Seasonal Patterns of Fat Deposits in Relation to Migratory Strategy in Facultative Migrants

Abstract

Physiological preparations for migration generally reflect migratory strategy. Migrant birds fuel long-distance flight primarily with lipids, but carrying excess fuel is costly; thus, the amount of fat deposited prior to departure often reflects the anticipated flight duration or distance between refueling bouts. Seasonal pre-migratory deposition of fat is well documented in regular seasonal migrants, but is less described for more facultative species. We analyze fat deposits of free-living birds across several taxa of facultative migrants in the songbird subfamily Carduelinae, including house finches (Haemorhous mexicanus), American goldfinches (Spinus tristis), pine siskins (Spinus pinus) and four different North American ecotypes of red crossbills (Loxia curvirostra), to evaluate seasonal fat deposition during facultative migratory periods. Our data suggest that the extent of seasonal fat deposits corresponds with migratory tendency in these facultative taxa. Specifically, nomadic red crossbills with a seasonally predictable annual movement demonstrated relatively large seasonal fat deposits coincident with the migratory periods. In contrast, pine siskins, thought to be more variable in timing and initiation of nomadic movements, had smaller peaks in fat deposits during the migratory season, and the partial migrant American goldfinch and the resident house finch showed no peaks coincident with migratory periods. Within the red crossbills, those ecotypes that are closely associated with pine habitats showed larger peaks in fat deposits coincident with autumn migratory periods and had higher wing loading, whereas those ecotypes associated with spruces, Douglas-fir and hemlocks showed larger peaks coincident with spring migratory periods and lower wing loading. We conclude that population averages of fat deposits do reflect facultative migration strategies in these species, as well as the winter thermogenic challenges at the study locations. A difference in seasonal fattening and wing loading among red crossbill ecotypes is consistent with the possibility that they differ in their migratory biology, and we discuss these differences in light of crossbill reproductive schedules and phenologies of different conifer species.

Does Age, Residency, or Feeding Guild Coupled with a Drought Index Predict Avian Health during Fall Migration?

“…Fat scores were based on the standardized categories set forth by the Institute for Bird Populations and include “none”, “trace”, “light”, “half”, “filled”, “bulging”, “greatly bulging”, and “very excessive” [ 2 ]. Fat scores provide a more direct measure of fuel stores than measures based on body mass [ 18 ], which is known to fluctuate seasonally independently of fat scores [ 27 ].…”

“…We chose to group the data in this manner to avoid zero-inflation problems in the statistical analyses and to evenly distribute the response variable terms, because approximately half of fat scores were categorized as “none”. Studies have shown that fat scores throughout the annual life cycle are higher during fall migration [ 18 , 28 ]. We, therefore, hypothesized that a migrant could be deemed as in better physiological condition if they had a fat score greater than zero during fall migration.…”

“…For example, McKinnon et al [ 13 ] examined life history traits during fall migration for a single species using geolocator data, and Rousseau et al [ 14 ] used fall banding data to assess the demographics of the Rufous Hummingbird ( Selasphorus rufus ). Body condition estimates, often using the amount of fat in a migrating bird, have been used to assess fall migration timing in relation to weather [ 15 , 16 , 17 , 18 ].…”

American Robin (Turdus Migratorius) Blood Lead Levels May Reflect Elevated Soil Lead Levels: Further Consequences of the Flint Water Crisis

“…Morphometrics were measured (un attened wing chord, length of metatarsus and keel) with analog calipers to the nearest 0.1 mm. Mass was measured using a Pesola spring scale to the nearest 0.5 g, and fat deposits were scored on a scale from 0-4 in the furcular and abdominal cavities and summed for a total fat score (following Helms and Drury 1960;Cornelius et al 2021). Birds were equipped with a unique U.S.G.S.…”

Advance social information allows red crossbills ( Loxia curvirostra ) to better conserve body mass and intestinal mass during food stress

“…Specifically, I predict that red crossbills with predictive social information will better conserve body mass during time-limited foraging trials through increased food intake, build-up of fat reserves and preservation of intestinal mass. Further, given that food challenge in the wild may require increased searching behaviour or-in the worstcase scenario-escape migrations to an alternative habitat [41][42][43], I also predict protection of the main muscle for flight locomotion (i.e. the pectoral muscle) during food restriction in those birds with predictive social information.…”