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The effect of breeding onset on molt intensity of migratory birds breeding at high latitude

Black, Nina (2022) BIOM02 20212
Degree Projects in Biology
Abstract
Increasing global temperatures have been shown to advance the breeding phenology of migratory birds, not least in temperate regions, causing birds to adapt the timing of other life-history events such as molt. At high latitudes, summers are shorter, and the birds must complete their post-breeding molt in the few weeks available before the conditions in the breeding site become unfavorable. I examined if and how birds adjust their molt in response to the timing of breeding onset. In years when breeding started late, we could expect birds to molt a couple of feathers during the breeding period, molt more feathers at once, or to interrupt molt to accommodate for a shortened post-breeding period. Molt data was collected over 37 years from 7... (More)
Increasing global temperatures have been shown to advance the breeding phenology of migratory birds, not least in temperate regions, causing birds to adapt the timing of other life-history events such as molt. At high latitudes, summers are shorter, and the birds must complete their post-breeding molt in the few weeks available before the conditions in the breeding site become unfavorable. I examined if and how birds adjust their molt in response to the timing of breeding onset. In years when breeding started late, we could expect birds to molt a couple of feathers during the breeding period, molt more feathers at once, or to interrupt molt to accommodate for a shortened post-breeding period. Molt data was collected over 37 years from 7 species breeding in a sub-alpine birch forest in Swedish Lapland. Each individual was given a ‘raggedness’ score, as a measure of molt intensity. There was no evidence to support that more feathers are molted at once when the breeding season was delayed, nor was there a significantly greater proportion of birds employing alternative molt strategies depending on the date of breeding onset. The lack of variation in molt intensity with timing of breeding suggests that birds breeding at high latitudes do not adjust their molt intensity because they are already molting as quickly as possible, or molt may not be as flexible as suspected. (Less)
Popular Abstract
Drop it like it’s hot (Feathers, that is)

For migratory birds, life is a cycle of migration, breeding, and molt. The timing of these events is critical for reproductive success and is especially important for birds breeding at high latitude where breeding onset is dictated by snow melt. Variation in annual snow melt may advance or delay the breeding onset date, subsequently delaying the onset of post-breeding molt, which must be completed to survive the fall migration. Thus, if breeding onset is delayed, birds face a greater time pressure to molt and may need to develop alternative molting strategies to complete the molt faster. I examined molt of migratory passerines breeding at high latitudes to see whether variations in the breeding... (More)
Drop it like it’s hot (Feathers, that is)

For migratory birds, life is a cycle of migration, breeding, and molt. The timing of these events is critical for reproductive success and is especially important for birds breeding at high latitude where breeding onset is dictated by snow melt. Variation in annual snow melt may advance or delay the breeding onset date, subsequently delaying the onset of post-breeding molt, which must be completed to survive the fall migration. Thus, if breeding onset is delayed, birds face a greater time pressure to molt and may need to develop alternative molting strategies to complete the molt faster. I examined molt of migratory passerines breeding at high latitudes to see whether variations in the breeding onset affect the post-breeding molt.

I investigated three possible molt strategies: 1) a ‘cheat’ molt- the bird molts a couple flight feathers at the beginning of the breeding season, stops when breeding, and then continues molting the remaining feathers after chicks have fledged; 2) a bird could molt more ‘intensely’, dropping and replacing more feathers at once than it normally would, creating a larger gap in the wing between feathers but faster replacement of all feathers; 3) an interrupted molt- a bird molts as it normally would, but leaves its last remaining flight feathers unmolted. I expected to find more instances of cheat molt and interrupted molt, and larger gaps in the wings due to more feathers being dropped at once when the breeding onset was delayed.

I used ringing and molt data of 7 birds species caught over 38 years in Ammarnäs, Swedish Lapland. When a molting adult was captured, the main flight feathers—the primaries—were assigned two values called the ‘primary score’, which describes the growth progression, and the ‘raggedness value’, which describes how much of a gap is created in the wing by the missing feathers. These two scores can then be used to generate a measurement called ‘Residual Raggedness Value’ (RRV), which allows the intensity of molt to be measured regardless of the stage of molt a bird is in. I used breeding onset data from a long-term nest box study of pied flycatchers in the surrounding area to compare RRV between years with advanced and delayed breeding onset.

There was no difference in the frequency of cheat molt and interrupted molt in years with delayed breeding onset, nor was there any difference in RRV. There was a difference in RRV with breeding onset between the sexes for two species—willow warbler and Eurasian siskin—with males of both species having a greater decrease in RRV with delayed breeding onset, suggesting that males reduce parental investment and begin molting faster when breeding is delayed. These results suggest that birds breeding at high latitudes already molt as quickly as possible.

Master’s Degree Project in Biology, 30 credits, 2022
Department of Biology, Lund University

Advisor: Åke Lindström
Department of Biology (Less)
Please use this url to cite or link to this publication:
author
Black, Nina
supervisor
organization
course
BIOM02 20212
year
type
H2 - Master's Degree (Two Years)
subject
language
English
id
9078649
date added to LUP
2022-04-22 14:51:54
date last changed
2022-04-22 14:51:54
@misc{9078649,
  abstract     = {{Increasing global temperatures have been shown to advance the breeding phenology of migratory birds, not least in temperate regions, causing birds to adapt the timing of other life-history events such as molt. At high latitudes, summers are shorter, and the birds must complete their post-breeding molt in the few weeks available before the conditions in the breeding site become unfavorable. I examined if and how birds adjust their molt in response to the timing of breeding onset. In years when breeding started late, we could expect birds to molt a couple of feathers during the breeding period, molt more feathers at once, or to interrupt molt to accommodate for a shortened post-breeding period. Molt data was collected over 37 years from 7 species breeding in a sub-alpine birch forest in Swedish Lapland. Each individual was given a ‘raggedness’ score, as a measure of molt intensity. There was no evidence to support that more feathers are molted at once when the breeding season was delayed, nor was there a significantly greater proportion of birds employing alternative molt strategies depending on the date of breeding onset. The lack of variation in molt intensity with timing of breeding suggests that birds breeding at high latitudes do not adjust their molt intensity because they are already molting as quickly as possible, or molt may not be as flexible as suspected.}},
  author       = {{Black, Nina}},
  language     = {{eng}},
  note         = {{Student Paper}},
  title        = {{The effect of breeding onset on molt intensity of migratory birds breeding at high latitude}},
  year         = {{2022}},
}