Effects of seasonal migration on the latitudinal distribution of west Palaearctic bird species

This paper examines seasonal changes in the latitudinal distribution of birds (excluding seabirds) that breed in the western Palaearctic. Some resident species occupy ranges that span less than 5° of latitude year-round, while some migrant species range over more than 120° of latitude during the course of a year. Among migrant species of land and freshwater habitats, the latitudinal spans of breeding and wintering ranges are correlated. In general, species that breed over a narrow span of latitude also winter over a narrow span of latitude, and vice versa. Among both groups, for any given span of breeding range, species that winter partly in Eurasia and partly in Africa winter over a wider latitudinal span than those that winter entirely within Eurasia or entirely within Africa. Among coastal birds, there is no correlation between the latitudinal spans of breeding and winter areas; most shorebird species breed over a narrow span of (northern) latitude and winter over a wide span distributed linearly along coastlines. Several migration patterns can be distinguished, from complete overlap of breeding and wintering ranges in year-round residents, through partial separation of summer and winter ranges, to complete separation of summer and winter ranges in different geographical regions. In some such species, the gap between breeding and wintering ranges spans up to 55° of latitude (6000 km).     

The spatial habitat structure of host populations explains the pattern of rejection behavior in hosts and parasitic adaptations in cuckoos

In this article we present tentative support for predictionsderived from a spatial habitat structure hypothesis arguingthat common cuckoos Cuculus canorus, the most common obligatebrood parasite in Europe, only breed in areas where they haveaccess to vantage points in trees. Thus, species in which somepopulations breed near trees while other populations breed farther from trees have a different cuckoo—host population dynamicthan species in which all populations always breed in the vicinityof trees. Parasitism rate, mimicry of brood parasite eggs withthose of the hosts, and rejection behavior of hosts varieswith the host breeding habitat. Cuckoos are best adapted toexploit species in which some populations breed near trees while other populations breed in open areas because such hosts arenot always accessible to cuckoos, and thus gene flow amongunparasitized and parasitized populations delays the evolutionof host adaptations. Adaptive behavior in cuckoos as well asin their hosts can be predicted from the spatial habitat structurehypothesis.     

Factors influencing occupancy and density of salt marsh songbirds in northeast Florida

Salt marshes and the organisms that depend on them are subject to a variety of anthropogenic threats. In Florida, Worthington’s Marsh Wrens (Cistothorus palustris griseus) and MacGillivray’s Seaside Sparrows (Ammospiza maritima macgillivraii) are species of concern that inhabit a small, narrow range of salt marsh in the northeastern corner of the state, an area of increasing human development. The historic ranges of these subspecies encompassed salt marshes in five counties, but their ranges had contracted to just two counties by the early 2000s and their populations declined. We surveyed the historic ranges of the two subspecies during the breeding seasons of 2014 and 2015 to document their distributions, identify habitat features that influenced occupancy and density, and assess whether any recolonization had occurred in areas previously abandoned. We found that the ranges of both subspecies remained relatively stable compared to the early 2000s, with no signs of either further contraction or recolonization. Both Marsh Wrens and Seaside Sparrows were more likely to occupy areas farther from uplands. Marsh Wren occupancy was positively associated with marshes dominated by smooth cordgrass (Spartina alterniflora) and negatively associated with marshes dominated by black needlerush (Juncus roemerianus). Seaside Sparrows were more likely to occur at sites of moderate elevation. We found greater densities of both subspecies in areas farther from uplands, with moderate elevations, and dense vegetation. Marsh Wren density also increased in smooth cordgrass marshes, whereas sparrow numbers increased in areas of moderate vegetation height. Despite these differences between subspecies, the need for dense vegetation away from uplands highlights the importance of smooth cordgrass marshes in the region.     

Relationship between breeding and wintering ranges in Palaearctic-African migrants

During the northern winter, Palaearctic migrant species are not evenly distributed within sub-Saharan Africa. Species numbers are greatest in a belt of savannah, lying south of the Sahara, and decline southwards. For any one latitude, species numbers are also greater in the east of Africa than in the west. Only about 3% of 187 species winter exclusively south of the equator, but other species migrate from north to south during the course of the northern winter.
For 62 Palaearctic species which winter entirely in Africa, the areas of breeding and wintering ranges are strongly correlated. With some exceptions, species with the largest breeding ranges also have the largest wintering ranges. However, in 69% of species, the breeding range is larger than the wintering range, whereas in 31% of species the wintering range is larger. On average, the wintering ranges of 57 landbird species cover about two-thirds the area of their breeding ranges, and in many species only parts of the wintering range may be occupied at any one time. This implies that the per area carrying capacity of African wintering areas is greater than that of Eurasian breeding areas.
The general correlation between the sizes of breeding and wintering ranges may have its basis in ecology, with generalists able to occupy wider areas than specialists in both breeding and winter quarters. At the same time, the correlation may result partly from an effect of numbers on range size, in that species which have a wide range at one time of the year may then achieve large numbers which occupy a wide range at the other time of year.     

