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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 focussed 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, which all 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 has 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 hypothesise 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 (Betuala 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 to flexibly 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 gene flow is not clear.

Advances in spring phenology are among the clearest biological responses to climate warming. In the ephemeral temperate deciduous forest food webs, at the vanguard of research on temperature’s effect on trophic interactions, most work has focused on the average timing of phenological events. In comparison, effects of temperature on the abundance of individuals and their seasonal spread is understudied, despite the potential for profound impacts on trophic interactions. Here we use a new method to show that for the guild of forest caterpillars, warmer spring conditions not only advance the timing of the phenological distribution of abundance by -4.96 days oC-1, but also increase its height by 34% oC-1. This increase in the maximum density of caterpillars with rising temperatures is likely to have major implications for both herbivory pressure and the resources available to secondary consumers.

The authors also thank funding sources: the Natural Environment Research Council (NERC; UK National Capability award number NE/R016429/1 as part of the UKSCaPE programme); Joint Nature Conservatio Committee (JNCC); Environment and Climate Change Canada; Natural Resources Canada; New Bedford Harbor Trustee Council; The Norwegian Environment Agency (and its predecessors), the SEAPOP programme (www.seapop.no) and its key institutions: The Norwegian Institute for Nature Research, The Norwegian Polar Institute and Tromsø University Museum and the French Polar Institute. 1. 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 (i) shared across species at a range of spatial scales, (ii) shared across populations of a species, or (iii) idiosyncratic to populations. 2. 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. 3. 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. 4. 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. 5. 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. Postprint

As temperatures rise, timing of reproduction is changing at different rates across trophic levels, potentially resulting in asynchrony between consumers and their resources. The match-mismatch hypothesis (MMH) suggests that trophic asynchrony will have negative impacts on average productivity of consumers. It is also thought to lead to selection on timing of breeding, as the most asynchronous individuals will show the greatest reductions in fitness.Using a 30-year individual-level dataset of breeding phenology and success from a population of European shags on the Isle of May, Scotland, we tested a series of predictions consistent with the hypothesis that fitness impacts of trophic asynchrony are increasing.These predictions quantified changes in average annual breeding success and strength of selection on timing of breeding, over time and in relation to rising sea surface temperature (SST) and diet composition.Annual average (population) breeding success was negatively correlated with average lay date yet showed no trend over time, or in relation to increasing SST or the proportion of principal prey in the diet, as would be expected if trophic mismatch was increasing. At the individual level, we found evidence for stabilising selection and directional selection for earlier breeding, although the earliest birds were not the most productive. However, selection for earlier laying did not strengthen over time, or in relation to SST or slope of the seasonal shift in diet from principal to secondary prey. We found that the optimum lay date advanced by almost 4 weeks during the study, and that the population mean lay date tracked this shift.Our results indicate that average performance correlates with absolute timing of breeding of the population, and there is selection for earlier laying at the individual level. However, we found no fitness signatures of a change in the impact of climate-induced trophic mismatch, and evidence that shags are tracking long-term shifts in optimum timing. This suggests that if asynchrony is present in this system, breeding success is not impacted. Our approach highlights the advantages of examining variation at both population and individual levels when assessing evidence for fitness impacts of trophic asynchrony.

An accumulating number of studies are reporting severe biomass, abundance and/or species richness declines of insects (Hallmann et al., 2017; Lister & Garcia, 2018; Seibold et al., 2019; Sánchez-Bayo & Wyckhuys, 2019). Collectively these studies aim to quantify the net change in invertebrate populations and/or community composition over time and to establish whether such changes can be attributed to anthropogenic drivers (Macgregor, Williams, Bell, & Thomas, 2019; Saunders, Janes, & O’Hanlon, 2019; Thomas, Jones, & Hartley, 2019; Montgomery et al., 2020; van Klink et al., 2020). Seibold et al. 2019 analysed a dataset of arthropod biomass, abundance and species richness from forest and grassland plots in a region of Germany and report significant declines of up to 78% over the time period of 2008 to 2018 (Seibold et al., 2019). However, their analysis did not account for the confounding effects of temporal pseudoreplication of observations from the same years. We show that simply by including a year random effect in the statistical models and thereby accounting for the common conditions experienced by observations from proximal sites in the same years, four of the five reported declines become non-significant out of six tests overall. To place their estimated effect sizes and those of other recent studies of insect declines in a broader geographic context, we analysed invertebrate biomass, abundance and species richness over time from 640 time series from 1167 sites around the world. We found that the average trend across the terrestrial and freshwater realms was not significantly distinguishable from no net change. Shorter time series that are likely to be most affected by sampling error variance – such as those reported in Seibold et al. 2019 – yielded the most extreme estimates of decline or increase. We suggest that the uncritical media uptake of extreme negative trends from short time series may be serving to exaggerate the speed of "insect Armageddon" and could eventually undermine public confidence in biodiversity research. We advocate that future research include all available data and use model structures that account for uncertainties to build a more robust understanding of biodiversity change during the Anthropocene and its variation among regions and taxa (Kunin, 2019; Saunders et al., 2019; Thomas et al., 2019; Didham et al., 2020; Dornelas & Daskalova, 2020).

