More Like this

1. Intra‐specific variation in migration phenology of American Kestrels ( Falco sparverius ) in response to spring temperatures

   https://doi.org/10.1111/ibi.12953  

   Powers, Breanna F. ; Winiarski, Jason M. ; Requena‐Mullor, Juan M. ; Heath, Julie A. ( March 2021 , Ibis)

   null (Ed.)

   Abstract In migratory birds, among- and within-species heterogeneity in response to climate change may be attributed to differences in migration distance and environmental cues that affect timing of arrival at breeding grounds. We used eBird observations and a within-species comparative approach to examine whether migration distance (with latitude as a proxy) and weather predictors can explain spring arrival dates at the breeding site in a raptor species with a widespread distribution and diverse migration strategies, the American Kestrel Falco sparverius. We found an interactive effect between latitude and spring minimum temperatures on arrival dates, whereby at lower latitudes (short-distance migrants) American Kestrels arrived earlier in warmer springs and later in colder springs, but American Kestrels at higher latitudes (long-distance migrants) showed no association between arrival time and spring temperatures. Increased snow cover delayed arrival at all latitudes. Our results support the hypothesis that short-distance migrants are better able to respond to conditions on the breeding ground than are long-distance migrants, suggesting that long-distance migrants may be more vulnerable to shifts in spring conditions that could lead to phenological mismatch between peak resources and nesting. 

   more » « less
2. Seasonal Patterns of Fat Deposits in Relation to Migratory Strategy in Facultative Migrants

   https://doi.org/10.3389/fevo.2021.691808  

   Cornelius, Jamie M. ; Hahn, Thomas P. ; Robart, Ashley R. ; Vernasco, Ben J. ; Zahor, Dorothy L. ; Glynn, Kenneth J. ; Navis, Corrie J. ; Watts, Heather E. ( June 2021 , Frontiers in Ecology and Evolution)

   null (Ed.)

   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. 

   more » « less
3. Interspecific competition between resident and wintering birds: experimental evidence and consequences of coexistence

   https://doi.org/10.1002/ecy.3208  

   Powell, Luke L. ; Ames, Elizabeth M. ; Wright, James R. ; Matthiopoulos, Jason ; Marra, Peter P. ( January 2021 , Ecology)

   The contribution of interspecific competition to structuring population and community dynamics remains controversial and poorly tested. Interspecific competition has long been thought to influence the structure of migrant–resident bird communities in winter, yet experimental evidence remains elusive. The arrival of billions of songbirds into Neotropical habitats, where they co‐exist with residents, provides a unique opportunity to assess interspecific competition and its consequences. Working in 15 ha of Jamaican black mangrove forest, we used removal experiments to test whether dominant resident Yellow Warblers compete interspecifically with subordinate wintering American Redstarts; we also used observational evidence (interspecific territorial overlap) to understand whether this coexistence influences physical condition, spring departure dates or annual return rates. Consistent with interspecific competition, after experimental removal of the resident, yearling male Redstarts (but not females or adult males) immediately moved into vacated Yellow Warbler territories, increasing their overlap with the space by 7.3%. Yearling Redstarts also appeared to adjust their territorial space use by actively avoiding Yellow Warblers; for example, Redstarts departing the wintering grounds as yearlings and returning the following winter shifted such that their territories overlapped 32% less with those of Yellow Warblers. Adult Redstarts showed no such territorial flexibility. Adult male Redstarts also showed evidence supporting the consequences of coexistence: territorial overlap with Yellow Warblers was negatively correlated with body condition and annual return rates. Adult male Redstarts with <25% territorial overlap with Yellow Warblers were more than three times as likely to return between seasons than those with 100% overlap. We propose that the territorial inflexibility of adult male Redstarts produces these consequences, which may be due to their years‐long investment in that particular territory. More generally, the temporary nature of migrant–resident interspecific competition is likely what allows coexistence during winter, the most resource‐poor time of year. Interspecific competition and the consequences of coexistence are likely age‐ and sex‐specific and the product of intraspecific dominance hierarchy in Redstarts. Our observations suggest that interspecific coexistence has measurable consequences, and our experiments support the long‐held, but previously untested belief that resident birds compete interspecifically with wintering migrants.