Correlates of timing of spring migration in birds: a comparative study of trans-Saharan migrants

The evolution of migratory strategies in birds is likely to have been influenced by ecological as well as socio-sexual factors in both wintering and breeding areas. In this comparative study, we analysed timing of spring passage of 38 long-distance migratory bird species that winter south of the Sahara desert and breed in Europe, in relation to wintering and breeding latitudes, moult strategy, nesting site (open vs. cavity), and sexual dimorphism in size and coloration, which may reflect intensity of sexual selection. We employed a large data set consisting of more than 190 000 individuals ringed during spring migration in the Mediterranean Sea. We found that the species that migrated earlier were those wintering farther north, nesting in cavities and showing the largest degree of sexual size dimorphism (SSD). However, sexual dichromatism was not related to migration date. Among passerine species, moulting wing-feathers in Africa delayed migration. We found no support for the energetic constraint hypothesis, which proposes that early arrival selects for large male size, since early arriving species were not larger than late arriving ones. Thus, the observed associations suggest that variation in migration schedules at the interspecific level may have evolved in relation to ecological factors and SSD, possibly reflecting the intensity of mating competition.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 85 , 199–210.     

Reversal of breeding season by lowland birds at higher altitudes in western Cameroon

Evidence of breeding during the dry season is given for 31 species of lowland birds in montane and semi-montane areas in western Cameroon. At least 17 of these species are shown to breed in the wet season at lower altitudes in West Africa. This reversal of breeding season may be due to the unusually heavy rainfall and high humidities which cause temperatures to decrease more rapidly with altitude than on mountains with drier climates. Low temperatures and heavy rainfall during the wet season prevent almost all montane species from breeding then, and affect similarly the lowland birds whose ranges overlap with those of montane birds. Breeding seasons of some lowland species in Cameroon have previously been considered prolonged, but separation of breeding records by altitude reveals clear seasonality.     

The geographical distribution of rarity

Summary This paper asks the question: are most species that are censused as rare in particular localities rare throughout most of their geographic ranges, or are they common in substantial portions of their ranges elsewhere? The first alternative is labeled suffusive rarity and the second diffusive rarity. To answer this and similar questions, rarity can be measured as the fraction of censuses from some locality (e.g., a quadrat) in which a species occurs (occurrence rarity), or the relative or absolute abundance of the species averaged over all censuses from some locality (abundance rarity). The question was analyzed for occurrence-rarity data from Australian terrestrial birds distributed over 1° (10⁴-km²) quadrats. The great majority of species that are rare in a particular quadrat are not rare and are often common in a substantial number of other quadrats, i.e., these avian species are much closer to the diffusive than suffusive portion of the rarity continuum. The data also show that 1) the distribution of sizes of geographic ranges, whether breeding or total, is highly skewed, appearing exponential to more concave; 2) species are much rarer in their nonbreeding than breeding ranges; 3) more widespread species, whether breeding or total ranges are considered, tend to occur more rarely in a slightly but significantly greater fraction of their ranges; and 4) hawks and owls, typified by high abundance rarity, show occurrence rarity in a greater fraction of their ranges than the average nonraptorial species. Although continental birds may be especially predilected toward diffusive rarity, the present analysis points to identification of centers of abundance as major ways of preserving those species contributing most to recorded instances of rarity. Similar analyses with other kinds of organisms would be most welcome.     