Tables A1-A4, Figs A1-A3, supplementary methods and results

The nesting phenology and productivity of hole‐nesting woodland passerines, such as tit species (Paridae), has been the subject of many studies and played a central role in advancing our understanding of the causes and consequences of trophic mismatch. However, as most studies have been conducted in mature, oak‐rich (Quercus sp.) woodlands, it is unknown whether insights from such studies generalise to other habitats used by woodland generalist species. Here we applied spatial mixed models to data collected over three years (2014‐2016) from 238 nestboxes across 40 sites – that vary in woodland habitat and elevation – along a 220km transect in Scotland. We evaluate the importance of habitat, biogeography and food availability as predictors of mesoscale among‐site variation in blue tit (Cyanistes caeruleus) nestbox occupancy and two components of productivity (clutch size and fledging success). We found that habitat was not a significant predictor of occupancy or clutch size but that occupancy exhibited pronounced biogeographic trends, declining with increasing latitude and elevation. However, fledging success, defined as the proportion of a clutch that fledged, was positively correlated with site level availability of birch, oak and sycamore, and tree diversity. The lack of correspondence between the effects of habitat on fledging success versus occupancy and clutch size may indicate that blue tits do not accurately predict the future quality of their breeding sites when selecting territories and laying clutches. We found little evidence of spatial autocorrelation in occupancy or clutch size, whereas spatial autocorrelation in fledging success extends over multiple sites, albeit non‐significantly. Taken together, our findings suggest that the relationship between breeding decisions and breeding outcomes varies among habitats, and we urge caution when extrapolating inferences from one habitat to others.

The rise in spring temperatures over the past half‐century has led to advances in the phenology of many nontropical plants and animals. As species and populations differ in their phenological responses to temperature, an increase in temperatures has the potential to alter timing‐dependent species interactions. One species‐interaction that may be affected is the competition for light in deciduous forests, where early vernal species have a narrow window of opportunity for growth before late spring species cast shade. Here we consider the Marsham phenology time series of first leafing dates of thirteen tree species and flowering dates of one ground flora species, which spans two centuries. The exceptional length of this time series permits a rare comparison of the statistical support for parameter‐rich regression and mechanistic thermal sensitivity phenology models. While mechanistic models perform best in the majority of cases, both they and the regression models provide remarkably consistent insights into the relative sensitivity of each species to forcing and chilling effects. All species are sensitive to spring forcing, but we also find that vernal and northern European species are responsive to cold temperatures in the previous autumn. Whether this sensitivity reflects a chilling requirement or a delaying of dormancy remains to be tested. We then apply the models to projected future temperature data under a fossil fuel intensive emissions scenario and predict that while some species will advance substantially others will advance by less and may even be delayed due to a rise in autumn and winter temperatures. Considering the projected responses of all fourteen species, we anticipate a change in the order of spring events, which may lead to changes in competitive advantage for light with potential implications for the composition of temperate forests.

There are wide reports of advances in the timing of spring migration of birds over time and in relation to rising temperatures, though phenological responses vary substantially within and among species. An understanding of the ecological, life-history and geographic variables that predict this intra- and inter-specific variation can guide our projections of how populations and species are likely to respond to future climate change.Here, we conduct phylogenetic meta-analyses addressing slope estimates of the timing of avian spring migration regressed on (i) year and (ii) temperature, representing a total of 413 species across five continents. We take into account slope estimation error and examine phylogenetic, ecological and geographic predictors of intra- and inter-specific variation.We confirm earlier findings that on average birds have significantly advanced their spring migration time by 2.1 days decade-1 and 1.2 days ºC-1. We find that over time and in response to warmer spring conditions short-distance migrants have advanced spring migratory phenology by more than long-distance migrants. We also find that larger bodied species show greater advance over time compared to smaller bodied species. Our results did not reveal any evidence that interspecific variation in migration response is predictable on the basis of species’ habitat or diet. We detected a substantial phylogenetic signal in migration time in response to both year and temperature, suggesting that some of the shifts in migratory phenological response to climate are predictable on the basis of phylogeny. However, we estimate high levels of species and spatial variance relative to phylogenetic variance, which is consistent with plasticity in response to climate evolving fairly rapidly and being more influenced by adaptation to current local climate than by common descent. On average, avian spring migration times have advanced over time and as spring has become warmer. While we are able to identify predictors that explain some of the true among-species variation in response, substantial intra- and inter-specific variation in migratory response remains to be explained.

