Niche construction through a Goldilocks principle maximizes fitness for a nest-sharing brood parasite

Abstract

Generalist brood parasites that share nests with host nestlings can optimize resource acquisition from host parents by balancing the benefits that host nest-mates provide, including attracting increased provisions to the nest, against the costs of competing with the same host young over foster parental resources. However, it is unclear how parasitic chicks cope when faced with more nest-mates than are optimal for their survival upon hatching. We suggest that, in the obligate brood parasitic brown-headed cowbird ( Molothrus ater ), chicks use a niche construction strategy and reduce larger, more competitive host broods to maximize the parasites' survival to fledging. We experimentally altered brood sizes to test for Goldilocks principle patterns (i.e. a ‘just right’ intermediate brood size) of cowbird survival in nests of prothonotary warbler ( Protonotaria citrea ) hosts. We found that intermediate brood sizes of two host nestlings maximized cowbird fledging success relative to 0 or 4 host nest-mates at hatching. Specifically, cowbird nestlings lowered host brood sizes towards this optimum when placed in broods with more host nestlings. The results suggest that cowbirds reduce, but do not eliminate, host broods as a niche construction mechanism to improve their own probability of survival.

Does a niche construction strategy adaptation really exist in brown‐headed cowbirds?

“…The subject of brood parasitism has become the focus of widespread attention for the numerous finely tuned adaptations that have been discovered in coevolving specialist brood parasite–host systems (Davies, 2000; Krüger, 2007; Soler, 2017a). A recent study by Antonson et al (2022) claims to have found one of these fascinating adaptations, posing the exciting conclusion that brown‐headed cowbird ( Molothrus ater ) chicks use a niche construction strategy (alteration of its own environment for its own fitness benefit) in prothonotary warbler ( Protonotaria citrea ) host nests. In this experimental study, the authors have found that the selective brood reduction strategy driven by cowbird nestlings is reducing, but not eliminating host broods.…”

“…This also implies an active manipulation of their environment (the nest), and therefore, this strategy could also be considered niche construction. However, Antonson et al (2022) suggest that in the brown‐headed cowbird—protonotary warbler system, the brood parasite selectively manipulates brood reduction favouring the presence of two warbler nestlings in the nest. This suggestion is based on comparisons of data on nestling warbler mortality in four experimental groups, with two parasitized and two unparasitized treatments.…”

“…In addition, this study selectively cites publications and raises several other key questions. Antonson et al (2022) base their study on the ‘begging assistance hypothesis’ (Kilner et al, 2004), which states that brown‐headed cowbird nestlings sharing the nest with host nestlings grow faster than cowbird nestlings raised alone because parasitic nestlings take advantage of the begging calls produced by host nestlings to procure resources. That is, begging displays of host nestlings compel their parents to increase the rate of food delivery, and this extra food would be consumed by the cowbird nestling due to its higher competitive ability (Kilner et al, 2004).…”

“…Further, Gloag et al (2012) reported that the begging assistance hypothesis was only supported in a small host species, but not in a larger one. Unfortunately, Antonson et al (2022) did not mention any of the experimental studies that did not support the predictions of the begging assistance hypothesis in an evictor brood parasite (Grim et al, 2009; Hauber & Moskát, 2008; Martín‐Gálvez et al, 2005). Antonson et al (2022) also did not seem to be aware of other studies (e.g.…”

“…Unfortunately, Antonson et al (2022) did not mention any of the experimental studies that did not support the predictions of the begging assistance hypothesis in an evictor brood parasite (Grim et al, 2009; Hauber & Moskát, 2008; Martín‐Gálvez et al, 2005). Antonson et al (2022) also did not seem to be aware of other studies (e.g. Bolopo et al, 2015; Rivers et al, 2010; Soler & de Neve, 2013) and a review (Soler, 2017b) concluding that in several nest‐sharing brood parasite species larger nestlings successfully compete for extra food, regardless of whether they are hosts or parasites.…”