Linking the wintering and breeding grounds of warblers along the Pacific Flyway

Long‐distance migration is a behavior that is exhibited by many animal groups. The evolution of novel migration routes can play an important role in range expansions, ecological interactions, and speciation. New migration routes may evolve in response to selection in favor of reducing distance between breeding and wintering areas, or avoiding navigational barriers. Many migratory changes are likely to evolve gradually and are therefore difficult to study. Here, we attempt to connect breeding and wintering populations of myrtle warblers (Setophaga coronata coronata) to better understand the possible evolution of distinct migration routes within this species. Myrtle warblers, unlike most other warblers with breeding ranges primarily in eastern North America, have two disjunct overwintering concentrations—one in the southeastern USA and one along the Pacific Coast—and presumably distinct routes to‐and‐from these locations. We studied both myrtle and Audubon's warblers (S. c. auduboni) captured during their spring migration along the Pacific Coast, south of the narrow region where these two taxa hybridize. Using stable hydrogen isotopes and biometric data, we show that those myrtle warblers wintering along the southern Pacific Coast of North America are likely to breed at high latitudes in Alaska and the Yukon rather than in Alberta or further east. Our interpretation is that the evolution of this wintering range and migration route along the Pacific Coast may have facilitated the breeding expansion of myrtle warblers into northwestern North America. Moreover, these data suggest that there may be a migratory divide within genetically similar populations of myrtle warblers.     

Why small cold-blooded insects pose different conservation problems to birds in modern landscapes

Butterflies and other insects have declined more rapidly than birds over parts of southwest Europe this century, and early attempts to conserve them often failed. Failure occurred mainly because reserve managers did not cater for three differences in their needs compared with those of vertebrates: (1) Many insects occupy very narrow specific niches within their biotopes, often associated with an ephemeral successional stage; (2) although an insect population can sometimes be supported by a small (<1 ha) patch of its specific habitat, an individual habitat patch may remain suitable for no more than 3–10 years and (3) several insect species are too sedentary to colonize new patches of habitat that arise farther than 300 m-1 km from old ones during the period that each remains suitable for breeding. Further complications arise from the dependency of some insects on narrow temperature bands, especially in northwest Europe where many species reach their edges of range and are locked into warm, anthropogenic habitats. The same species have broader or different niches farther south, and their habitats may require very different management across geographical ranges. Despite these problems, conservationists have had considerable success in maintaining populations of threatened insects within nature reserves, once knowledge of their ecology and dynamics was applied to site management.     

Diurnal space use and nocturnal roost‐site selection by male Cerulean Warblers during the breeding season

Detailed information about space use during the breeding season is limited for most Nearctic‐Neotropical migratory species of songbirds because of their small size and often cryptic behaviors. We monitored male Cerulean Warblers (Setophaga cerulea), a species of conservation concern, using radio‐telemetry during the 2006–2008 breeding seasons in northern Alabama to better understand their space use and habitat selection. We estimated diurnal home range and core areas using information theoretic criteria, located nocturnal roost sites, and related day and evening locations to surrounding landscape habitat, including features representative of canopy disturbances. Mean home range size was 6.7 ha (N = 10), and home ranges included an average of at least 2 core areas encompassing 0.7 ha. We located 53 nocturnal roost sites that were an average 159.0 m from the center of the nearest core area. More than one‐third (36.6%) of roost sites were located outside the diurnal home ranges of male Cerulean Warblers; only 13.6% were located in core areas. Males in our study moved much farther than reported in previous studies, with some singing in areas > 300 m from previously used song perches, a behavior suggesting pursuit of extra‐pair copulations. Cerulean Warblers in our study preferentially selected a heavily forested landscape composed of mesic, floodplain bottomlands with little man‐made disturbance. Within their home ranges, diurnal locations of males in core areas were located significantly closer to a creek than locations outside of core areas. Our results suggest that male Cerulean Warblers require much larger areas than previously reported and underscore the importance of a predominately forested landscape in their habitat selection process. Although edge habitats appeared to influence space use by male Cerulean Warblers in our study, the extent to which this is an essential requirement is unclear. Our results and those of previous studies suggest that specific habitat requirements of this species can vary at the local scale throughout its breeding range.     

No downregulation of immune function during breeding in two year‐round breeding bird species in an equatorial East African environment

