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An investigation into the quality of carrion crow Corvus corone feathers in England and Wales

Hawthorn, A (2022) An investigation into the quality of carrion crow Corvus corone feathers in England and Wales. Doctoral thesis, Liverpool John Moores University.

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Flight feathers are crucial for foraging, predator avoidance and large-scale movements in most avian populations. However, the structural integrity of these feathers can be compromised by growth defects, negatively impacting flight ability and survival. Poor feather condition is characterised by the presence of fault bars, which are weak areas displayed as translucent bands that appear through the width of the feather vane. Fault bars occur as a result of stressful or adverse environmental conditions during feather growth. The thesis investigated the macroscopic and microscopic characteristics of this growth defect in relation to current formation theories, assessed different feather quality measures and explored possible causes and consequences of fault bars. The study was carried out at RSPCA Stapeley Grange Wildlife Centre, which receive a large number of carrion crow Corvus corone admissions displaying poor feather condition each year.

Firstly, the macroscopic and microscopic characteristics of fault bars were observed, reviewing existing fault bar formation theories and imagery with the use of advanced technology. Unique observational evidence was presented from this, identifying a ‘squeezed’ appearance to the barbules within fault bars, supporting the hypothesis of muscular constriction around the growing feather pin. Moreover, for the first time, images of fault bar occurrence within the growing feather pin were presented.

Relationships between different measures of feather quality were then assessed, accounting for differences between feather type (primary, secondary and tail), in reference to the ‘fault bar allocation’ hypothesis (Jovani & Blas, 2004). In this, fault bars occur on feathers that are least important for flight, resulting in the majority of fault bars being located on the tail feathers, with the lowest numbers in the primary feathers. A variety of feather quality measures were assessed here, including the number of fault bars, average width of fault bars, feather iridescence and strength, in addition to the number of snapped and white feathers. A key finding in this study was the relationship between the average width of fault bars and average feather iridescence across all feather types. This information strengthens our knowledge of how dull feather portray honest communication signals of low fitness. Moreover, average feather strength was found to be an independent measure of quality, with generally no relationship found with other measures of quality.

Average fault bar width measurements were used to investigate the causes of fault bar production in relation to chemical profile of feathers, parasite burden, sex and age (study aim 3). This made a valuable and novel discovery, identifying a possible link between calcium deficiency and fault bar occurrence. Calcium an essential element in skeletal mineralisation and eggshell formation. Therefore, the results of this study add to the knowledge of calcium and its role in fitness, expanding to feather quality. This study also found a potential trade-off between costly immune defences facilitated at the cost of feather quality, where low numbers of endoparasite species associated with wide fault bars in the wing feathers. Poor feather quality was not found to vary between sexes, as carrion crows are monomorphic and non-migratory. In regard to age differences, the tail feathers of younger individuals were found to have the widest fault bars. This supports many other studies in highlighting the vulnerability of juveniles during the feather growth period.

Lastly, average feather strength measurements were used to investigate the consequences of poor feather quality in relation to the chemical profile of feathers, endoparasite burden, sex and age (study aim 4). A key finding here was that stress resistant bases were associated with a high proportion of chlorine in primary flight feathers. Links to parasite burden and sex were not identified; however, in line with the above findings, younger individuals were found to have low stress tolerance in the primary feathers compared to adults. Differences in feather strength in relation to fault bar occurrence was also reviewed. Contrary to predictions, no differences in strength were found between feather regions with fault bar occurrence and those with fault bar absence.

Future research in this field could be extended to nestlings, an age group that was unfortunately excluded from this study due to the presentation of pin feathers. Moreover, research could also be broadened to additional species, as fault bars are found to impact a wide variety of passerine and non-passerine individuals. This may then open opportunities in understanding stressors faced by vulnerable species, aiding future conservation efforts.

Item Type: Thesis (Doctoral)
Uncontrolled Keywords: Feather; Quality; Carrion crow
Subjects: Q Science > QH Natural history > QH301 Biology
Q Science > QL Zoology
Divisions: Biological & Environmental Sciences (from Sep 19)
SWORD Depositor: A Symplectic
Date Deposited: 13 Jul 2022 13:42
Last Modified: 13 Jul 2022 13:42
DOI or ID number: 10.24377/LJMU.t.00017201
Supervisors: Mettke-Hofmann, C, Gunn, A and Rae, R
URI: https://researchonline.ljmu.ac.uk/id/eprint/17201
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