Can vector summation describe the orientation system of juvenile ospreys and honey buzzards? – An analysis of ring recoveries and satellite tracking

Juvenile bird migrants are generally believed to use a clock‐and‐compass migratory orientation strategy. According to such a strategy migrants accomplish their migration by flying a number of successive flight steps with direction and number of steps controlled by an endogenous programme. One powerful way of testing this is by comparing predictions from a model of such a strategy with observed patterns. We used data from ringing and satellite‐based radio telemetry to investigate the orientation system of juvenile ospreys (Pandion haliaetus) and honey buzzards (Pernis apivorus) migrating from Sweden to tropical west Africa. The ring recoveries showed a much larger scatter in the orientation of ospreys than of honey buzzards, but there was only a slight such difference in the satellite tracks. These tracks of individuals of both species were rather straight with a high directional concentration per step. The honey buzzard data showed a close fit to a simple vector summation model, which is expected if birds follow a clock‐and‐compass strategy. However, the osprey data did not fit such a simple model, as ring recoveries showed a significantly greater deviation at short distances than predicted on the basis of long distance data. Satellite tracking also indicated less concentrated orientation on short distances. The pattern observed for the osprey can generally be explained by an extended vector summation model, including an important element of pre‐migration dispersal. The existence of extensive dispersal in the osprey stands in contrast to the apparent absence of such dispersal in the honey buzzard. The explanation for this difference between the species is unclear. The model of orientation by vector summation is very sensitive to the existence of differences in mean direction between individuals. Assuming such differences, as tentatively indicated by the satellite tracking data, makes simple compass orientation by vector summation inconsistent with the distribution of ring recoveries at long distances, with a high proportion of misoriented birds falling outside the normal winter range.     

Spring migratory routes of eight trans-Saharan passerines through the central and western Mediterranean; results from a network of insular and coastal ringing sites

Detailed information on spring migration routes of songbirds across the Mediterranean is still scanty. Results are presented here from a study on the occurrence of eight Palaearctic-African migrants across the western and central Mediterranean based on ringing data collected during the Progetto Piccole Isole, a co-ordinated project based on standardized mist netting at 21 islands and coastal stations. The species were Melodious Warbler Hippolais polyglotta , Icterine Warbler Hippolais icterina, Bonelli's Warbler Phylloscopus bonelli , Wood Warbler Phylloscopus sibilatrix , Pied Flycatcher Ficedula hypoleuca , Collared Flycatcher Ficedula albicollis , Red-backed Shrike Lanius collurio and Woodchat Shrike Lanius senator . Their capture frequencies were expressed as the frequency of each species relative to the total number of migrants ringed at each station in order to correct for differences in capture effort between stations. For most species, the detailed, quantitative results confirmed the scanty information available so far, whereas in the case of Melodious Warbler and Bonelli's Warbler, evidence of undescribed and unexpected migration routes was found. These results suggest that standardized mist netting provides an important method for studying the migratory routes of small birds, in particular for those species which are not frequently ringed in their breeding and wintering grounds.     

Sex and age-specific differences in wing pointedness and wing length in blackcaps Sylvia atricapilla migrating through the southern Baltic coast

The blackcap Sylvia atricapilla shows a complex migratory pattern and is a suitable species for the studies of morphological migratory syndrome, including adaptations of wing shape to different migratory performance. Obligate migrants of this species that breed in northern, central, and Eastern Europe differ by migration distance and some cover shorter distance to the wintering grounds in the southern part of Europe/North Africa or the British Isles, although others migrate to sub-Saharan Africa. Based on ˃40 years of ringing data on blackcaps captured during autumn migration in the Southern Baltic region, we studied age- and sex-related correlations in wing pointedness and wing length of obligate blackcap migrants to understand the differences in migratory behavior of this species. Even though the recoveries of blackcaps were scarce, we reported some evidence that individuals which differ in migration distance differed also in wing length. We found that wing pointedness significantly increased with an increasing wing length of migrating birds, and adults had longer and more pointed wings than juvenile birds. This indicates stronger antipredator adaptation in juvenile blackcaps than selection on flight efficiency, which is particularly important during migration. Moreover, we documented more pronounced differences in wing length between adult and juvenile males and females. Such differences in wing length may enhance a faster speed of adult male blackcaps along the spring migration route and may be adaptive when taking into account climatic effects, which favor earlier arrival from migration to the breeding grounds.     