One consequence of rising spring temperatures is that the optimum timing of key life history events may advance. Where this is the case, a population’s fate may depend on the degree to which it is able to track a change in the optimum timing either via plasticity or via adaptation. Estimating the effect that temperature change will have on optimum timing using standard approaches is logistically challenging, with the result that very few estimates of this important parameter exist. Here we adopt an alternative statistical method that substitutes space for time to estimate the temperature-sensitivity of the optimum timing of 22 plant species based on >200,000 spatiotemporal phenological observations from across the UK. We find that first leafing and flowering dates are sensitive to forcing (spring) temperatures, with optimum timing advancing by 3 days°C-1 and plastic responses to forcing between -3 and -8 days°C-1. Chilling (autumn/winter) temperatures and photoperiod tend to be important cues for species with early and late phenology respectively. For most species we find that plasticity is adaptive and for seven species plasticity is sufficient to track geographic variation in the optimum phenology. For four species we find that plasticity is significantly steeper than the optimum slope that we estimate between forcing temperature and phenology, and we examine possible explanations for this countergradient pattern, including local adaptation.

BackgroundSympatric speciation is today generally viewed as plausible, and some well-supported examples exist, but its relative contribution to biodiversity remains to be established. We here quantify geographic overlap of sister species of heliconiine butterflies, and use age-range correlations and spatial simulations of the geography of speciation to infer the frequency of sympatric speciation. We also test whether shifts in mimetic wing colour pattern, host plant use and climate niche play a role in speciation, and whether such shifts are associated with sympatry.ResultsApproximately a third of all heliconiine sister species pairs exhibit near complete range overlap, and analyses of the observed patterns of range overlap suggest that sympatric speciation contributes 32 %–95 % of speciation events. Müllerian mimicry colour patterns and host plant choice are highly labile traits that seem to be associated with speciation, but we find no association between shifts in these traits and range overlap. In contrast, climatic niches of sister species are more conserved.ConclusionsUnlike birds and mammals, sister species of heliconiines are often sympatric and our inferences using the most recent comparative methods suggest that sympatric speciation is common. However, if sister species spread rapidly into sympatry (e.g. due to their similar climatic niches), then assumptions underlying our methods would be violated. Furthermore, although we find some evidence for the role of ecology in speciation, ecological shifts did not show the associations with range overlap expected under sympatric speciation. We delimit species of heliconiines in three different ways, based on “strict and ” “relaxed” biological species concepts (BSC), as well as on a surrogate for the widely-used “diagnostic” version of the phylogenetic species concept (PSC). We show that one reason why more sympatric speciation is inferred in heliconiines than in birds may be due to a different culture of species delimitation in the two groups. To establish whether heliconiines are exceptional will require biogeographic comparative studies for a wider range of animal taxa including many more invertebrates.

Projecting the fates of populations under climate change is one of global change biology’s foremost challenges. Here we seek to identify the contributions that temperature-mediated local adaptation and plasticity make to spatial variation in British bird nesting phenology, a phenotypic trait showing strong responses to warming. We apply a mixed modeling framework to a Britain-wide spatiotemporal dataset comprising >100,000 records of first egg dates from four single-brooded passerine bird species. The average temperature during a specific time period (sliding-window) strongly predicts spatiotemporal variation in lay date. All four species exhibit phenological plasticity, advancing lay date by 2–5 days°C-1. The initiation of this sliding-window is delayed further north, which may be a response to a photoperiod threshold. Using clinal trends in phenology and temperature we are able to estimate the temperature sensitivity of selection on lay date (B), but our estimates are highly sensitive to the temporal position of the sliding-window. If the sliding-window is of fixed duration with a start date determined by photoperiod we find B is tracked by phenotypic plasticity. If, instead, we allow the start and duration of the sliding-window to change with latitude, we find plasticity does not track B, though at odds with theoretical expectations, our estimates of B differ across latitude versus longitude. We argue that a model combining photoperiod and mean temperature is most consistent with current understanding of phenological cues in passerines, the results of which suggest that each species could respond to projected increases in spring temperatures through plasticity alone. However, our estimates of B require further validation.

Island biotas emerge from the interplay between colonisation, speciation and extinction and are often the scene of spectacular adaptive radiations. A common assumption is that insular diversity is at a dynamic equilibrium, but for remote islands, such as Hawaii or Galápagos, this idea remains untested. Here, we reconstruct the temporal accumulation of terrestrial bird species of the Galápagos using a novel phylogenetic method that estimates rates of biota assembly for an entire community. We show that species richness on the archipelago is in an ascending phase and does not tend towards equilibrium. The majority of the avifauna diversifies at a slow rate, without detectable ecological limits. However, Darwin's finches form an exception: they rapidly reach a carrying capacity and subsequently follow a coalescent-like diversification process. Together, these results suggest that avian diversity of remote islands is rising, and challenge the mutual exclusivity of the non-equilibrium and equilibrium ecological paradigms.