Abstract Identifying how and why species vary in their ability to adjust to rapidly changing climates is a key challenge in ecology. While phenological shifts are well documented for birds and often studied in the context of tracking resource availability, less is known about the extent to which adjustments in phenology allow populations to track a consistent thermal niche. In particular, there has been little examination of how the extent of phenological thermal niche tracking compares over time versus space; a comparison that has the potential to inform on the underlying mechanisms. Here, we use data on breeding phenology derived from BTO Nest Record Scheme data, to examine the extent to which 13 passerine bird species track a consistent incubation thermal niche across years (both interannually and a year gradient) and along latitudinal and elevational gradients, and whether migrant and resident species differ in their tracking ability. Overall, we found support across species for partial tracking, with all species showing trends consistent with partial tracking across one or more axis, though for one species we could not reject the null hypothesis of no tracking. When we looked at average trends across species, we found significant tracking across interannual variation, latitude, and elevation, but not across a year trend. However, we found no evidence that tracking differs between residents and migrants, and for only a few species did we found evidence that species incubation thermal niche impacts on fitness. Taken together, our findings highlight the extent to which shifts in phenology can allow birds to track a thermal niche in a changing climate. The timing of a thermal niche provides a useful and widely-applicable yardstick to examine how changes in climate will impact on the abiotic conditions that populations experience.

Climate warming is altering the timing of seasonal events across ecosystems, impacting the temporal synchrony of interactions among species (1, 2). For trophic interactions, the match-mismatch hypothesis predicts that when consumers become phenologically asynchronous with key ephemeral resources their fitness will decline (3-5). Most studies of mismatch focus on single resource-consumer species pairs, and implicitly assume trophic specialisation. However, many consumers exploit more than one resource species, giving rise to several mechanisms whereby the negative impacts of mismatch on individuals and populations could be buffered (6). Here we experimentally manipulate phenological asynchrony across 48 plant-caterpillar interactions in a spring woodland food-web system and assay caterpillar performance. As asynchrony increases, we find strong evidence for a decline in survival that generalises across host-caterpillar interactions, whereas caterpillar growth and development are largely unaffected. We also show that focus in the literature on a single model interaction (Oak-Winter Moth) has likely overestimated the general impact asynchrony in this system. The strength of the effect of mismatch varies markedly among host-plants, caterpillars, and their interactions - with a small number of interactions showing little or no decline in consumer performance despite substantial asynchrony. Our results demonstrate that the fitness consequences of phenological mismatch are widespread but interaction-specific, revealing substantial heterogeneity in how trophic interactions are expected to respond to climate-driven shifts in seasonal timing. This variation in response could allow resource diversity and resource switching to buffer consumer guilds against the phenological impacts of ongoing climate change, stabilising the abundance of caterpillars for higher trophic levels.

Many arthropods show earlier seasonal activity with warming, but these responses cannot continue indefinitely. Identifying such phenological thresholds is crucial for understanding limits to climate tracking and species persistence, but few studies test for breakpoints that may indicate physiological or ecological constraints. Using a 28-year time series, we examined breakpoint responses to snowmelt and temperature across 15 arthropod taxa in seven plots from high-Arctic Greenland, a region experiencing pronounced warming. Our meta-analysis found breakpoint responses in two of six phenological driver and event combinations: onset and peak activity advanced with earlier snowmelt until a threshold, beyond which the relationship levelled off. A breakpoint for peak activity in response to temperature disappeared when snowmelt was included in the model, underscoring the importance of considering several environmental cues to prevent incorrect inferences about plasticity limits. Most responses showed no evidence of a breakpoint in phenological sensitivity, instead exhibiting continued tracking of cues over the study period. Our findings suggest that while many Arctic arthropods remain responsive to climate change, some may be approaching limits, potentially altering ecological interactions and vulnerability to abiotic cues. Our findings highlight the need for broader assessments of phenological thresholds to refine predictions of species responses to environmental change.

