Year-round breeding equatorial Larks from three climatically-distinct populations do not use rainfall, temperature or invertebrate biomass to time reproduction

Collapse of an insular bird species driven by a decrease in rainfall

Arid island environments harbour a unique biota characterised to have adaptive features that enable them to thrive in such harsh habitats. However, our understanding of how anthropogenic climate change compromises the biodiversity and sustainability of these ecosystems is greatly unknown. Here we used fine-grained field data to evaluate the effects of extreme weather on the population size, distribution, and habitat preferences of an endemic bird species inhabiting an arid Atlantic island, across two temporal windows spanning approximately 20 years (2005-2024). Population size declined sharply (63 %-70 %) between periods, according to a distance-based sampling design and a habitat suitability modelling approach, with the number of individuals estimated in 2024 being 4650 (CI 95 %: 3600-5950) and 4150 (CI 95 %: 3600-4800) respectively. The density of this species in 2024 was reduced by approximately three times compared to the previous study period. The results revealed that in 2024 a larger island area (246 km2 and 514 km2) was necessary to encompass 50 % and 75 % of all individuals of this species, respectively, compared to the previous period (195 km2 and 434 km2). Such a population collapse was associated with the decrease in rainfall on the island, which is closely related to the reproductive success in this species. We recorded a pattern of continuous decrease in rainfall since 2005-2006, which included several extremely dry years immediately before the 2024 study. We also found strong evidence for the population decline of other native bird species of the same foraging guild. Overall, our data emphasizes the significance of recurrent and extreme climatic events on arid island bird species in driving population declines and reducing their distribution ranges; and highlights how fragile such unique taxa can be under longer dry periods, with uncertain consequences for their future viability.

Cloacal microbiota are biogeographically structured in larks from desert, tropical and temperate areas

BACKGROUND

In contrast with macroorganisms, that show well-documented biogeographical patterns in distribution associated with local adaptation of physiology, behavior and life history, strong biogeographical patterns have not been found for microorganisms, raising questions about what determines their biogeography. Thus far, large-scale biogeographical studies have focused on free-living microbes, paying little attention to host-associated microbes, which play essential roles in physiology, behavior and life history of their hosts. Investigating cloacal gut microbiota of closely-related, ecologically similar free-living songbird species (Alaudidae, larks) inhabiting desert, temperate and tropical regions, we explored influences of geographical location and host species on α-diversity, co-occurrence of amplicon sequence variants (ASVs) and genera, differentially abundant and dominant bacterial taxa, and community composition. We also investigated how geographical distance explained differences in gut microbial community composition among larks.

RESULTS

Geographic location did not explain variation in richness and Shannon diversity of cloacal microbiota in larks. Out of 3798 ASVs and 799 bacterial genera identified, 17 ASVs (< 0.5%) and 43 genera (5%) were shared by larks from all locations. Desert larks held fewer unique ASVs (25%) than temperate zone (31%) and tropical larks (34%). Five out of 33 detected bacterial phyla dominated lark cloacal gut microbiomes. In tropical larks three bacterial classes were overrepresented. Highlighting the distinctiveness of desert lark microbiota, the relative abundances of 52 ASVs differed among locations, which classified within three dominant and 11 low-abundance phyla. Clear and significant phylogenetic clustering in cloacal microbiota community composition (unweighted UniFrac) showed segregation with geography and host species, where microbiota of desert larks were distinct from those of tropical and temperate regions. Geographic distance was nonlinearly associated with pairwise unweighted UniFrac distances.

CONCLUSIONS

We conclude that host-associated microbiota are geographically structured in a group of widespread but closely-related bird species, following large-scale macro-ecological patterns and contrasting with previous findings for free-living microbes. Future work should further explore if and to what extent geographic variation in host-associated microbiota can be explained as result of co-evolution between gut microbes and host adaptive traits, and if and how acquisition from the environmental pool of bacteria contributes to explaining host-associated communities.