    

   more » « less
4. Perceived wintering latitude determines timing of song output in a migratory bird

   https://doi.org/10.1002/ece3.5922  

   Brewer, Dustin E ; McGill, Clint A ; Fudickar, Adam M. ( December 2019 , Ecology and evolution)

   Migratory bird populations frequently consist of individuals that overwinter variable distances from the breeding site. Seasonal changes in photoperiod, which varies with latitude, underlie seasonal changes in singing frequency in birds. Therefore, migratory populations that consist of individuals that overwinter at different latitudes with large overwintering ranges could experience within-population variation in seasonal production of song. To test the influence of overwintering latitude on intrapopulation variance in song production in the spring, we subjected two groups of Eastern Song Sparrows (Melospiza melodia melodia) from the same partially migratory breeding population to different photoperiodic schedules associated with a 1,300-km difference in overwintering location. One group remained on the natural photoperiodic schedule of the breeding site (resident group) while the other group experienced a nonbreeding photoperiod that mimicked a southern migration in the fall followed by a northern migration back to the breeding site in the spring (migratory group). We compared song output between the two groups in three different stages (nonbreeding, prebreeding, and breeding). Little singing occurred during nonbreeding stage sample dates (20 November, 6 December) for the resident group, and no singing occurred for the migrant group. During the prebreeding stage (27 January, 7 February), significantly more singing occurred in the resident group than in the migrant group. During the breeding stage (21 March, 4 April), after a simulated migration for the migrants, song output was similar in both groups. These results suggest that within-population variation in wintering latitude may contribute to variation in seasonal changes in singing behavior, which may covary with readiness to breed. Studies utilizing confirmed migrants and residents, rather than merely simulated migrants and residents, are also needed to better understand these processes. 

   more » « less
5. Breeding latitude predicts timing but not rate of spring migration in a widespread migratory bird in South America

   https://doi.org/10.1002/ece3.5159  

   Jahn, Alex E. ; Cereghetti, Joaquín ; Cueto, Víctor R. ; Hallworth, Michael T. ; Levey, Douglas J. ; Marini, Miguel Â. ; Masson, Diego ; Pizo, Marco A. ; Sarasola, José Hernán ; Tuero, Diego T. ( April 2019 , Ecology and Evolution)

   Identifying the processes that determine avian migratory strategies in different environmental contexts is imperative to understanding the constraints to survival and reproduction faced by migratory birds across the planet.

   We compared the spring migration strategies of Fork‐tailed Flycatchers (Tyrannus s. savana) that breed at south‐temperate latitudes (i.e., austral migrants) vs. tropical latitudes (i.e., intratropical migrants) in South America. We hypothesized that austral migrant flycatchers are more time‐selected than intratropical migrants during spring migration. As such, we predicted that austral migrants, which migrate further than intratropical migrants, will migrate at a faster rate and that the rate of migration for austral migrants will be positively correlated with the onset of spring migration.

   We attached light‐level geolocators to Fork‐tailed Flycatchers at two tropical breeding sites in Brazil and at two south‐temperate breeding sites in Argentina and tracked their movements until the following breeding season.

   Of 286 geolocators that were deployed, 37 were recovered ~1 year later, of which 28 provided useable data. Rate of spring migration did not differ significantly between the two groups, and only at one site was there a significantly positive relationship between date of initiation of spring migration and arrival date.

   This represents the first comparison of individual migratory strategies among conspecific passerines breeding at tropical vs. temperate latitudes and suggests that austral migrant Fork‐tailed Flycatchers in South America are not more time‐selected on spring migration than intratropical migrant conspecifics. Low sample sizes could have diminished our power to detect differences (e.g., between sexes), such that further research into the mechanisms underpinning migratory strategies in this poorly understood system is necessary.

    

   more » « less