Species distribution models (SDMs) are frequently used to project species' ranges under future climate conditions. Such space-for-time substitutions rely on the assumptions that spatial climate-distribution relationships are causal and that relationships are equivalent over space and time. These assumptions have rarely been tested. Using UK populations of the Orange-tip butterfly (Anthocharis cardamines) as a case study, we demonstrate an approach to test whether these key assumptions are met. First, we identified the seasonal periods over which temperature and precipitation variables best explained the Orange-tip butterfly's abundance. Then, we compared the effects of climate variables on both occupancy and abundance over space versus time. We found that, over both space and time, temperature in the previous year's flight period has a positive effect on the occupancy (i.e. presence) and abundance of the Orange-tip butterfly, whereas precipitation appeared not to have a causal effect. We found that temporal effects of temperature on abundance did not differ from spatial effects at colder sites, but at warmer UK sites, the magnitude of these effects significantly differed. Conversely, spatial and temporal effects of temperature on occupancy did not differ at warmer (mid-range) sites, with effect magnitudes significantly differing at colder sites. Our results demonstrate the importance of identifying causal climate-distribution relationships before making projections. We also show that the reliability of SDM projections over time can be highly context dependent, even when considering a single species. Therefore, where data are available over space and time, the space versus time modelling approach presented here should be incorporated into the SDM statistical repertoire to improve the reliability of projections.Read the free Plain Language Summary for this article on the Journal blog.

Phenological shifts are well documented biological responses to warming. While many studies have focused on the mean timing of an event, there is growing appreciation that the height and width of the phenological distribution will also impact on species interactions. A temperate deciduous forest food chain of oak trees - arboreal caterpillars - insectivorous passerines has become paradigmatic in research on phenological mismatch. This focus on oak-dominated woodlands means that we have limited insight into whether 1) caterpillar phenological distributions vary among tree taxa and habitats and 2) oak is an exceptional host, which has implications for the potential for buffering of interactions on a local and landscape scale. Here, we survey caterpillar abundance and mass throughout spring on 10 tree taxa for 10 years across 44 Scottish woodland sites. We found substantial variation in caterpillar abundance among host taxa, with oak, birch and willow yielding similarly high numbers of caterpillars, and evidence that caterpillar abundance increases with the density of oak foliage within a woodland stand, but not with the density of other taxa. Considering variation in the phenological distribution of caterpillars on different host taxa, we found the main axis of variation to be the maximum abundance/total biomass reached, which was highest on oak. We found significant variation in the mean timing of abundance and duration of abundance and total biomass among hosts, though effect sizes were quite small, and little evidence for among host variation in the phenological distribution of individual caterpillar mass. In woodlands where oak is abundant, our findings are consistent with the presence of other tree taxa providing little local buffering of phenological mismatch. Whereas, in the absence of oak, birch and willow have the potential to support similarly substantial caterpillar abundances. These findings have implications for conservation, resilient forestry planting and management decisions.

AbstractUncovering the patterns and structure in species interactions is central to understanding community assembly and dynamics. Species interact via their phenotypes, but identifying and quantifying the traits that structure species-specific interactions (links) can be challenging. Where these traits show phylogenetic signal, link properties (such as which species interact and how often) may be predictable using models that incorporate phylogenies in place of trait data. However, quantification of phylogenetic patterns in link properties is conceptually and methodologically challenging because it requires coestimation of multiple phylogenetic and nonphylogenetic pattern types in interaction data for multiple sites while controlling for confounding effects and making biologically plausible assumptions about which species can interact. Here we show how this can be done in a Bayesian mixed modeling framework, using data for trophic interactions between oak cynipid galls and parasitoid natural enemies. We find strong signatures of cophylogeny (i.e., related parasitoids attack related host galls) in both link incidence (presence/absence) and link frequency data, alongside patterns in link incidence/richness and identity across sites that are independent of either parasitoid or gall wasp phylogeny. Our results are robust to substantially reduced sample completeness and are consistent with structuring of trophic interactions by a combination of phylogenetically conserved and phylogenetically labile traits in both trophic levels. We show that incorporation of phylogenetic relationships into analyses of species interactions has substantial explanatory power even in the absence of trait data, with potential applied use in prediction of natural enemies of invading pests and nontarget hosts of biocontrol agents.

