I was sent a photograph of a bluebird sitting on a nest of eggs in a bird box. The sitter was definitely a male; it's fairly easy to distinguish the genders with this songbird species. The sender had been monitoring the bird box and knew there were eggs under this bird. So, what was this male doing there? Incubating the eggs while the female foraged, or covering the eggs in a protective posture during the female’s absence? Is that a distinction without a difference? (The picture does not appear here because I don't have permission to use it since it's appearing elsewhere.)
Incubation is the process intended to keep the embryos within eggs in the temperature range that supports their development. The opportunity for males to be involved in this effort arises for two reasons: (1) birds are oviparous, that is, egg laying, and (2) the eggs, once outside the female’s body, require incubation. These two factors are the driving force behind the development of male and female pair bonds, the arrangement for most birds during the breeding season. (See, Bart Kempenaers, Mating Systems in Birds, Current Biology, Volume 32, October 24, 2022.) Taking care of eggs and, subsequently, tending to any nestlings is hard, demanding work, something many birds have evolved to deal with by including both partners. Tomasz Wesołowski put it this way:
It seems that conditions for the evolution of biparental care arose only when eggs had become so dependent on care that they could not develop normally without it. (On the Origin of Parental Care and the Early Evolution of Male and Female Parental Roles in Birds, The American Naturalist, Volume 143, Number 1, 1994, p. 53.)
Identifying male involvement in incubation of eggs is a challenge. Consider the case at hand: the bluebird male in the picture appeared to be incubating by virtue of sitting on the eggs, but was he actually helping to regulate the temperature of the eggs? A couple of texts I had immediately at hand drew me into an ornithological black hole. One source stated forcefully that incubation was solely the province of the bluebird female (Hal Harrison, A Field Guide to the Birds’ Nests: United States East of the Mississippi River, 1975, p. 164), while another countered that incubation was a shared responsibility among bluebird males and females, though it was performed “mainly by the female” (Paul J. Baicich and Colin J.O. Harrison in their A Guide to the Nests, Eggs, and Nestlings of North American Birds, 1997, second edition, p. 252).
To arbitrate this issue, I turned to the Cornell Lab of Ornithology, specifically its Birds of the World, an online resource bringing together the research on over 11,000 bird species. (The full resource resides behind a paywall.) In the eastern bluebird entry last updated in 2015, Patricia A. Gowaty and Jonathan H. Plissner have this to say about bluebird incubation (this particular text is behind the paywall):
By female only; scattered claims of male incubation probably based on observations of male attentiveness to incubating females or to nesting cavities.
Hard to square that assertion with the photograph that started this exploration. Possibly a better, though perhaps less authoritative, place to come down is the somewhat more nuanced position taken by Lawrence Zeleny, founder of the North American Bluebird Society:
The male bluebird sometimes sits on the eggs while the female is out of the box for brief periods, but it is doubtful that he contributes much to the incubation process since he does not have a brood patch. (The Remarkable Process of Incubation, Bluebird, Spring 2021, an edited version of an article that first appeared in 1983, p. 23.)
The brood patch is a featherless area on the abdomen of a bird that develops to efficiently and effectively use the bird’s body heat to incubate eggs. This anatomical feature is almost exclusively sported by females during the nest season. In some species both genders develop brood patches, and in a very few, only the males do.
I take Zeleny's statement about male incubation to be based on his assumption that, because the bluebird male lacks a brood patch, he cannot be much of a partner in maintaining eggs in the optimal temperature range. A fairly logical conclusion, but is it true?
I have come to believe it is not. In much of the rest of the literature on avian nesting, if a male is seen to be sitting on a nest or present in a nesting cavity or box when the female is absent, the male is often deemed to be incubating the eggs, whether or not his precise impact on egg temperature is even measured. Further, the presence or absence of a brood patch apparently isn't dispositive.