Some equatorial environments exhibit substantial within‐location variation in environmental conditions throughout the year and yet have year‐round breeding birds. This implies that breeding in such systems are potentially unrelated to the variable environmental conditions. By breeding not being influenced by environmental conditions, we become sure that any differences in immune function between breeding and non‐breeding birds do not result from environmental variation, therefore allowing for exclusion of the confounding effect of variation in environmental conditions. This create a unique opportunity to test if immune function is down‐regulated during reproduction compared to non‐breeding periods. We compared the immune function of sympatric male and female chick‐feeding and non‐breeding red‐capped Calandrella cinerea and rufous‐naped larks Mirafra africana in equatorial East Africa. These closely‐related species occupy different niches and have different breeding strategies in the same grassland habitat. Red‐capped larks prefer areas with short grass or almost bare ground, and breed during low rainfall periods. Rufous‐naped larks prefer areas of tall grass and scattered shrubs and breed during high rainfall. We measured the following immune indices: nitric oxide, haptoglobin, agglutination and lysis, and measured total monthly rain, monthly average minimum (T_min) and maximum (T_max) temperatures. Contrary to our predictions, we found no down‐regulation of immune function during breeding; breeding birds had higher nitric oxide than non‐breeding ones in both species, while the other three immune indices did not differ between breeding phases. Red‐capped larks had higher nitric oxide concentrations than Rufous‐naped larks, which in turn had higher haptoglobin levels than red‐capped larks. T_max was higher during breeding than during non‐breeding for red‐capped larks only, suggesting potential confounding effect of T_max on the comparison of immune function between breeding and non‐breeding birds for this species. Overall, we conclude that in the two year‐round breeding equatorial larks, immune function is not down‐regulated during breeding.     

Breeding variation in female kakapo (

We investigated why some mature females of New?Zealand?s critically endangered parrot, the kakapo (Strigops habroptilus), did not attempt to breed during the 2005 breeding season on Codfish Island. At a population level, the initiation of kakapo breeding appears to correspond with years of mast fruiting of rimu (Dacrydium cupressinum) trees, with the proportion of females that breed each season dependent on the quantity of rimu fruit available. This research investigates possible links between habitat quality within individual home ranges and the breeding status of adult females during 2005, when the abundance of available rimu fruit was low. We assessed the importance of both home range size and habitat characteristics in determining breeding attempts. Foraging home ranges were characterised using radio-tracking and triangulation techniques. The relative importance of habitat variables in optimal breeding habitat was assessed using ecological niche factor analysis. Our results show that female kakapo breeding in 2005 had, on average, home ranges twice the size of those females that did not breed that season and the ranges contained a significantly greater quantity of mature rimu forest. Multivariate analysis illustrates female kakapo were effectively partitioning available habitat, as breeders? foraging locations were positively correlated with high-abundance rimu forest with a tall canopy, described as optimal breeding habitat. In contrast non-breeders? locations were weakly correlated with short forest containing little or no mature rimu forest. To maximise reproductive output each breeding season, conservation managers need to ensure that all breeding-aged females occupy optimal breeding habitat on Codfish Island. Removal to other islands of kakapo not required in the breeding population may enable females to increase their home range size and occupy better breeding habitat.     

Reproduction and survival of suricates (Suricata suricatta) in the southern Kalahari

In most respects, the demography of Kalahari suricates (Suricata suricatta) resembles that of other social mongooses. Average group size varies from four to nine, and groups typically include several mature females, of which one is responsible for the majority of breeding attempts. Breeding females show a postpartum oestrus; gestation is around 60 days; litter size is three to five pups at emergence and females rarely breed before the age of 24 months. In contrast, annual survival rates (0.20 for pups and 0.43 for animals over one year old) are lower than those recorded in other species. Breeding frequency is related to rainfall and breeding can cease altogether when rainfall is unusually low. In a year when this occurred, group size eroded rapidly and over 60% of groups became extinct. Total numbers were slow to recover during the following year because emigration by females was infrequent and new groups did not form in vacant ranges created by the extinction of groups. High rates of group extinction have been found in other cooperative breeders and may occur because breeding success and survival show inverse density dependence.     

Drosophila reared from the stinkhorn, Phallus impudicus, near Zurich, Switzerland

Drosophila species were reared from, baited with and netted over sporophores of the fungal species Phallus impudicus. The presence and proportions of species varied between years, areas and methods. In the first year, 110 sporophores were used for breeding, and in the second year, 16 for baiting and subsequent breeding. 6417 specimens of 10 Drosophila species were obtained. Drosophila suhohscura was reported from England to breed in stinkhorn, but did not breed in or was even attracted to it in Switzerland, suggesting a remarkable geographical variation in breeding substrate utilization.     

MIGRATION PHENOLOGY OF WADERS (CHARADRII) ON THE TAIMYR PENINSULA,NORTHERN RUSSIA

Syroechkovski, E.E. Jr. & Lappo, E.G. 1994. Migration phenology of waders (Charadrii) on the Taimyr Peninsula, northern Russia. Ostrich 65:181–190.