Revealing processes that structure species interactions is central to understanding community assembly and dynamics. Species interact via their phenotypes, but identifying and quantifying the traits that structure species-specific interactions (links) can be challenging. Where these traits show phylogenetic signal, however, link properties may be predictable using models that incorporate phylogenies in place of trait data. We analysed variation in link richness, frequency, and species identity in a multi-site dataset of interactions between oak cynipid galls and parasitoid natural enemies, using a Bayesian mixed modelling framework allowing concurrent fitting of phylogenetic effects of both trophic levels. In both link incidence (presence/absence) and link frequency datasets, we identified strong signatures of cophylogeny (related parasitoids attack related host galls) alongside patterns independent of either phylogeny. Our results are robust to simulations of substantially reduced sample completeness, and are consistent with the structuring of trophic interactions by a combination of phylogenetically conserved and convergently evolving traits in both trophic levels. We discuss our results in light of phenotypic traits thought to structure gall-parasitoid interactions and consider wider applications of this approach, including inference of underlying community assembly processes and prediction of economically important trophic interactions. ### Competing Interest Statement The authors have declared no competing interest.

The deciduous tree‐herbivorous caterpillar‐insectivorous bird food chain is a well‐studied system for investigating the impacts of climate change across trophic levels. To date, across Europe, most attention has focused on the impacts of increasing spring temperature on changes to phenology in Oak‐dominated (Quercus spp.) woodlands. Paridae species and Pied Flycatcher Ficedula hypoleuca are the most studied secondary consumers, all of which demonstrate an advancement in reproductive phenology with increases in spring temperature. Shifts in climate and phenology may also impact on reproductive investment in clutch size, and the effects of climate on phenology and clutch size may vary depending on woodland composition. To date, the effects of among‐habitat variation in phenology and reproductive investment have received little attention. Insectivorous birds inhabiting woodlands that differ in tree composition may differ in the timing of breeding, due to local tree leafing phenology acting as a cue for egg‐laying date and/or clutch size. Moreover, for most insectivorous birds, woodland composition within a territory is likely to be the main determinant of food availability for both adults and chicks. Consequently, if warming springs affect the temporal patterns of food availability differently across different woodland compositions, this may affect the optimal average local phenology for nesting birds. Here, using data from 34 long‐term (mean 15 years) nest monitoring sites across the UK, we investigate the effect of woodland tree composition and temperature on Blue Tit Cyanistes caeruleus first egg date (FED) and clutch size. We supplemented the nest monitoring data by quantifying woodland composition, at a site level, through modified point counts. We predict that birds breeding in woodlands with greater proportions of late‐leafing species, such as Oak and Ash Fraxinus excelsior, will breed later than those breeding in woodlands with greater proportions of early‐leafing species, such as Birch Betula spp. and Beech Fagus sylvatica. We found no evidence for differences in Blue Tit FED or clutch size in relation to the proportion of any of the tree species investigated, after controlling for temperature and latitude (FED: −3.4 and 2.2, clutch size: −0.4 and − 0.2 eggs for one‐unit increase in temperature and latitude, respectively). In recent decades and across all sites, clutch size has decreased as spring temperatures have increased, a strategy which could allow birds flexibly to adjust their breeding phenology such that nestling demand coincides with peak food availability. The lack of an effect of woodland composition on Blue Tit phenology suggests Blue Tits do not fine‐tune their reproductive phenology to the local tree composition. Whether this lack of evidence for phenological divergence is due to an absence of divergent selection on breeding phenology and clutch size or to gene flow is not clear.

Over the past four decades, rising temperatures have impacted the breeding phenology of many bird species, in some cases with consequences for their reproductive success. Migratory birds face particular challenges in shifting breeding phenology to track warmer springs, and understanding the impacts of rising spring temperatures on migratory birds' breeding is urgent. Here, we use over 4000 UK observations of Common Redstart nests, and spring temperature data from 1974 to 2020, to examine the effect of spring temperatures on laying date, clutch size and brood size. We use a sliding window approach to detect periods over which traits are most sensitive to temperature, and compare phenotypic responses to temperature over space and time with the aim of identifying causal effects of temperature and inferring the contributions of plasticity and local adaptation. We found that redstart laying date was sensitive to spring temperature from mid-April to late May, with a relatively shallow response of 1-2 days/degrees C that was similar across space and time, but shallower than the phenological response of many of the resource species. Over the study period, laying date has advanced by more than 11 days, which is substantially more than can be explained based on the temperature plasticity estimates we obtained. Spring temperature had a weak, but positive, impact on clutch size, but with no evidence of an effect of spatial variation in temperature. The rate of brood size reduction from hatching to fledging became more negative at higher temperatures, but after taking into account a non-significant but positive effect of temperature on brood size at hatching, there was no net effect of temperature on fledging success. Taken together, we found little evidence that higher temperatures in the UK lead to lower reproductive output.