Montane forest birds in winter: do they regularly move to lower altitudes? Observations from the Eastern Cape,South Africa

Seasonal altitudinal migration to lower altitudes including the coast has been ascribed to a number of forest birds, of which 14 species occur at Fort Fordyce Reserve in the Eastern Cape, South Africa. Based on our observations and ringing at this site (2007–2017), as well as concurrent data from the South African Bird Atlas Project (SABAP2), we suggest that in this region only three species, the African Dusky Flycatcher Muscicapa adusta, White-starred Robin Pogonocichla stellata and Barratt's Warbler Bradypterus barratti, are regular altitudinal migrants. For two other species, the Grey Cuckooshrike Coracina caesia and Yellow-throated Woodland Warbler Phylloscopus ruficapilla, local movements apparently occur, but these may take place within the coastal zone rather than between the coast and inland forests.     

“Closer‐to‐home” strategy benefits juvenile survival in a long‐distance migratory bird

Human‐induced changes in the climate and environment that occur at an unprecedented speed are challenging the existence of migratory species. Faced with these new challenges, species with diverse and flexible migratory behaviors may suffer less from population decline, as they may be better at responding to these changes by altering their migratory behavior. At the individual level, variations in migratory behavior may lead to differences in fitness and subsequently influence the population's demographic dynamics. Using lifetime GPS bio‐logging data from 169 white storks (Ciconia ciconia), we explore whether the recently shortened migration distance of storks affects their survival during different stages of their juvenile life. We also explore how other variations in migratory decisions (i.e., time, destination), movement activity (measured using overall body dynamic acceleration), and early life conditions influence juvenile survival. We observed that their first autumn migration was the riskiest period for juvenile white storks. Individuals that migrated shorter distances and fledged earlier experienced lower mortality risks. In addition, higher movement activity and overwintering “closer‐to‐home” (with 84.21% of the tracked individuals stayed Europe or North Africa) were associated with higher survival. Our study shows how avian migrants can change life history decisions over only a few decades, and thus it helps us to understand and predict how migrants respond to the rapidly changing world.     

Process in the evolution of bird migration and pattern in avian ecogeography

Current ideas about the evolution of bird migration equate its origin with the first appearance of fully migratory populations, and attribute its evolution to a selective advantage generated by increased breeding success, gained through temporary emigration from resident populations to breed in under-exploited seasonal areas. I propose an alternative hypothesis in which migration first appears as a temporary directional shift away from the breeding site outside the reproductive period, in response to seasonal variation in the direction and/or severity of environmental gradients. Fully migratory populations then appear through either extinction of sedentary phenotypes, or colonisation of vacant seasonal areas by migrants. Where colonisation occurs, resident ancestral populations can be driven to extinction by competition from migrants which invade their range outside the breeding season, resulting in fully migratory species. An analogous process drives the evolution of migration between high latitudes and the tropics, since extension of breeding range into higher latitudes may drive low latitude populations to extinction, resulting in an overall shift of breeding range. This process can explain reverse latitudinal gradients in avian diversity in the temperate zone, since the breeding ranges of migratory species concentrate in latitudes where they enjoy the highest breeding success. Near absence of forest-dwelling species among Palaearctic-African migrants is attributable to the lack of forest in northern Africa for much of the Tertiary, which has precluded selection both for southward extension of migration by west Palaearctic forest species, and northward breeding colonisation by African forest species.     

