Climate‐Driven Changes in Resources Shift Reproductive Season and Effort of Insectivorous Montane Bird Species

Climate change has long been recognized as a threat to biodiversity in tropical regions through a variety of mechanisms (Thomas et al. 2004). Phenological shifts due to climate change have been well documented, but it is more difficult to discern how these shifts (particularly in plants) may influence interaction networks of dependent consumers like herbivores, pollinators, and frugivores (Sandor et al. 2021). This is further complicated by the fact that changing climate can also alter bottom‐up trophic effects by affecting primary productivity (Lu et al. 2024). Rainfall in particular is a key environmental variable that influences both plant productivity and phenology in tropical regions. Shifting seasonal timing and magnitude of rainfall due to climate change (Held and Soden 2006) may therefore alter species interactions and disrupt complex tropical food webs. It is thus critical to understand how climate variables such as temperature and precipitation affect the multi‐trophic interaction networks of flora and fauna in tropical regions around the world (Marquis et al. 2025). One way to disentangle these climate‐driven effects is to intensively sample climate data along with phenology and abundance data across multiple trophic levels.

A recently published work in Global Change Biology by Newell et al. (2026), is a prime example of the huge effort it takes to begin to understand the community‐level effects of climate change in diverse tropical systems. Through an intensive field study of eight study sites over 5 years, they collected in situ data on precipitation, flowering and fruiting, arthropod abundances, and the timing of bird reproductive seasons across a network of tropical montane cloud forests. Although such intensive studies are necessarily limited in both geographic and temporal scope, the comprehensive and integrative data generated reveal the numerous effects of climate change on the networks of complex interactions within biological communities. This work represents an exemplar of the type of intense, multi‐taxa in situ field work that reveals both dramatic and nuanced effects of climate change.

Newell et al. (2026) examined the diverse effects of changing climate on tropical birds species across trophic levels. Newell and colleagues, with the efforts of many dedicated field assistants, worked in eight cloud forest study landscapes across 10,000 km² over 5 years (2015–2019) in the Andes Mountains of northern Peru. Together, these areas varied from 1700 to 3100 m in elevation and experienced between 1000 and 2500 mm rainfall annually. Based on intensive seasonal measurements of food resources (arthropods, fruit, and nectar), in conjunction with in situ weather data (temperature, humidity, rainfall, and cloud cover), this study found pronounced differences in the impacts of changes in wet and dry seasons across the study system contingent upon the trophic level of the bird species under investigation. Earlier publications by the same authors summarized details on local and regional climate and rainfall‐driven changes in arthropod biomass in the same study areas (citations in Newell et al. 2026). As a group, these publications provide in‐depth data documenting changes in rainfall extremes, and their timing, and how these changes affect availability of the resources on which the bird species inhabiting the study sites depend.

Over the 5 years of the project, Newell and co‐authors mist‐netted birds at each site, recording evidence of breeding, captures of juveniles, and feather molt. They netted over 8000 individual birds of 133 bird species of 28 families and 94 genera. Of these captures, Newell et al. (2026) also recorded close to 4000 reproductive and related events (including data on 391 nests) that allowed them to analyze breeding effort across the annual cycle, contingent upon trophic feeding category and elevation.

Newell et al. (2026) proceeded under a framework informed by existing literature and the modular breeding hypothesis, expecting that the timing of breeding of the different trophic guilds would largely reflect long‐term climate means. Specifically, they predicted that (1) insectivorous and omnivorous bird species would nest as the rainy season commences to exploit increasing arthropod abundances; (2) frugivorous bird species would nest during wet season peak rains when fruit abundances tend to peak; (3) nectarivorous bird species would nest during the dry season when bird‐pollinated flowers reach their greatest abundance.

In brief, Newell and colleagues report that changing patterns of rainfall across the annual cycle had limited to no effects on plant‐based avian consumers (frugivores, nectarivores). As expected from previous research (Dunham et al. 2018; McLaren and McDonald 2005), frugivores and nectarivores coincided breeding with maximal availability of their respective food resources (wet season for frugivores; beginning of dry season for nectarivores), even as seasonal rainfall patterns shifted from long‐term averages. In stark contrast, insectivores shifted their breeding efforts by as much as 5 months, with 30 species switching reproduction from before to after the dry season. As noted by the investigators, owing to the low‐latitude location of their study sites, such shifts cannot be attributed to seasonal differences in temperature, daylength, or leaf‐flush. Rainfall is thus the paramount driver of the documented shifts in the timing of breeding insectivores.

This publication by Newell and coauthors joins a growing number of recent publications examining tropical bird communities worldwide (e.g., Sherry 2021; Blake and Loiselle 2024, among others). A common finding is that many bird species in tropical habitats around the world are declining, and declines seem most pronounced amongst insectivorous bird species. The exact mechanisms prompting these declines can be difficult to determine because detailed data on arthropod resources are often not available at many study sites. One impressive aspect of Newell et al.'s contribution is how intensively they sampled arthropod, floral, and fruit resources consumed by the bird species within their montane study sites. This was, no doubt, a difficult and time‐consuming task. Newell et al. (2026) convincingly demonstrate that climate change driven shifts in seasonal rainfall in their montane study sites directly alter arthropod biomass and thus indirectly impact seasonal patterns of nesting in avian insectivores by 5 months in under a hundred kilometers. These seasonal changes in breeding are accompanied by an apparent decrease in reproductive effort.

Despite their detailed data on resource abundance and bird reproductive effort across the annual cycle at their study sites, Newell et al. do not report changes in vital rates, like fecundity and year‐to‐year survivorship, that would directly link resource availability to insectivore population dynamics. Although the authors report decreased reproductive effort in this study, no direct evidence indicates that reduced reproductive effort has led to population declines. Still, reduced reproductive effort over many years clearly has the potential to contribute to population declines.

Newell and co‐authors provide a valuable contribution to help advance our understanding of the complex effects of climate change in a region of high biodiversity. However, data across a longer time series will be necessary to examine the effects of long‐term climate trends. This may be especially important in regions (such as the one in this study) that are highly impacted by the El Niño Southern Oscillation (ENSO), which shifts every 2 to 7 years. To draw reliable conclusions in these regions, studies should reflect data from both El Niño and La Niña periods. Future studies could also build on this work by explicitly determining how changes in phenology link to population dynamics and assessing population resilience to reduced reproductive rates.

In the global effort to document the varied and cascading effects of climate change, the study by Newell et al. underscores our need for intense, detailed, in situ field studies. Such data reveal nuanced effects, such as those that differ among trophic groups, which might otherwise be overlooked with coarser‐grain, large spatial scale data. As the authors contend, their comprehensive data, to some extent, begin to untangle Darwin's “entangled bank” of complex food webs, with their myriad interactions, and emphasize the importance of a detailed understanding of climate‐mediated energy flows.

Author Contributions

Christopher J. Whelan: writing – original draft, writing – review and editing. Megan Garfinkel: writing – original draft, writing – review and editing.

Funding

The authors have nothing to report.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The authors have nothing to report.