The potential for climate change to disrupt phenology-mediated interactions in interaction networks has attracted considerable attention in recent decades. Frequently, studies emphasize the fragility of ephemeral seasonal interactions, and the risks posed by phenological asynchrony. Here, we argue that the fitness consequences of asynchrony in phenological interactions may often be more buffered than is typically acknowledged. We identify three main forms that buffering may take: (i) mechanisms that reduce asynchrony between consumer and resource; (ii) mechanisms that reduce the costs of being asynchronous; and (iii) mechanisms that dampen interannual variance in performance across higher organizational units. Using synchrony between the hatching of winter moth caterpillars and the leafing of their host-plants as a case study, we identify a wide variety of buffers that reduce the detrimental consequences of phenological asynchrony on caterpillar individuals, populations, and meta-populations. We follow this by drawing on examples across a breadth of taxa, and demonstrate that these buffering mechanisms may be quite general. We conclude by identifying key gaps in our knowledge of the fitness and demographic consequences of buffering, in the context of phenological mismatch. Buffering has the potential to substantially alter our understanding of the biotic impacts of future climate change—a greater recognition of the contribution of these mechanisms may reveal that many trophic interactions are surprisingly resilient, and also serve to shift research emphasis to those systems with fewer buffers and towards identifying the limits of those buffers.

In an epoch of rapid environmental change, understanding and predicting how biodiversity will respond to a changing climate is an urgent challenge. Since we seldom have sufficient long-term biological data to use the past to anticipate the future, spatial climate-biotic relationships are often used as a proxy for predicting biotic responses to climate change over time. These 'space-for-time substitutions' (SFTS) have become near ubiquitous in global change biology, but with different subfields largely developing methods in isolation. We review how climate-focussed SFTS are used in four subfields of ecology and evolution, each focussed on a different type of biotic variable - population phenotypes, population genotypes, species' distributions, and ecological communities. We then examine the similarities and differences between subfields in terms of methods, limitations and opportunities. While SFTS are used for a wide range of applications, two main approaches are applied across the four subfields: spatial in situ gradient methods and transplant experiments. We find that SFTS methods share common limitations relating to (i) the causality of identified spatial climate-biotic relationships and (ii) the transferability of these relationships, i.e. whether climate-biotic relationships observed over space are equivalent to those occurring over time. Moreover, despite widespread application of SFTS in climate change research, key assumptions remain largely untested. We highlight opportunities to enhance the robustness of SFTS by addressing key assumptions and limitations, with a particular emphasis on where approaches could be shared between the four subfields.

A classic system for studying trophic mismatch focuses on the timing of the spring caterpillar peak in relation to the breeding time and productivity of woodland passerine birds. Most work has been conducted in single-site oak woodlands and little is known about how insights generalise to other woodland types or across space. Here we present the results of a three-year study on the species composition and temporal distribution of the spring caterpillar peak on different tree taxa across 40 woodland sites spanning two degrees of latitude in Scotland. We used molecular barcoding to identify 62 caterpillar species, with winter moth (Operophtera brumata) the most abundant, comprising a third of the sample. Oak (Quercus sp.) and willow (Salix sp.) hosted significantly higher caterpillar abundances than other tree taxa, with winter moth exhibiting similar trends and invariantly proportionate across tree taxa. Caterpillar peak phenology was broadly similar between tree taxa. While latitude had little effect, increasing elevation increased the height of the caterpillar peak and retarded timing by 3.7 days/100m. These findings extend our understanding of how mismatch may play out spatially, with caterpillar peak date varying with elevation, and tree taxa varying in the caterpillar resource that they host.

