The interactions between avian obligate interspecific brood parasites and their hosts provide tractable systems for studying coevolutionary processes in nature. This review highlights recent advances in understanding coevolution in these systems. First, we discuss the evolution and phylogenetic history of avian brood parasitism. Next, we examine coevolved adaptations and counteradaptations in brood parasites and hosts at all stages of the host nesting cycle: those that precede laying of the parasitic egg and those at the egg, chick, and fledgling stages. We then consider the factors that affect the evolution of offense and defense portfolios (the suites of adaptations and counteradaptations across the nesting cycle), and the outcomes of coevolutionary interactions between brood parasites and hosts. Ongoing efforts to document the diversity of host defenses and parasite offenses will facilitate understanding of coevolutionary processes and the ecological and evolutionary consequences of species interaction...

/pdf/advances-in-the-study-of-coevolution-between-avian-brood-1irqrs1nkg.pdf

Advances in the Study of Coevolution Between Avian Brood Parasites and Their Hosts

It is unclear how birds differentiate their own eggs from cuckoo’s eggs that parasitize their nests. Here, the authors develop a computer vision tool that simulates how brains process pattern information and show that cuckoos’ hosts have evolved unique egg patterns to distinguish their own eggs from a cuckoo’s.

/pdf/pattern-recognition-algorithm-reveals-how-birds-evolve-1cw2gypcgk.pdf

Pattern recognition algorithm reveals how birds evolve individual egg pattern signatures

Cuckoo eggs famously mimic those of their foster parents to evade rejection from discriminating hosts. Here we test whether parasites benefit by repeatedly parasitizing the same host nest. This should make accurate rejection decisions harder, regardless of the mechanism that hosts use to identify foreign eggs. Here we find strong support for this prediction in the African tawny-flanked prinia (Prinia subflava), the most common host of the cuckoo finch (Anomalospiza imberbis). We show experimentally that hosts reject eggs that differ from an internal template, but crucially, as the proportion of foreign eggs increases, hosts are less likely to reject them and require greater differences in appearance to do so. Repeated parasitism by the same cuckoo finch female is common in host nests and likely to be an adaptation to increase the probability of host acceptance. Thus, repeated parasitism interacts with egg mimicry to exploit cognitive and sensory limitations in host defences.

/pdf/repeated-targeting-of-the-same-hosts-by-a-brood-parasite-56om0p1e46.pdf

Repeated targeting of the same hosts by a brood parasite compromises host egg rejection

Avian brood parasites lay their eggs in other birds’ nests and impose considerable fitness costs on their hosts. Historically and scientifically, the best studied example of circumventing host defences is the mimicry of host eggshell colour by the common cuckoo (Cuculus canorus). Yet the chemical basis of eggshell colour similarity, which impacts hosts’ tolerance towards parasitic eggs, remains unknown. We tested the alternative scenarios that (i) cuckoos replicate host egg pigment chemistry, or (ii) cuckoos use alternative mechanisms to produce a similar perceptual effect to mimic host egg appearance. In parallel with patterns of similarity in avian-perceived colour mimicry, the concentrations of the two key eggshell pigments, biliverdin and protoporphyrin, were most similar between the cuckoo host-races and their respective hosts. Thus, the chemical basis of avian host–parasite egg colour mimicry is evolutionarily conserved, but also intraspecifically flexible. These analyses of pigment composition reveal a novel proximate dimension of coevolutionary interactions between avian brood parasites and hosts, and imply that alternative phenotypes may arise by the modifications of already existing biochemical and physiological mechanisms and pathways.

https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2011.1718

A shared chemical basis of avian host-parasite egg colour mimicry.

Most theoretical models of coevolution between brood parasites, whether interspecific or conspecific, and their hosts explicitly assume consistent individual behaviour in host egg-rejection responses. Accordingly, hosts cast as acceptors always accept, whereas ejectors always reject parasitic eggs when exposed to stable ecological conditions. To date, only few studies have attempted to test this critical assumption of individual repeatability in egg-rejection responses of hosts. Here, we studied the repeatability of egg rejection in blackbirds (Turdus merula) and song thrush (T. philomelos), species in which females are reported to reject simulated, non-mimetic foreign eggs at intermediate frequencies at the population level. However, intermediate rates of acceptance and rejection can be consistent with either or both intra- and interindividual variability in rejection behaviours. Our experiments revealed generally high individual consistency in these hosts’ responses to experimentally introduced non-mimetic and mimetic model foreign eggs. Individuals also responded faster on average to second than to first trials within the same breeding attempts, but the difference was statistically significant only in blackbirds. These results are consistent with the critical assumption of co-evolutionary models, that statistically egg rejection is mostly individually repeatable, but also reveal that some individuals in both species change their responses even within the short time-window of one breeding attempt. The data suggest that individuals reject foreign eggs faster when perceived parasitism risk is greater because of repeated

Repeatability of Foreign Egg Rejection: Testing the Assumptions of Co-Evolutionary Theory

Many avian hosts have evolved antiparasite defence mechanisms, including egg rejection, to reduce the costs of brood parasitism. The two main alternative cognitive mechanisms of egg discrimination are thought to be based on the perceived discordancy of eggs in a clutch or the use of recognition templates by hosts. Our experiments reveal that the great reed warbler (Acrocephalus arundinaceus), a host of the common cuckoo (Cuculus canorus), relies on both mechanisms. In support of the discordancy mechanism, hosts rejected their own eggs (13%) and manipulated ('parasitic') eggs (27%) above control levels in experiments when manipulated eggs were in the majority but when clutches also included a minority of own eggs. Hosts that had the chance to observe the manipulated eggs daily just after laying did not show stronger rejection of manipulated eggs than when the eggs were manipulated at clutch completion. When clutches contained only manipulated eggs, in 33% of the nests hosts showed rejection, also supporting a mechanism of template-based egg discrimination. Rejection using a recognition template might be more advantageous because discordancy-based egg discrimination is increasingly error prone with higher rates of multiple parasitism.

/pdf/discordancy-or-template-based-recognition-dissecting-the-1yjpke9jxl.pdf

Rim W. G. Lucassen

Papers

Discordancy or template-based recognition? Dissecting the cognitive basis of the rejection of foreign eggs in hosts of avian brood parasites