The overlooked complexity of avian brood parasite-host relationships

Abstract

The relationships between avian brood parasites and their hosts are widely recognised as model systems for studying coevolution. However, while most brood parasites are known to parasitise multiple species of host and hosts are often subject to parasitism by multiple brood parasite species, the examination of multispecies interactions remains rare. Here, we compile data on all known brood parasite-host relationships and find that complex brood parasite-host systems, where multiple species of brood parasites and hosts coexist and interact, are globally commonplace. By examining patterns of past research, we outline the disparity between patterns of network complexity and past research emphases and discuss factors that may be associated with these patterns. Drawing on insights gained from other systems that have embraced a multispecies framework, we highlight the potential benefits of considering brood parasite-host interactions as ecological networks and brood parasitism as a model system for studying multispecies interactions. Overall, our results provide new insights into the diversity of these relationships, highlight the stark mismatch between past research efforts and global patterns of network complexity, and draw attention to the opportunities that more complex arrangements offer for examining how species interactions shape global patterns of biodiversity.

Keywords: bird; brood parasitism; coevolution; cowbird; cuckoo; ecology; evolution; multispecies interactions.

FIGURE 1

A diagram depicting the networks considered in this study: One‐to‐one (e.g. species A → C); one‐to‐many (e.g. species A → C and D); many‐to‐one (e.g. species A and B → C); and many‐to‐many (e.g. species A and B → C and D)

FIGURE 2

World map depicting field site locations utilised in brood parasitism studies. The size of each bubble correlates with the number of studies conducted at each field site, with larger bubbles indicating that more studies were conducted

FIGURE 3

A breakdown of the number of studies: (a) investigating the 10 most frequently studied species of brood parasites; (b) by continent for each decade between 1981 and 2020, and studies pre‐1981; (c) by system type for each decade between 1981 and 2020, and studies pre‐1981. The continent of Antarctica is omitted as no brood parasites breed in the region. One‐to‐one refers to a system with one species of brood parasite and one species of host, one‐to‐many refers to a system with one species of brood parasite and multiple species of host, many‐to‐one refers to a system with multiple species of brood parasites and one species of host, and many‐to‐many refers to a system with multiple species of brood parasites and multiple species of hosts

FIGURE 4

Plots describing the relationship between system complexity measured as the linkage density of potential brood parasite–host systems for each respective land hexagon, and three metrics of species richness: (a) number of species of brood parasites; (b) number of species of hosts; (c) number of bird species. Each dot represents one land hexagon

FIGURE 5

Heatmaps describing (a) global patterns of brood parasite species diversity; (b) host species diversity; and (c) brood parasite–host system complexity. System complexity is measured as the linkage density of potential brood parasite–host systems for each respective land hexagon. Grey land areas represent regions where either no species of brood parasites (a and c) or hosts (b) occur