High juvenile mortality during migration in a declining population of a long‐distance migratory raptor

Many populations of long‐distance migrants are declining and there is increasing evidence that declines may be caused by factors operating outside the breeding season. Among the four vulture species breeding in the western Palaearctic, the species showing the steepest population decline, the Egyptian Vulture Neophron percnopterus, is a long‐distance migrant wintering in Africa. However, the flyways and wintering areas of the species are only known for some populations, and without knowledge of where mortality occurs, effective conservation management is not possible. We tracked 19 juvenile Egyptian Vultures from the declining breeding population on the Balkan Peninsula between 2010 and 2014 to estimate survival and identify important migratory routes and wintering areas for this species. Mortality during the first autumn migration was high (monthly survival probability 0.75) but mortality during migration was exclusively associated with suboptimal navigation. All birds from western breeding areas and three birds from central and eastern breeding areas attempted to fly south over the Mediterranean Sea, but only one in 10 birds survived this route, probably due to stronger tailwind. All eight birds using the migratory route via Turkey and the Middle East successfully completed their first autumn migration. Of 14 individual and environmental variables examined to explain why juvenile birds did or did not successfully complete their first migration, the natal origin of the bird was the most influential. We speculate that in a declining population with fewer experienced adults, an increasing proportion of juvenile birds are forced to migrate without conspecific guidance, leading to high mortality as a consequence of following sub‐optimal migratory routes. Juvenile Egyptian Vultures wintered across a vast range of the Sahel and eastern Africa, and had large movement ranges with core use areas at intermediate elevations in savannah, cropland or desert. Two birds were shot in Africa, where several significant threats exist for vultures at continental scales. Given the broad distribution of the birds and threats, effective conservation in Africa will be challenging and will require long‐term investment. We recommend that in the short term, more efficient conservation could target narrow migration corridors in southern Turkey and the Middle East, and known congregation sites in African wintering areas.     

Migration patterns of Hoopoe Upupa epops and Wryneck Jynx torquilla: an analysis of European ring recoveries

For many bird species, recovery of ringed individuals remains the best source of information about their migrations. In this study, we analyzed the recoveries of ringed European Hoopoe (Upupa epops) and the Eurasian Wryneck (Jynx torquilla) from 1914 to 2005 from all European ringing schemes. The aim was to define general migration directions and to make inferences about the winter quarters, knowing that hardly any recoveries are available from sub-Saharan Africa. For the autumn migration, there is evidence of a migratory divide for the Hoopoe in Central Europe, at approximately 10–12°E. Autumn migration directions of Wrynecks gradually change from SW to SE depending on the longitude (west to east) of the ringing place. In both species, only a few recoveries were available indicating spring migration directions, but they showed similar migration axes as for autumn migration, and hence no evidence for loop-migration. Due to a paucity of recoveries on the African continent, we can make only limited inferences about wintering grounds: extrapolating migration directions are only indicative of the longitude of the wintering area. The directions of autumn migration indicate a typical pattern observed in European long-distance migrants: west-European Hoopoes and Wrynecks are likely to winter in western Africa, while central- and east-European birds probably winter more in the east. Due to the migratory divide, for the Hoopoe, this phenomenon is more pronounced.     

Biometry and phenology of two sibling Phylloscopus warblers on their circum-Mediterranean migrations

