A partial migrant relies upon a range-wide cue set but uses population-specific weighting for migratory timing

Background Many birds species range over vast geographic regions and migrate seasonally between their breeding and overwintering sites. Deciding when to depart for migration is one of the most consequential life-history decisions an individual may make. However, it is still not fully understood which environmental cues are used to time the onset of migration and to what extent their relative importance differs across a range of migratory strategies. We focus on departure decisions of a songbird, the Eurasian blackbird Turdus merula, in which selected Russian and Polish populations are full migrants which travel relatively long-distances, whereas Finnish and German populations exhibit partial migration with shorter migration distances. Methods We used telemetry data from the four populations (610 individuals) to determine which environmental cues individuals from each population use to initiate their autumn migration. Results When departing, individuals in all populations selected nights with high atmospheric pressure and minimal cloud cover. Fully migratory populations departed earlier in autumn, at longer day length, at higher ambient temperatures, and during nights with higher relative atmospheric pressure and more supportive winds than partial migrants; however, they did not depart in higher synchrony. Thus, while all studied populations used the same environmental cues, they used population-specific and locally tuned thresholds to determine the day of departure. Conclusions Our data support the idea that migratory timing is controlled by general, species-wide mechanisms, but fine-tuned thresholds in response to local conditions.

Differential Response of Migratory Guilds of Birds to Urbanization

Species richness and density of native fauna in urban parks and greenspaces (“parks”) is often lower than in surrounding areas. Understanding the causes of these differences requires a hierarchical approach that integrates factors across multiple scales. In 2003 we surveyed bird communities in 48 forested parks in Portland, Oregon, USA, to identify the relative contributions of park size, shape, and connectivity, landscape composition, and variation in local habitat to differences in richness and density of long-distance migrant, short-distance/partial migrant, and resident birds. All surveyed parks contained highly structured understories comprised primarily of native vegetation and lacked development beyond trails. The bird guilds responded differently to environmental factors. Richness and density of long-distance migrants increased with park area and the abundance of small, mostly native, tree species. Resident species richness also increased with the abundance of small trees. Richness of residents and short-distance migrants was independent of park area, and resident density declined with increasing park area. Park shape, connectivity, and landscape composition did not contribute to differences in richness or density of different migratory guilds. Most long-distance migrants were forest-dependent species. An analysis of all forest-dependent species of all migratory guilds suggested that few exist in parks below 10 ha, and minimum area requirements for maintaining populations of forest-dependent species are estimated to be 30 to 40 ha. Without such parks most long-distance migrants would likely disappear from Portland’s landscape.

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

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.

Individual variation in migratory behavior in a subarctic partial migrant shorebird

Migratory behavior can differ markedly amongst individuals within populations or species. Understanding the factors influencing this variation is key to understanding how current environmental changes might influence migratory propensity and the distribution and abundance of migratory species across their range. Here, we investigate variation in migratory behavior of the partially migratory Eurasian oystercatcher (Haematopus ostralegus) population breeding in Iceland. We use the resightings of color-ringed adults and stable isotopes to determine whether individuals migrate or remain in Iceland during winter and test whether individual migratory strategies vary in relation to sex, body size, and breeding location. We also explore individual consistency in migratory strategy and test whether assortative mating with respect to strategy occurs in this population. The proportion of migrants and residents varied greatly across breeding locations but not with respect to sex or body size. Individuals were consistent in migratory strategy between years and there was no evidence of assortative mating by migratory strategy. We use these findings to explore factors underlying the evolution and maintenance of partial migration at high latitudes.

Local avian density influences risk of mortality from window strikes

Up to a billion birds die per year in North America as a result of striking windows. Both transparent and reflective glass panes are a cause for concern, misleading birds by either acting as invisible, impenetrable barriers to desired resources, or reflecting those resources over a large surface area. A high number of window strikes occur during migration, but little is known about the factors of susceptibility, or whether particular avian taxa are more vulnerable than others. We report on a study of window strikes and mist-netting data at the Virginia Zoological Park (Norfolk, Virginia, USA), conducted in the autumn of 2013 and 2014. We focused on three factors likely to contribute to an individual’s predisposition to collide with windows: (i) taxonomic classification, (ii) age, and (iii) migrant vs. resident status. Thrushes, dominated by the partial migrant American Robin (Turdus migratorius), were significantly less likely to strike glass than be sampled in mist nets (χ2= 9.21, p = 0.002), while wood-warblers (Parulidae) were more likely to strike than expected (χ2= 13.55, p < 0.001). The proportion of juveniles striking windows (45.4%) was not significantly different (χ2= 0.05, p = 0.827) than the population of juvenile birds naturally occurring at the zoo (48.8%). Migrants, however, were significantly more susceptible to window strikes than residents (χ2= 6.35, p = 0.012). Our results suggest that resident birds are able to learn to avoid and thus reduce their likelihood of striking windows; this intrinsic risk factor may help explain the apparent susceptibility of certain taxa to window strikes.