐analysis. It revealed that assortative mating was moderately positive for both ornaments. It also showed that mating patterns differed among populations and especially between two neighboring populations that displayed phenotypic divergence. Our results therefore underscore that long‐term studies are needed to draw broad conclusions about mating patterns in natural populations. They also call for studying the potential role of assortative mating in local adaptation and evolution of ornaments in both sexes.

Does it always pay to defend one's nest? A case study in African penguin

“…Previous studies have documented signs on both dis‐assorted (great tits, Dingemanse et al., ) and assorted mating (zebra finches, Schuett, Dall, et al., ; dumpling squid Euprymna tasmanica, Sinn, Apiolaza, & Moltschaniwskyj, ) in risk‐taking behaviour (i.e., exploration behaviour and boldness). However, other studies on mate assortment, whether related to behaviour or other individual characteristics (e.g., plumage colour), indicated either clear pair assortment (Grist et al., ) or a fluctuation in pair assortment between years (Fargevieille et al., ). Adult African penguins tended to assort positively by behaviour with their partner (i.e., similar index of mobility and similar number of threats), but the relations were weak, which suggests an overall diversity of pair assortment.…”

“…In the wild, mate choice relies on a complex mechanism in which environmental conditions must be considered as pair assortment may vary from a year to another depending on the benefit of being assorted or dis‐assorted (Fargevieille et al., ; Schuett et al., ). For example, exhibiting an energetically expensive level of nest defence might be an advantage to secure a nest site and/or a mate, but be mal‐adaptive when food is scarce (Careau et al., ).…”

“…In monogamous species with bi‐parental care, individuals’ choice for their partner is crucial because both parents share the responsibility of offspring survival during early stages of its life (Davies, Krebs, & West, ). Preference for a mate can be made through behaviour or other individual characteristics (Fargevieille, Grégoire, Charmantier, Rey Granado, & Doutrelant, ; Schuett, Tregenza, & Dall, ), which results in pairs being either assorted (i.e., similar personality trait) or dis‐assorted (i.e., dis‐similar personality trait). For example, in a captive experiment, proactive female zebra finches (Taeniopygia guttata ), which were moderately or highly exploratory, preferred risk‐taking males (Schuett, Godin, & Dall, ).…”

“…However, Fargevieille et al. () demonstrated in a long‐term study on pair assortment that assortment may vary between years (i.e., across different environmental conditions) and, therefore, warn on the ease to generalize conclusions in short‐term studies. In addition, how assortment types influence breeding success may also vary with environmental conditions.…”

Scrutinizing assortative mating in birds

“…Such results may arise for both biological and technical reasons. For example, measures of plumage coloration may show temporal autocorrelation because plumage color gradually changes after moult due to wear or because of changes in the white balance used for calibration of hand-held photo spectrometers [37]. Also, depending on the research question, effects that induce temporal and spatial autocorrelation can be of biological interest (e.g., for ecologists), or it can be a confounding factor (e.g., if the interest lies in studying mate choice and mating competition).…”