Choice of nest attributes as a frontline defense against brood parasitism

Abstract

Breeding- and nest-site choice is a behavioral strategy often used to counter negative interactions. Site choices before breeding prevent costs of predation and competition but have been neglected in the context of brood parasitism. For hosts of brood parasites, the earlier brood parasitism is prevented in the breeding cycle the lower the future costs. Suitable nest-sites for cavity-nesting common redstarts (Phoenicurus phoenicurus), a host of the common cuckoo (Cuculus canorus), are a limited resource, but their cavity-nesting strategy could potentially deter predators and brood parasites. We altered the entrance size of breeding cavities and investigated redstart nest-site choice and its consequences to nest predation and brood parasitism risk, although accounting for potential interspecific competition for nest sites. We set-up paired nest-boxes and let redstarts choose between 7 cm and 5 cm entrance sizes. Additionally, we monitored occupancy rates in nest-boxes with 3 cm, 5 cm, and 7 cm entrance sizes and recorded brood parasitism and predation events. We found that redstarts preferred to breed in 5 cm entrance size cavities, where brood parasitism was eliminated but nest predation rates were comparable to 7 cm entrance size cavities. Only in 3 cm cavities both, brood parasitism and predation rates were reduced. In contrast to the other cavity-nesting species, redstart settlement was lowest in 3 cm entrance size cavities, potentially suggesting interspecific competition for small entrance size cavities. Nest-site choice based on entrance size could be a frontline defense strategy that redstarts use to reduce brood parasitism.

Common cuckoo female host selection is not determined by host quality but can affect cuckoo nestling growth when parasitising Common redstarts

“…Although we relied on estimating breeding areas of cuckoos around available nests, it is highly likely that these results captured host availability for four reasons. First, because natural crevices suitable for redstarts are scarce in our natural area (Moreras et al ., 2021, pers. obs.).…”

“…Third, because this situation allows us to accurately monitor redstart breeding cycle and record most redstart nests potentially available for cuckoo females. And four, because the few natural crevices that redstart might breed in are mostly woodpecker cavities, which are generally too small for the cuckoo female to lay her egg (Avilés et al, 2005; Moreras et al, 2021). Therefore, our results are strong evidence of quantitative and qualitative variation in potential host nest options for cuckoo females to parasitise.…”

Aggressive hosts are undeterred by a cuckoo's hawk mimicry, but probably make good foster parents

“…However, the very properties which make hosts attractive foster parents may also make them more difficult to parasitize. In particular, many birds defend their nests from both predators and parasites using 'frontline' defences that include nest placement and architecture, nesting aggregations, cooperative breeding, and parental aggression [10][11][12]. In each instance, avian brood parasites which are able to broach these defences may reap the benefits of increased survival of their offspring through decreased nest predation.…”

Breeding near heterospecifics as a defence against brood parasites: can redstarts lower probability of cuckoo parasitism using neighbours?

“…In Finland, redstarts suffer relatively high brood parasitism rates (20% -32%, Rutila et al, 2002;Samaš et al, 2016;Thomson et al, 2016), yet show weak egg-stage anti-parasitic strategies (Avilés et al 2005;Grim et al 2009;Thomson et al 2016). Redstarts may evade cuckoo parasitism through front-line defences, for example, choosing nest-sites inaccessible for the cuckoo (Moreras et al 2021). However, other front-line strategies are largely unexplored (but see, Tolvanen et al 2017a), as is also the case for many other brood parasite-host systems with apparent lack of host defence strategies.…”

“…Although great tits are currently believed to be a nonsuitable host for the cuckoo, this might be biased information, because almost all knowledge about great tits comes from nest-box populations, where only nest-boxes with small entrances (ca. 3 cm in diameter) that prevent cuckoo parasitism are used (Grim et al 2014;Moreras et al 2021).…”