Pair-bond duration leads to convergence in feather iridescence in the spotless starling (Sturnus unicolor)
(2026) BIOM02 20252Degree Projects in Biology
- Abstract
- Assortative mating describes reproductive patterns in which individuals pair based on their phenotypic similarity. Although this has often been explained as a result of mutual mate choice, recent reviews suggest that it may instead arise from indirect ecological mechanisms or methodological biases. Here, I studied a colony of spotless starlings (Sturnus unicolor) to analyse the relationship between feather reflectance, age, and sex. Reflectance was a dimorphic, age-dependent trait: males showed overall higher reflectance than females, but the relationship between reflectance and age differed between the sexes. Analysing the population as a whole, I found no evidence of assortative mating for any of the ornaments studied. However, there was... (More)
- Assortative mating describes reproductive patterns in which individuals pair based on their phenotypic similarity. Although this has often been explained as a result of mutual mate choice, recent reviews suggest that it may instead arise from indirect ecological mechanisms or methodological biases. Here, I studied a colony of spotless starlings (Sturnus unicolor) to analyse the relationship between feather reflectance, age, and sex. Reflectance was a dimorphic, age-dependent trait: males showed overall higher reflectance than females, but the relationship between reflectance and age differed between the sexes. Analysing the population as a whole, I found no evidence of assortative mating for any of the ornaments studied. However, there was significant positive assortment based on feather reflectance in pairs with stable bonds, but not in recently formed pairs. This pattern remained robust after applying inverse probability weighting to correct for biases arising from differential capture probability. These findings run counter to predictions of assortative mate choice and instead support the hypothesis that reflectance is a condition-dependent trait, with phenotypic convergence over time driven by shared environmental influences. (Less)
- Popular Abstract
- Many animals do not choose their partners at random. In some cases, individuals tend to pair with others that are similar to them in traits such as age, color or body size. This has traditionally been explained as the result of active mate choice, with individuals preferring partners that are similar. However, recent studies show that this is not always the case. Similarity between pair members can also emerge through indirect processes, such as ecological factors or methodological issues.
In this project, I studied a colony of spotless starlings (Sturnus unicolor), a bird species in which both males and females display a shiny black plumage. I wanted to analyse whether the reflectance of their feathers changes with age and sex, and... (More) - Many animals do not choose their partners at random. In some cases, individuals tend to pair with others that are similar to them in traits such as age, color or body size. This has traditionally been explained as the result of active mate choice, with individuals preferring partners that are similar. However, recent studies show that this is not always the case. Similarity between pair members can also emerge through indirect processes, such as ecological factors or methodological issues.
In this project, I studied a colony of spotless starlings (Sturnus unicolor), a bird species in which both males and females display a shiny black plumage. I wanted to analyse whether the reflectance of their feathers changes with age and sex, and whether birds choose their partners based on how reflective these feathers are.
I found that feather reflectance differed between males and females, and that it also changed with age. Overall, males had more reflective feathers than females, but the relationship between reflectance and age was different between the sexes. In males, the youngest and the oldest birds exhibited the most reflective feathers, while in females feather reflectance increased with age. This suggests that feather reflectance may play different roles in males and females, depending on the pressures and conditions experienced by each sex.
To test whether birds chose similar partners, I compared newly formed pairs (first year breeding together) and stable pairs (one or more years breeding together). If birds actively preferred partners with similar feather reflectance, I expected to see this pattern in both groups, as new pairs represent mate choice at the beginning. However, this is not what I found, as newly formed pairs did not show any similarity in feather reflectance.
In contrast, birds in stable pairs were much more similar to each other. Part of this pattern was explained by age, as partners also tended to be of similar age, and feather reflectance is influenced by age. However, even after accounting for age, partners in stable pairs still showed similar levels of feather reflectance.
This suggests that the similarity between partners observed in these pairs does not come from initial mate choice, but instead develops gradually over time. Because these birds stay together throughout the year, not only during the breeding season, their plumage might become more alike over time as they experience similar environmental conditions (e.g., diet, climate or parasites).
Overall, my results suggest that feather reflectance in spotless starlings is linked to the bird’s age and body condition. Besides, when pair members are similar, it does not necessarily mean they paired with each other for that reason, but that similarity can also emerge later as a result of sharing the same environment. (Less)
Please use this url to cite or link to this publication:
https://lup.lub.lu.se/student-papers/record/9226121
- author
- García Díaz, Carmen
- supervisor
- organization
- course
- BIOM02 20252
- year
- 2026
- type
- H2 - Master's Degree (Two Years)
- subject
- language
- English
- id
- 9226121
- date added to LUP
- 2026-05-12 15:02:31
- date last changed
- 2026-05-12 15:02:31
@misc{9226121,
abstract = {{Assortative mating describes reproductive patterns in which individuals pair based on their phenotypic similarity. Although this has often been explained as a result of mutual mate choice, recent reviews suggest that it may instead arise from indirect ecological mechanisms or methodological biases. Here, I studied a colony of spotless starlings (Sturnus unicolor) to analyse the relationship between feather reflectance, age, and sex. Reflectance was a dimorphic, age-dependent trait: males showed overall higher reflectance than females, but the relationship between reflectance and age differed between the sexes. Analysing the population as a whole, I found no evidence of assortative mating for any of the ornaments studied. However, there was significant positive assortment based on feather reflectance in pairs with stable bonds, but not in recently formed pairs. This pattern remained robust after applying inverse probability weighting to correct for biases arising from differential capture probability. These findings run counter to predictions of assortative mate choice and instead support the hypothesis that reflectance is a condition-dependent trait, with phenotypic convergence over time driven by shared environmental influences.}},
author = {{García Díaz, Carmen}},
language = {{eng}},
note = {{Student Paper}},
title = {{Pair-bond duration leads to convergence in feather iridescence in the spotless starling (Sturnus unicolor)}},
year = {{2026}},
}