The Mediterranean Sea is known as an ecological barrier for numerous migratory birds flying from European breeding grounds to African wintering sites. Birds generally avoid migration over open sea and fly over land. In the Mediterranean Basin, few land bridges or bottlenecks for migratory birds exist. The narrowest are at the western and eastern extremes: the Strait of Gibraltar and Israel. Comparative studies between these locations are extremely rare to date. Therefore, in order to elucidate the differences between the two flyways, we compared data collected simultaneously for two sister leaf warbler species, the Bonelli’s Warbler complex, Phylloscopus bonelli and Phylloscopus orientalis, at ringing stations in the western Mediterranean Basin Gibraltar, and the eastern Eilat, Israel. Data on biometrics and passage dates of individuals trapped at Gibraltar and Eilat were used, and it was found that mean arrival date of Western Bonelli’s Warblers at Gibraltar was 15 days later than Eastern Bonelli’s Warblers at Eilat. Furthermore, Western Bonelli’s Warblers had shorter wings than Eastern Bonelli’s Warblers. On the other hand, birds in Eilat were in poorer body condition than individuals in Gibraltar. The comparison between geographically distant stop-over sites contributes to furthering our understanding of the development of migration strategies across ecological barriers in sibling species. Our study showed that populations that breed in southwestern Europe migrate through Gibraltar and winter in West Africa are able to accomplish migration in comparatively good body condition. This is in contrast to those that winter in East Africa, migrate through Israel and have to endure the combined challenge of crossing the Sahel, Sahara and Sinai deserts before reaching their breeding grounds across southeast Europe and southwest Asia. Hence, the discrepancies described between the western and the eastern flyway suggest that individuals in the west, in general, migrate shorter distances, have a physiologically less demanding crossing of the North African deserts and appear to stage before their crossing the Strait of Gibraltar, a privilege unavailable to the migrants of the eastern flyway.     

Geolocator reveals migratory and winter movements of a Prothonotary Warbler

Prothonotary Warblers (Protonotaria citrea) are Nearctic‐Neotropic migrants that have experienced declining populations over the past 50 yr. Determining their migration routes and wintering areas are critical steps in identifying habitats used by species and populations of conservation concern. We captured a male Prothonotary Warbler on its breeding territory in Louisiana in June 2013 and attached a geolocator. We recaptured this male in March 2014 and analysis of the geolocator data revealed that the male traveled an estimated 7950 km through seven countries. Fall migration was characterized by movements south into Central America on 13 August 2014, then east to the Greater Antilles for about 1 mo, followed by a second movement south to Panama or Colombia on 15 November 2014 where the male remained for the rest of the winter. Spring migration occurred rapidly, with the bird leaving its southern wintering area on 4 March 2014 and returning to its breeding territory on 23 March 2014. The movements of this male Prothonotary Warbler add to accumulating evidence that many migrant birds exhibit either prolonged stopover behavior or secondary migratory movements. Factors contributing to prolonged stopover behavior or additional migratory movements of the Prothonotary Warbler in our study warrant further investigation.     

Passerine migration strategies and body mass variation along geographic sectors across East Africa, the Middle East and the Arabian Peninsula

Using the body mass estimates of 12 long-distance migrating Palearctic passerine species monitored at successive sites across the Eastern Africa flyway, we tested whether birds modulate their body mass according to specific seasonal demands across different geographic sectors. We compared body mass estimates across latitudinal distances and geographic sectors in Europe, the desert, Northeast Africa and East Africa. Our results show that, depending on the species and season considered, the average body mass increase or decrease is variable at and among different geographic sectors. By comparing the variation in body mass between different ecological sectors, we were able to show when and where migrants accumulate their migratory fuel reserves during migration.     

Across a migratory divide: divergent migration directions and non‐breeding grounds of Eurasian reed warblers revealed by geolocators and stable isotopes