Palearctic waders from six flyways breed on the Taimyr Peninsula. They can be observed in winter from Europe to South Africa and Australia. The interval between pre- and postbreeding movements of waders in Taimyr is short. Breeding waders, which spend from 50–70 days on the tundra have a arrival tight schedule, dependent mostly on weather conditions. Most species arrive during the first half of June, with the more northern areas occupied several days later than southern areas. However, Turnstone, Dunlin and Curlew Sandpiper arrive in the optimal art of their breeding ground first, and subsequently occupy sup-optimal areas to the north and the south. In autumn waders migrate in small groups across the tundra. The schedule depends greatly on the breeding success of the year. Large numbers of Ruff and Bartailed Godwit concentrate in high arctic areas, 300–500 km north of their breeding grounds, before commencing southwards migration. Autumn concentrations of waders in intertidal areas are rare.     

Invasion trajectory of alien trees: the role of introduction pathway and planting history

Global change is driving a massive rearrangement of the world's biota. Trajectories of distributional shifts are shaped by species traits, the recipient environment and driving forces with many of the driving forces directly due to human activities. The relative importance of each in determining the distributions of introduced species is poorly understood. We consider 11 Australian Acacia species introduced to South Africa for different reasons (commercial forestry, dune stabilization and ornamentation) to determine how features of the introduction pathway have shaped their invasion history. Projections from species distribution models (SDMs) were developed to assess how the reason for introduction influences the similarity between climatic envelopes in native and alien ranges. A lattice model for an idealized invasion was developed to assess the relative contribution of intrinsic traits and introduction dynamics on the abundance and extent over the course of simulated invasions. SDMs show that alien populations of ornamental species in South Africa occupy substantially different climate space from their native ranges, whereas species introduced for forestry occupy a similar climate space in native and introduced ranges. This may partly explain the slow spread rates observed for some alien ornamental plants. Such mismatches are likely to become less pronounced with the current drive towards ‘eco gardens’ resulting in more introductions of ornamental species with a close climate match between native and newly introduced regions. The results from the lattice model showed that the conditions associated with the introduction pathway (especially introduction pressure) dominate early invasion dynamics. The placement of introduction foci in urban areas limited the extent and abundance of invasive populations. Features of introduction events appear to initially mask the influence of intrinsic species traits on invasions and help to explain the relative success of species introduced for different purposes. Introduction dynamics therefore can have long‐lasting influences on the outcomes of species redistributions, and must be explicitly considered in management plans.     

Characterizing patterns of introgressive hybridization between two species of Tyrannus following concurrent range expansion

Hybridization between two species typically occurs when allopatric or ecologically dissimilar species expand into areas of secondary contact or habitat transitions. However, as species' ranges shift rapidly in response to environmental change, the potential for novel types of ephemeral hybrid zones exists. Here, we document and describe the occurrence, prevalence and symmetry of a previously undocumented hybrid zone involving two sympatric, ecologically similar sister species that have been expanding their ranges eastward in the central USA: Scissor‐tailed Flycatchers Tyrannus forficatus and Western Kingbirds Tyrannus verticalis. We identified cases of hybridization and introgression using analyses of eight microsatellite DNA loci and a single mitochondrial gene. We also evaluated short‐term reproductive consequences of hybridization for both species by surveying for both species and potential hybrids at the periphery of their ranges in northeastern Arkansas and western Tennessee, USA. Genetic data revealed bi‐directional backcrossing at the periphery of the species’ ranges, including a cryptic hybrid. We also analysed DNA of putative ‘pure’ individuals from other parts of their ranges and detected two cryptic admixed individuals, suggesting backcrossed individuals from the periphery may be dispersing to breed or that hybridization events have occurred in the core. Finally, our results suggest that there are no short‐term reproductive consequences of hybridization for the two species. In total, hybrid zones that occur at the edges of expanding, sympatric ranges may be ephemeral; we suggest they play an important role in introgression and may have long‐standing impacts for sympatric sister species. Exploring the extent of hybrid zones such as this for other range‐expanding taxa will elucidate whether this type of hybrid zone is unique or a common occurrence.     

