Global drivers of variation in cup nest size in passerine birds

Abstract

The size of a bird's nest can play a key role in ensuring reproductive success and is determined by a variety of factors. The primary function of the nest is to protect offspring from the environment and predators. Field studies in a number of passerine species have indicated that higher-latitude populations in colder habitats build larger nests with thicker walls compared to lower-latitude populations, but that these larger nests are more vulnerable to predation. Increases in nest size can also be driven by sexual selection, as nest size can act as a signal of parental quality and prompt differential investment in other aspects of care. It is unknown, however, how these microevolutionary patterns translate to a macroevolutionary scale. Here, we investigate potential drivers of variation in the outer and inner volume of open cup nests using a large dataset of nest measurements from 1117 species of passerines breeding in a diverse range of environments. Our dataset is sourced primarily from the nest specimens at the Natural History Museum (UK), complemented with information from ornithological handbooks and online databases. We use phylogenetic comparative methods to test long-standing hypotheses about potential macroevolutionary correlates of nest size, namely nest location, clutch size and variables relating to parental care, together with environmental and geographical factors such as temperature, rainfall, latitude and insularity. After controlling for phylogeny and parental body size, we demonstrate that the outer volume of the nest is greater in colder climates, in island-dwelling species and in species that nest on cliffs or rocks. By contrast, the inner cup volume is associated solely with average clutch size, increasing with the number of chicks raised in the nest. We do not find evidence that nest size is related to the length of parental care for nestlings. Our study reveals that the average temperature in the breeding range, along with several key life-history traits and proxies of predation threat, shapes the global interspecific variation in passerine cup nest size. We also showcase the utility of museum nest collections-a historically underused resource-for large-scale studies of trait evolution.

Keywords: differential allocation hypothesis; museum collections; nest size; parental investment; passerine nests; predation threat.

FIGURE 1

Schematic drawing of (a) outer and (b) inner nest size measurements of museum specimens. D/d, diameter; H/h, height; Vol, volume; the bar above a letter denotes the mean value of four measurements of nest diameter (D_1–4, d_1–4) or two measurements of nest height (H_1–2, h_1–2). Outer and inner nest volumes, highlighted in the blue box, are the main focus of the study. Background images of nests in courtesy of the Trustees of the Natural History Museum, London.

FIGURE 2

Distribution of outer nest volume (cm³, log‐transformed) and body mass (g) in passerines across a single topology from birdtree.org, using the Hackett backbone (Jetz et al., 2012), n = 827 species, left panel. The ancestral state reconstructions of nest volume are visualised on the tree structure while the grey bars on the outside represent the corresponding body mass per species. For ease of interpretation, only the names of families with records for 15 species or more have been displayed. Four species are further highlighted in the right panel; all images sourced from the Macaulay Library at the Cornell Lab of Ornithology: (a) Corvus corax or common raven (asset ML343861691) builds the largest nest in absolute terms while (b) Rhipidura leucophrys or willie wagtail (ML189527291), along with other fantails, has one of the smallest nest volumes; (c) Sphecotheres vieilloti or Australasian figbird (ML292333431) builds a relatively small nest for its body mass while (d) Regulus regulus or goldcrest (ML712648) has the largest nest with respect to its weight (<6 g).

FIGURE 3

Geographical distribution of the mean outer nest volume divided by body mass per 0.25° grid cell (n = 827 species). Grid cells with fewer than four species have been removed from the visualisation and therefore appear blank. The subpanels (a–c) illustrate variation in nest size in different island groups, with all grid cells containing at least one species displayed. The same colour scale has been retained for all panels to ensure comparability; values along the y‐ and x‐axes correspond to the latitude and longitude, respectively.

FIGURE 4

Predictors of (a) outer nest volume (n = 1002 records, n = 827 species) and (b) inner nest volume (n = 1218 records, n = 965 species) calculated with a Bayesian phylogenetic mixed model. Significant predictors can be identified by a substantial shift from 0; significant positive and negative associations highlighted in red and blue, respectively. See Tables S3 and S4 for further information.

FIGURE 5

(a) Tukey box and whisker plot of variation in outer nest volume by the number of nest‐building parents (n = 584 records, n = 434 species). The ends of the grey boxes correspond to the first and third quartiles of data distribution; the line in the middle represents the median value. Whiskers indicate the minimum and maximum values excluding outliers, which are calculated as first and third quartiles ±1.5 times the interquartile range. (b) Relationship between nestling period and outer nest volume, with LOWESS regression line displayed (n = 374 species).