Migratory divides represent narrow zones of overlap between parapatric populations with distinct migration directions and, consequently, expected divergent non‐breeding distributions. The composition of the mixed population at a migratory divide and the corresponding non‐breeding ranges remain, however, unknown for many Palaearctic‐African migrants. Here, we used light‐level geolocation to track migration direction and non‐breeding grounds of Eurasian reed warblers Acrocephalus scirpaceus from three breeding populations across the species’ migratory divide. Moreover, by using feathers grown at non‐breeding grounds, we quantified stable isotope composition for individuals with known southwestern (SW) and southeastern (SE) migration directions. On a larger sample per population, we then assessed the proportions of SW‐ and SE‐migrating phenotypes in each of the three populations. All tracked reed warblers from Germany and two thirds of the birds tagged from the Czech population headed initially SW. Nevertheless, about one third of the birds from the Czech site migrated towards SE. No tracking data have been obtained for the Bulgarian population. The initial migration direction determined by geolocators was a strong predictor of the non‐breeding region, with SW migrants staying in west Africa and SE migrants in central Africa. Feather δ³⁴S and δ¹⁵N values confirmed the predominance of SW migrants in the German population, the co‐occurrence of SW and SE migrants in the Czech population, and indicated a high (72%) proportion of SE migrants in the Bulgarian population. Thus, the combined approach of geolocator tracking and stable isotopic assignments provided clear evidence for the existence of a migratory divide in the southeast of central Europe and predicted non‐breeding range in central and central‐eastern Africa for the eastern population.     

Rapid moult among palaearctic passerines in West Africa- an adaptation to the oncoming dry season?

Moult of some palaearctic migrant passerines was studied at a wintering site in northern Ghana, West Africa. We present data on moult pattern and estimated moult duration for a sample of species. Savi's Warbler Locustella luscinioides , Woodchat Shrike Lanius senator and probably Grasshopper Warbler Locustella naevia deviated from the ordinary descendent moult sequence. Different measures of moult speed (moult speed of retrapped individuals, growth of individual feathers, wing raggedness and first date of completion of moult) showed unexpectedly rapid moult among individuals of several species, notably the Great Reed Warbler Acrocephalus arundinaceus and the Sedge Warbler A. schoenobaenus. The rapid moult might be an adaptation to a limited period of abundant food which comes to an end at the beginning of the dry season.     

Extreme genetic similarity does not predict non‐breeding distribution of two closely related warblers

Detailed knowledge of migratory connectivity can facilitate effective conservation of Neotropical migrants by helping biologists understand where and when populations may be most limited. We studied the migratory behavior and non‐breeding distribution of two closely related species of conservation concern, the Golden‐winged Warbler (Vermivora chrysoptera) and Blue‐winged Warbler (Vermivora cyanoptera). Although both species have undergone dynamic range shifts and population changes attributed to habitat loss and social interactions promoting competition and hybridization, full life‐cycle conservation planning has been limited by a lack of information about their non‐breeding ecology. Because recent work has demonstrated that the two species are nearly identical genetically, we predicted that individuals from a single breeding population would have similar migratory timing and overwintering locations. In 2015, we placed light‐level geolocators on 25 males of both species and hybrids in an area of breeding sympatry at the Fort Drum Military Installation in Jefferson and Lewis counties, New York. Despite extreme genetic similarity, non‐breeding locations and duration of migration differed among genotypes. Golden‐winged Warblers (N = 2) overwintered > 1900 km southeast of the nearest Blue‐winged Warbler (N = 3) and spent nearly twice as many days in migration; hybrids (N = 2) had intermediate wintering distributions and migratory timing. Spring migration departure dates were staggered based on distance from the breeding area, and all birds arrived at the breeding site within 8 days of each other. Our results show that Golden‐winged Warblers and Blue‐winged Warblers in our study area retain species‐specific non‐breeding locations despite extreme genetic similarity, and suggest that non‐breeding locations and migratory timing vary along a genetic gradient. If the migratory period is limiting for these species, our results also suggest that Golden‐winged Warblers in our study population may be more vulnerable to population decline than Blue‐winged Warblers because they spend almost twice as many days migrating.     