When Jared Verner and Mary F. Willson reported the results of a massive review of the literature available to them (their paper was published in 1969) on how male passerines (members of the Passeriformes order that includes the oscines, i.e., the songbirds) participated in the nesting period, they noted it could not typically be determined from published papers whether a male was “simply covering the eggs or was actually applying heat to them.” (Mating Systems, Sexual Dimorphism, and the Role of Male North American Passerine Birds in the Nesting Cycle, Ornithological Monographs, Number 9, 1969, p. 35) Nevertheless, they considered the two actions to be roughly comparable, positing that “simply covering the eggs, however, can significantly reduce their heat loss and thus help in their development.” (p. 35) (No source cited in support of this conclusion.)
Frankly, there is no clear picture currently available of the impact on egg temperatures of a songbird male, lacking a brood patch, sitting on a nest. The results from the few studies I've read that do measure egg temperature during male attendance suggest that there can be some impact, though it's not necessarily straightforward or consistent. For example, in 1983, Gregory F. Ball published the results of research on barn swallow males sitting on nests. (Functional Incubation in Male Barn Swallows, The Auk, Volume 100, Issue 4, 1983.) Barn swallow males lack a brood patch. Ball placed eggs with internal temperature devices in a series of barn swallow nests and determined that “when male Barn Swallows sit tightly on the nest, they can incubate as effectively as the female despite the lack of a brood patch.” (p. 999) He did acknowledge, though, that the brood patch might be necessary to maintain egg temperatures at night, when ambient temperatures were likely to fall (a period of “thermal stress” as he described it), thus this was a time when only females sat on the nest.
Some 25 years later, Margaret A. Voss, et al., reached a somewhat different set of conclusions, though they still found this male activity had positive consequences for egg temperatures. (Adaptive Value of Thermally Inefficient Male Incubation in Barn Swallow (Hirundo Rustica), The Auk, Volume 125, Number 3, July 2008.) As did Ball, they placed eggs with temperature gauges in barn swallow nests. They found that, in those nests from which they could gather data, the temperature in the eggs often fell when males were sitting on them, and only rose when females did the sitting. Though hardly the same results that Ball obtained in his study, Voss et al. actually found a subtle postive impact. This male action slowed the rate of temperature loss compared to what transpired when the eggs were left unattended. As a result, when males sat on the nest, the eggs were kept from dropping below the temperature level needed for embryo development. Does this constitute male incubation? I think it does. Further, the authors concluded that this male involvement enabled females able to spend more time away from the nest foraging and rebuilding their physical reserves, without endangering the eggs. They posited that this could make the females less likely to abandon their nests.
Of particular interest, one study fairly recent study found that incubation by the one species' males lacking a brood patch did actually raise the temperature of the eggs appreciably. (Sonya K. Auer, et al., Biparental Incubation in the Chestnut-Vented Tit-Babbler Parisoma subcaeruleum: Mates Devote Equal Time, But Males Keep Eggs Warmer, Journal of Avian Biology, Volume 389, 2007.) The authors asserted: "To the best of our knowledge, our results are the first to demonstrate significantly higher egg incubation temperatures in males relative to females." (p. 281)
The proposition that sitting is incubating does make things neater and probably reflects reality. Regardless of what the actual temperature effect is of a nest-sitting male that lacks a brood patch, a significant portion of all passerine species exhibit some degree of such male participation in incubation (as broadly defined in the literature). Daiping Wang, et al., analyzed the variation in parental engagement in three aspects of the breeding cycle, including incubation, as those variations were reported in Birds of the World for over 1,500 bird species. (Sex Differences in Avian Parental Care Patterns Vary Across the Breeding Cycle, Nature Communications, Volume 14, 2023.) With regard to incubation, they reported that, in slightly fewer than half (48%) of the species they covered, incubation was reported to be a shared activity between males and females, while in about 50% of species it was solely a female responsibility, and in fewer than 2% was it only the male providing the care. (The presence or absence of a brood patch in males was not considered, though, as already noted, most males lack that anatomical feature in the nesting season.)
Given how widespread male involvement is in incubation, it arguably is of some benefit for the nesting success of these species that practice it. Males appear to be a valuable part of the process, however complicated or variable their contribution might be.
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