Effects of errors in range maps on estimates of historical species richness of mammals in Canadian national parks

Aim Tests for faunal relaxation in reserves, particularly for mammals, have relied on comparisons of current species richness with estimates of species richness derived from historical range maps. However, any range map reflects the extent of occurrence of species and not necessarily the area of occupancy. Thus, estimates of historical species richness might be prone to error introduced by ‘false positives’, that is, a species might be considered to have been present in locations where it actually was not. The effect of such ‘false positives’ could bias statistical tests of faunal relaxation to type I error, and result in estimates of the extent of faunal relaxation in reserves greater than was actually the case. We evaluated the potential for errors in historical range maps to generate inflated estimates of historical species richness of mammals at sites that are reserves today. Location Canadian national parks in the Canadian portion of the Alleghenian‐‐Illinoian mammal province in south‐eastern Canada (the maritime region and parts of southern Québec, Ontario and Manitoba). Methods The effect of varying levels of error in range maps on estimates of historical species richness was tested using geographical information systems (GIS)‐based statistical sampling of simulated historical ranges. Species’ areas of occupancy were simulated to be only 25%, 75% and 95% of published historical species ranges. For each reserve, estimates of historical species richness from these simulated species ranges were then compared with similar, previously published estimates of richness based on published historical species ranges. Results Previous estimates of historical species richness for reserves were inversely and linearly related to the degree of inaccuracy of species ranges. If species ranges were, on average, 5% smaller than the accepted ranges, then estimates of historical species richness agreed with previous estimates in c. 90% of cases. However, if historical ranges were, on average, 25% smaller than those used in previous analyses, then previous historical estimates of species richness may be overestimates in c. 40% of cases. Main conclusions Estimates of the extent of faunal relaxation in reserves that use historical range maps to quantify past species richness appear to be sensitive to even small errors in the degree to which range maps may overestimate ‘area of occupancy’.     

The effect of conservation interventions on the abundance of breeding waders within nature reserves in the United Kingdom

Breeding populations of many wading birds have declined globally, primarily caused by habitat degradation and loss. In the UK, population declines have been particularly notable on lowland wet grasslands. In response, some areas of lowland wet grassland have been restored and are under ongoing management to improve the breeding conditions of target species. Here, we assess the efficacy of management measures using a Bayesian framework and controlling for confounding factors. We focus on four wader species, Northern Lapwing Vanellus vanellus, Eurasian Curlew Numenius arquata, Common Snipe Gallinago gallinago and Common Redshank Tringa totanus, that breed in numbers on wet grassland reserve sites in the UK. We collated annual site-specific climate variables, management information (e.g. the creation of wet features and predator control measures) and bird counts between 1994 and 2018. We found the effects of conservation actions varied between intervention types and species. For Lapwing and Redshank, excluding predators by predator-exclusion fencing, especially in combination with fox control, was generally associated with higher breeding counts. For all study species, sites with longer histories of management were associated with higher breeding numbers, with the effect of site age being particularly notable for management on former arable land. Our findings support the effectiveness of targeted conservation actions to achieve high numbers of breeding waders on lowland wet grassland reserves, and also highlight the value of consistent and reliable monitoring data.     

Temperature‐related geographical shifts among passerines: contrasting processes along poleward and equatorward range margins

Climate change is causing widespread geographical range shifts, which likely reflects different processes at leading and trailing range margins. Progressive warming is thought to relax thermal barriers at poleward range margins, enabling colonization of novel areas, but imposes increasingly unsuitable thermal conditions at equatorward margins, leading to range losses from those areas. Few tests of this process during recent climate change have been possible, but understanding determinants of species’ range limits will improve predictions of their geographical responses to climate change and variation in extinction risk. Here, we examine the relationship between poleward and equatorward range margin dynamics with respect to temperature‐related geographical limits observed for 34 breeding passerine species in North America between 1984–1988 and 2002–2006. We find that species’ equatorward range margins were closer to their upper realized thermal niche limits and proximity to those limits predicts equatorward population extinction risk through time. Conversely, the difference between breeding bird species’ poleward range margin temperatures and the coolest temperatures they tolerate elsewhere in their ranges was substantial and remained consistent through time: range expansion at species’ poleward range margins is unlikely to directly reflect lowered thermal barriers to colonization. The process of range expansion may reflect more complex factors operating across broader areas of species’ ranges. The latitudinal extent of breeding bird ranges is decreasing through time. Disparate responses observed at poleward versus equatorward margins arise due to differences in range margin placement within the realized thermal niche and suggest that climate‐induced geographical shift at equatorward range limits more strongly reflect abiotic conditions than at their poleward range limits. This further suggests that observed geographic responses to date may fail to demonstrate the true cost of climate change on the poleward portion of species’ distributions. Poleward range margins for North American breeding passerines are not presently in equilibrium with realized thermal limits.