Complex postbreeding molt strategies in a songbird migrating along the East Asian Flyway,the Pallas’s Grasshopper Warbler Locustella certhiola

Molt strategies have received relatively little attention in current ornithology, and knowledge concerning the evolution, variability and extent of molt is sparse in many bird species. This is especially true for East Asian Locustella species where assumptions on molt patterns are based on incomplete information. We provide evidence indicating a complex postbreeding molt strategy and variable molt extent among the Pallas's Grasshopper Warbler Locustella certhiola, based on data from six ringing sites situated along its flyway from the breeding grounds to the wintering areas. Detailed study revealed for the first time that in most individuals wing feather molt proceeds from the center both toward the body and the wing‐tip, a molt pattern known as divergent molt (which is rare among Palearctic passerines). In the Russian Far East, where both breeding birds and passage migrants occur, a third of the adult birds were molting in late summer. In Central Siberia, at the northwestern limit of its distribution, adult individuals commenced their primary molt partly divergently and partly with unknown sequence. During migration in Mongolia, only descendantly (i.e., from the body toward the wing‐tip) molting birds were observed, while further south in Korea, Hong Kong, and Thailand the proportion of potential eccentric and divergent feather renewal was not identifiable since the renewed feathers were already fully grown as expected. We found an increase in the mean number of molted primaries during the progress of the autumn migration. Moderate body mass levels and low‐fat and muscle scores were observed in molting adult birds, without any remarkable increase in the later season. According to optimality models, we suggest that an extremely short season of high food abundance in tall grass habitats and a largely overland route allow autumn migration with low fuel loads combined with molt migration in at least a part of the population. This study highlights the importance of further studying molt strategy as well as stopover behavior decisions and the trade‐offs among migratory birds that are now facing a panoply of anthropogenic threats along their flyways.     

Estimating variation among individuals in migration direction

Among-individual variation in mean migration directions is the basis of evolution of new migration routes and has important consequences for our understanding of the migratory orientation system. This variation in migration directions is also of interest for modelling of migratory flight paths. In test series with caged migrants, the variation among individuals is generally much smaller than the variation within individuals making the variation among individuals difficult to detect with small sample sizes. No methods exist for estimating among-individual variance for directional data. We therefore used simulations to estimate the variation among individual mean migration directions. Among-individual variation was found to be present in at least half of the 34 series analysed. In the 21 series with first-time migrants, our estimates of the variation among individuals ranged from r=0.20 (mean vector length) to >0.99, with median 0.93 and 86% less than 1. We found slightly more variation among individuals in displaced birds and in experiments with manipulated cues. Test series with experienced migrants, presumably having varying goal areas, showed similar estimates (median 0.94 and 77% less than 1).     

Does the migration programme constrain dispersal and range sizes of migratory birds?

Aim  It is generally believed that the migration programme constrains the dispersal and hence range sizes of migratory bird species. This conclusion is based on analyses of breeding ranges of migratory versus non-migratory (resident) terrestrial bird species, and rests on the assumption that there are no ecological or evolutionary constraints on extending the non-breeding range. To investigate this assumption, the abilities of migrant and resident terrestrial species to colonize new wintering areas were compared.
Location  Three major wintering regions of long-distance migrants: South America, sub-Saharan Africa and the Indian Subcontinent.
Methods  It was determined whether the relative numbers of residents and short- and long-distance migrants were the same in those species that have dispersed to a novel wintering region as in the source species pools.
Results  At the species level, long-distance migratory species are more likely to have non-breeding ranges that include more than one of the above regions than resident species. This indicates that the dispersal of migratory species is less constrained than that of resident species. The pattern holds irrespective of the inclusion or exclusion of species associated with coastal, freshwater and wetland habitats, and also holds for ecological groups such as aerial feeders. The pattern is most pronounced between the regions separated by the strongest dispersal barriers (South America and sub-Saharan Africa).
Main conclusions  It is unlikely that the migration programme per se constrains dispersal, but rather that difficulties in establishing new non-breeding areas prevent range expansions in migrant species.     