Immune function differs among tropical environments but is not downregulated during reproduction in three year-round breeding equatorial lark populations

Seasonal variation in immune function can be attributed to life history trade-offs, and to variation in environmental conditions. However, because phenological stages and environmental conditions co-vary in temperate and arctic zones, their separate contributions have not been determined. We compared immune function and body mass of incubating (female only), chick-feeding (female and male), and non-breeding (female and male) red-capped larks Calandrella cinerea breeding year-round in three tropical equatorial (Kenya) environments with distinct climates. We measured four immune indices: haptoglobin, nitric oxide, agglutination, and lysis. To confirm that variation in immune function between breeding (i.e., incubating or chick-feeding) and non-breeding was not confounded by environmental conditions, we tested if rainfall, average minimum temperature (T_min), and average maximum temperature (T_max) differed during sampling times among the three breeding statuses per location. T_min and T_max differed between chick-feeding and non-breeding, suggesting that birds utilized environmental conditions differently in different locations for reproduction. Immune indices did not differ between incubating, chick-feeding and non-breeding birds in all three locations. There were two exceptions: nitric oxide was higher during incubation in cool and wet South Kinangop, and it was higher during chick-feeding in the cool and dry North Kinangop compared to non-breeding birds in these locations. For nitric oxide, agglutination, and lysis, we found among-location differences within breeding stage. In equatorial tropical birds, variation in immune function seems to be better explained by among-location climate-induced environmental conditions than by breeding status. Our findings raise questions about how within-location environmental variation relates to and affects immune function.

Rapid plastic breeding response to rain matches peak prey abundance in a tropical savanna bird

Changes in climate are shifting the timing of life cycle events in the natural world. Compared to northern temperate areas, these effects are relatively poorly understood in tropical and southern regions, where there is limited information on how timing of breeding and food availability are affected by climatic factors, and where patterns of breeding activity are more unpredictable within and between years. Combining a new statistical modelling approach with 5 years of continuous individual-based monitoring of a monsoonal tropical insectivorous bird, we quantified (a) the proximate climatic drivers at two trophic levels: timing of breeding and abundance of arthropod prey; (b) the effect of climate variation on reproductive output and (c) the role of individual plasticity. Rainfall was identified as the main determinant of phenology at both trophic levels. Throughout the year, likelihood of egg laying increased very rapidly in response to even small amounts of rain during the preceding 0-3 weeks. Adult body mass and male sperm storage also increased rapidly after rain, suggesting high breeding preparedness. Additionally, females were flexible, since they were more likely to nest whether their previous attempt was longer ago and unsuccessful. Arthropod abundance also increased after rainfall, but more slowly, with a peak around 10 weeks. Therefore, the peak food availability coincided with the presence of dependent fledglings. Fitness benefits of nesting after more rain appeared to be linked to offspring quantity rather than quality: nest attempts following higher rainfall produced larger clutches, but showed no improvement in nestling mass or relative fledging success. The response of clutch size to rainfall was plastic, since repeated sampling showed that individual females laid larger clutches after more rain, possibly mediated by improved body mass. Rapid, individually flexible breeding in response to rainfall and slower increase in arthropod abundance also as a response to rainfall, might buffer insectivorous species living in tropical seasonal environments from climate change-induced phenological trophic mismatches.

Geographical and temporal variation in environmental conditions affects nestling growth but not immune function in a year-round breeding equatorial lark

Background

Variation in growth and immune function within and among populations is often associated with specific environmental conditions. We compared growth and immune function in nestlings of year-round breeding equatorial Red-capped Lark Calandrella cinerea from South Kinangop, North Kinangop and Kedong (Kenya), three locations that are geographically close but climatically distinct. In addition, we studied growth and immune function of lark nestlings as a function of year-round variation in breeding intensity and rain within one location. We monitored mass, wing, and tarsus at hatching (day 1) and at 4, 7, and 10 days post-hatch, and we quantified four indices of immune function (haptoglobin, agglutination, lysis and nitric oxide) using blood samples collected on day 10.

Results

Nestling body mass and size at hatching, which presumably reflect the resources that females allocated to their eggs, were lowest in the most arid location, Kedong. Contrary to our predictions, nestlings in Kedong grew faster than nestlings in the two other cooler and wetter locations of South and North Kinangop. During periods of peak reproduction within Kedong, nestlings were heavier at hatching, but they did not grow faster over the first 10 days post-hatch. In contrast, rainfall, which did not relate to timing of breeding, had no effect on hatching mass, but more rain did coincide with faster growth post-hatch. Finally, we found no significant differences in nestling immune function, neither among locations nor with the year-round variation within Kedong.

Conclusion

Based on these results, we hypothesize that female body condition determines nestling mass and size at hatching, but other independent environmental conditions subsequently shape nestling growth. Overall, our results suggest that environmental conditions related to food availability for nestlings are relatively unimportant to the timing of breeding in equatorial regions, while these same conditions do have consequences for nestling size and growth.