This blog post is provided by Karina Vanadzina and tells the #StoryBehindthePaper for the paper ‘Global drivers of variation in cup nest size in passerine birds‘, which was recently published in Journal of Animal Ecology. In their study they find that nest size in passerine bird species is closely linked to the conditions in their breeding habitat.

A parent bird tending to their chicks in a nest is one of the first examples that spring to mind when we think about parental care in animals. In contrast to insects and bony fishes where it is common for parents to desert the young shortly after mating, most bird species invest a lot of time and resources in their offspring by caring for them in specially constructed nests. A nest can provide great protection from cold, heat, rain and predators – but it is also very energetically taxing to build one. Measuring the size of the nest is a simple way to quantify how birds vary in their nest-building effort. But what determines the optimal size of a nest for each species on an evolutionary timescale?

Very little is known about between-species variation in nest size, particularly not at the global level. However, we have some ideas about why nests of some populations are bigger and some are smaller from field studies (see Fig. 1 for a summary). For example, researchers working on common temperate species such as blackbirds, robins and finches have crossed considerable distances to measure how nest characteristics vary depending on their location. They have found that birds nesting in the northern part of the species’ range where temperatures are lower build bulkier, better insulated nests compared to those in the south. Increased threat from visual predators and brood parasites, however, has the opposite effect on nest size as it leads to smaller nests that are harder to spot.  We also know that nest dimensions reflect the size of the parent bird, with larger birds generally building bigger nests. The number of offspring also has an influence, because larger clutches require more spacious nests. Nest size can furthermore reflect the quality of the mate because healthy, high-quality individuals tend to build larger nests. As finding a mate takes time and energy, members of a pair invest more in any offspring they have with an attractive individual that has great nest-building skills, e.g., by caring for their brood for longer. They care less when the available mate falls short. Some researchers have suggested that, on an evolutionary timescale, this would lead to longer periods of care in species with larger nests.

To understand what makes nests bigger or smaller at the macro scale, I needed measurements of nest sizes from a variety of species breeding in different environments. Even though passerines constitute more than half of all bird diversity, the majority of them build cup-shaped nests that are comparable in dimensions across many species. Some measurements I was able to get from the literature. For other species, I travelled to the Natural History Museum at Tring and measured 1,401 cup nests from their collections (Fig. 2).  I then assessed how nest size ties in with a range of ecological and life-history factors that characterise each species. I mainly focused on variation in the outer cup volume that best captures the amount of material included in the nest – but I also measured the volume of the inner cup that holds the developing eggs. As part of our analyses, we controlled for the potential effect of phylogeny, that is, the fact that more closely related species construct nests that are more similar to each other in size than distantly related ones. Altogether, we analysed data from 1,117 species of passerines from environments as varied as tropical forests, deserts or taiga.

We found several key results in this study. First, larger species do indeed build bigger nests at the macroevolutionary scale. Second, after controlling for body size differences, species breeding in colder climates construct bulkier nests with more nesting material than those breeding in warmer climates, meaning that findings from local field studies do scale up across a global sample of passerine species. In absolute terms, the largest nest in our dataset is built by the common raven – but it is the miniscule goldcrest (<5g) that constructs the bulkiest nest for its size, in order to keep its brood protected from the harsh nesting conditions in northern Europe. Third, we find that species that nest on islands or on cliffs – places with fewer predators – also build larger nests. The size of the inner cup that holds the offspring is, perhaps unsurprisingly, linked to the life-history traits rather than to the ecology of the species; larger clutches and bigger birds need larger cups to accommodate them. Finally, after controlling for all other variables, we fail to find evidence that species with big nests care for their nestlings for longer periods of time compared to species with small nests, which means that the impact of mate quality on nests made by individual birds doesn’t quite translate to the macro scale.

Our study demonstrates that, even on a global level, nest size in passerine species is closely linked to the conditions in their breeding habitat. This implies that a mismatch between the nest structure and the surrounding environment could endanger the survival of the species, especially when nesting birds are subjected to climate conditions that change rapidly and unpredictably. Experiments with zebra finches have shown that breeding pairs add more material to their nests when building in a cold lab compared to a warm one, but it is currently not known if other passerine species also possess an equal measure of plasticity in how they construct their nests. Furthermore, our results relied on specimens from natural history museum collections, highlighting just one way such resources can be used to understand biological responses to a changing world. While the nest represents a key parental investment in birds, nest specimens have not received as much research attention as collections of eggs or skins. Nest size is just one aspect of nest morphology that could be explored using museum collections, and a closer look at variation in nest shape or in materials used by birds could provide further insights into the relationship between nests and environment.

Read the full paper here: Vanazina, K., Street, S.E., Healy, S.D., Laland, K.N. & Sheard, C. (2022). Global drivers of variation in cup nest size in passerine birds. Journal of Animal Ecology, 00, 1–14. https://doi.org/10.1111/1365-2656.13815