Broad wintering range and intercontinental migratory divide within a core population of the near‐threatened pallid harrier

Aim To identify the migration routes and wintering grounds of the core populations of the near‐threatened pallid harrier, Circus macrourus, and highlight conservation needs associated with these phases of the annual cycle. Location Breeding area: north‐central Kazakhstan; Wintering areas: Sahel belt (Burkina Faso to Ethiopia) and north‐west India. Methods We used ring recovery data from Kazakhstan and satellite tracking data from 2007 to 2008 on six adults breeding in north‐central Kazakhstan to determine migration routes and locate wintering areas. In addition, one first‐year male was tagged in winter 2007–2008 in India. Results Data evidenced an intercontinental migratory divide within the core pallid harrier population, with birds wintering in either Africa or India. The six individuals tagged in north‐central Kazakhstan followed a similar route (west of the Caspian Sea and Middle East) towards east Africa, before spreading along the Sahel belt to winter either in Sudan, Ethiopia, Niger or Burkina Faso. Spring migration followed a shorter, more direct route, with marked interindividual variation. The bird tagged in India spent the summer in central Kazakhstan. Half of the signal losses (either because of failure or bird mortality) occurred on the wintering areas and during migration. Main conclusions Our study shows that birds from one breeding area may winter over a strikingly broad range within and across continents. The intercontinental migratory divide of pallid harriers suggests the coexistence of distinct migratory strategies within the core breeding population, a characteristic most likely shared by a number of threatened species in central Asia. Conservation strategies for species like the pallid harrier, therefore, require considering very large spatial scales with possibly area‐specific conservation issues. We highlight urgent research priorities to effectively inform the conservation of these species.     

Postbreeding elevational movements of western songbirds in Northern California and Southern Oregon

Migratory species employ a variety of strategies to meet energetic demands of postbreeding molt. As such, at least a few species of western Neotropical migrants are known to undergo short‐distance upslope movements to locations where adults molt body and flight feathers (altitudinal molt migration). Given inherent difficulties in measuring subtle movements of birds occurring in western mountains, we believe that altitudinal molt migration may be a common yet poorly documented phenomenon. To examine prevalence of altitudinal molt migration, we used 29 years of bird capture data in a series of linear mixed‐effect models for nine commonly captured species that breed in northern California and southern Oregon. Candidate models were formulated a priori to examine whether elevation and distance from the coast can be used to predict abundance of breeding and molting birds. Our results suggest that long‐distance migrants such as Orange‐crowned Warbler (Oreothlypis celata) moved higher in elevation and Audubon's Warbler (Setophaga coronata) moved farther inland to molt after breeding. Conversely, for resident and short‐distance migrants, we found evidence that birds either remained on the breeding grounds until they finished molting, such as Song Sparrow (Melospiza melodia) or made small downslope movements, such as American Robin (Turdus migratorius). We conclude that altitudinal molt migration may be a common, variable, and complex behavior among western songbird communities and is related to other aspects of a species’ natural history, such as migratory strategy.     

To breed or not to breed: a model of partial migration

Migration is used by a number of species as a strategy for dealing with a seasonally variable environment. In many migratory species, only some individuals migrate within a given season (migrants) while the rest remain in the same location (residents), a phenomenon called ‘partial migration’. Most examples of partial migration considered in the literature (both empirically and theoretically) fall into one of two categories: either species where residents and migrants share a breeding ground and winter apart, or species where residents and migrants share an overwintering ground and breed apart. However, a third form of partial migration can occur when non‐migrating individuals actually forgo reproduction, essentially a special form of low‐frequency reproduction. While this type of partial migration is well documented in many taxa, it is not often included in the partial migration literature, and has not been considered theoretically to date. In this paper we present a model for this partial migration scenario and determine under what conditions an individual should skip a breeding opportunity (resulting in partial migration), and under what conditions individuals should breed every chance they get (resulting in complete migration). In a constant environment, we find that partial migration is expected to occur when the mortality cost of migration is high, and when individuals can greatly increase their fecundity by skipping a year before breeding. In a stochastic environment, we find that an individual should skip migration more frequently with increased risk of a bad year (higher probability and severity), with higher mortality cost of migration, and with lower mortality cost of skipping. We discuss these results in the context of empirical data and existing life history theory.