Reed Warbler Hosts Do Not Fine-Tune Mobbing Defenses During the Breeding Season, Even When Cuckoos Are Rare

Deryk Tolman,Katja Rönkä,Daniela Campobello,Rose Thorogood,Edward Kluen

doi:10.3389/fevo.2021.725467

Deryk Tolman, Katja Rönkä + Show 3 more

Open Access

https://doi.org/10.3389/fevo.2021.725467

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Abstract

Hosts of brood parasitic cuckoos often employ mobbing attacks to defend their nests and, when mobbing is costly, hosts are predicted to adjust their mobbing to match parasitism risk. While evidence exists for fine-tuned plasticity, it remains unclear why mobbing does not track larger seasonal changes in parasitism risk. Here we test a possible explanation from parental investment theory: parents should defend their current brood more intensively as the opportunity to replace it declines (re-nesting potential), and therefore “counteract” any apparent seasonal decline to match parasitism risk. We take advantage of mobbing experiments conducted at two sites where reed warblers (Acrocephalus scirpaceus) experience (in Italy), or do not experience (in Finland), brood parasitism. We predicted that mobbing of cuckoos should be higher overall in Italy, but remain constant over the season as in other parasitised sites, whereas in Finland where cuckoos do not pose a local threat, we predicted that mobbing should be low at the beginning of the season but increase as re-nesting potential declined. However, while cuckoos were more likely to be mobbed in Italy, we found little evidence that mobbing changed over the season at either the parasitized or non-parasitized sites. This suggests that re-nesting potential has either little influence on mobbing behavior, or that its effects are obscured by other seasonal differences in ecology or experience of hosts.

Highlights

When encountering an intruder at the nest, many parent birds use noisy vocal and visual displays to defend their brood
Latency to approach the cuckoo showed little change over the season; the model containing the linear term for date was 1.2 AICc units larger than the null model (Table 2A) and the relationship was not significant
At the nests where reed warblers mobbed the cuckoo (N = 37), there was no change in their intensity of mobbing calls: no model was within 2 AICc of the null model (Table 2C, negative binomial generalized linear mixed effects models (GLMMs) of linear term: estimate = 0.012 ± 0.164, z = 0.076, p = 0.940; Figure 2D)

Summary

Introduction

When encountering an intruder at the nest, many parent birds use noisy vocal and visual displays to defend their brood. Indiscriminate mobbing behavior is rarely optimal (Montgomerie and Weatherhead, 1988) and depends on the benefits gained by protecting the survival of the current brood outweighing the risks it entails for a parent’s future reproduction (Trivers, 1972) or continued survival (Montgomerie and Weatherhead, 1988). In the case of hawk-mimicry, recognition errors can be fatal for adults (if a hawk is misidentified as a cuckoo and approached), whereas misidentifying a virulent cuckoo as a hawk can lead to elimination of the current brood Together these costs mean that, as in models of antipredator mobbing, hosts should adjust their mobbing to match local variation in parasitism risk

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