Advances in spring phenology are among the clearest biological responses to climate warming. There has been much interest in how climate impacts on phenology because the timings of key events have implications for species interactions, nutrient cycling and ecosystem services. To date most work has focused on only one aspect of population phenology, the effects of temperature on the average timing. In comparison, effects of temperature on the abundance of individuals and their seasonal spread are understudied, despite their potential to have profound impacts on species interactions.Here we develop a new method that directly estimates the effect of spring temperatures on the timing, height and width of the phenological distribution and apply it to temperate forest caterpillars, a guild that has been the focus of much research on phenology and trophic mismatch.We find that warmer spring conditions advance the timing of the phenological distribution of abundance by -4.96 days degrees C-1 and increase its height by 34% degrees C-1 but have no significant effect on the duration of the distribution. An increase in the maximum density of arboreal caterpillars with rising temperatures has implications for understanding climate impacts on forest food chains, both in terms of herbivory pressure and the resources available to secondary consumers.The new method we have developed allows the thermal sensitivity in the full phenological distribution to be modelled directly from raw data, providing a flexible approach that has broad applicability within global change research.Read the free Plain Language Summary for this article on the Journal blog. Read the free Plain Language Summary for this article on the Journal blog.image

Timing of breeding, an important driver of fitness in many populations, is widely studied in the context of global change, yet despite considerable efforts to identify environmental drivers of seabird nesting phenology, for most populations we lack evidence of strong drivers. Here we adopt an alternative approach, examining the degree to which different populations positively covary in their annual phenology to infer whether phenological responses to environmental drivers are likely to be (a) shared across species at a range of spatial scales, (b) shared across populations of a species or (c) idiosyncratic to populations. We combined 51 long-term datasets on breeding phenology spanning 50 years from nine seabird species across 29 North Atlantic sites and examined the extent to which different populations share early versus late breeding seasons depending on a hierarchy of spatial scales comprising breeding site, small-scale region, large-scale region and the whole North Atlantic. In about a third of cases, we found laying dates of populations of different species sharing the same breeding site or small-scale breeding region were positively correlated, which is consistent with the hypothesis that they share phenological responses to the same environmental conditions. In comparison, we found no evidence for positive phenological covariation among populations across species aggregated at larger spatial scales. In general, we found little evidence for positive phenological covariation between populations of a single species, and in many instances the inter-year variation specific to a population was substantial, consistent with each population responding idiosyncratically to local environmental conditions. Black-legged kittiwake Rissa tridactyla was the exception, with populations exhibiting positive covariation in laying dates that decayed with the distance between breeding sites, suggesting that populations may be responding to a similar driver. Our approach sheds light on the potential factors that may drive phenology in our study species, thus furthering our understanding of the scales at which different seabirds interact with interannual variation in their environment. We also identify additional systems and phenological questions to which our inferential approach could be applied.

Birds build nests primarily as a receptacle to lay their eggs in, but they can also provide secondary benefits including structural support, camouflage and adjustment of the microclimate surrounding the eggs and offspring. The factors underlying intraspecific variation in nest characteristics are poorly understood. In this study, we aim to identify the environmental factors that predict nest height variation and the duration of nest building in blue tits Cyanistes caeruleus, evaluating latitude, elevation, temperature and the timing of egg‐laying as predictors of nest height, while also taking into account female and male parental identity. Using 713 nest height observations collected over a period of five years along a 220 km transect in Scotland, we found that if the annual mean timing of egg‐laying was earlier, nests were taller. However, there was no correlation between nest height and elevation, latitude, the minimum temperature in the 14 days pre‐egg‐laying or the phenology of birds within a year. Female parental identity accounted for a large amount of variation in nest height, suggesting that individual behaviour has an influence on nest structure. We also found that nest building duration was shorter when egg laying occurred earlier in the year, and that across all observations taller nests took longer to build. Overall, our results show that blue tits are able to alter their nest characteristics based on environmental gradients like latitude (in the case of building duration) and the annual mean phenological variation of egg laying, and that birds build relatively taller nests faster.