I’ve heard since I was a kid that when a mated pair of birds is tending to a restful of eggs, the mama bird will sit on the eggs and incubate, and the papa bird will bring food to the mama bird (I’m pretty confident that not ALL species of bird behave this way). But here’s my question:
Is the papa bird motivated by the need to keep his offspring viable, or by the desire for the mama bird to mate with him again?
In my (very limited) experience with birds, none behave that way.
With chickens, the hen will incubate the eggs, and she will get up and leave for short periods to get food. The rooster is off doing rooster-stuff (like impregnating other hens).
With hummingbirds, the female incubates the eggs and the male is nowhere to be found. She also leaves the nest periodically to eat.
Harris’ Hawks often have two generations tending the nest. But although the female is doing the brooding, both sexes hunt and feed the young.
Courtship feeding is common in birds. I think this most often precedes egg laying, when the female will benefit from the extra nutrition, and when she may be less able to feed herself.
Courtship feeding is frequently seen in terns. For instance, in an effort to lure females to their territories in the nesting area, a male Common Tern carries a fish around the breeding colony and displays it to prospective mates. After a pair bond is formed, during the “honeymoon period” the male tern actually feeds the female, and soon thereafter they begin to copulate. During the following five to ten days, both sexes feed themselves, but the male also frequently feeds the increasingly dependent female. For the few days prior to egg laying the female is fed almost exclusively by the male, but this activity declines rapidly as the second and third eggs are laid.
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After the eggs are laid, I’m not sure if it’s quite so common for one bird to incubate the eggs and the other to feed it, but that article does mention it.
ETA: here you go
In species with female-only incubation,
males can help their partners by providing them with food
on the nest, a behavior which may enable females to spend
more time incubating and could, consequently, lead to
improved reproductive performance. In the study reported
here, we first investigated whether incubation feeding by
males affects nest attendance by females in Blue Tits
(Cyanistes caeruleus) and subsequently determined how
this incubation feeding affects reproductive performance.
We found that the female nest attendance tended to
increase with increasing amounts of food supplied by their
male partner. Thus, males may enable females to incubate
more when needed, as suggested by our observation that
male incubation feeding was more frequent when the
ambient temperature was lower, and especially so when
females incubated later in the breeding season (during the
study period the ambient temperature decreased rapidly
over the breeding season, which is exceptional). Although
female nest attendance did not result in a shorter time until
the eggs hatched or in higher hatching success, females that
attended the nest more produced heavier nestlings. We
suggest that the trade-off between self-maintenance and
meeting the demands of egg incubation likely tends to be
less when females received more assistance from their
partner during the egg incubation period, resulting in a
higher investment in offspring.
I think motivation for the male (sex or viable offspring) is a bit of chicken or the egg situation and probably can’t be teased apart. Natural selection works on successful reproduction and both of those factors are important to achieve that. Raptors, to take that example, just aren’t that bright - they have learned behaviors, but mostly they are operating on pure instinct.
More:
https://www.sciencedirect.com/science/article/abs/pii/S0003347288801453
Male marsh tits, Parus palustris , regularly feed their mates from the beginning of nest building until hatching… There was a significant negative relationship between the frequency with which the male fed the female in the nest during incubation and the length of the incubation period…feeding of the female by the male is a nutritional contribution and that the shorter incubation period and increased hatching success enhance the fitness of both parents…the male’s provisioning rate increased when ambient temperatures decreased.
https://onlinelibrary.wiley.com/doi/full/10.1111/ibi.12614
…during the incubation phase in passerines, females perform most or all of the incubation, while the male cares for the brood indirectly by feeding the female…The frequency of male incubation feeding varied between 0 and 74 times per day with an average of 12 feeds per day…males fed their females more often at the nest when ambient temperatures were low.
As to this specific question, that research shows that there’s a fitness advantage from incubation feeding to both parents in keeping the eggs warm. In addition to the male having an instinct for the behavior, we’d expect the female to be instinctively attracted to a male who exhibits the behavior - these are surely complementary, not mutually exclusive alternatives.
Apparently emperor penguin fathers brood the egg while the females are at sea, eating.
Emu females just lay the eggs and their job is done; the males incubate the eggs and raise the chicks.
Cassowaries, rheas, most kiwis and tinamous as well. In fact ostriches are kinda the exception to the rule among ratites and their close taxonomic kin (tinamou) and even with them the male still regularly participates in brooding (they cover the night shift).
The papa bird feels a biological urge to feed the chicks. There’s no compelling reason to believe complicated webs of motivation are involved.
Yeah, from the broader perspective, they do it because males who do that tend to have more descendants, who carry on the instinct-genes. From the perspective of an individual bird, if you were to somehow ask him, he’d probably just say “because that’s what you do”, or something similarly non-descriptive. “Motivation” is something that’s reserved for higher mental function, and even we probably lie to ourselves about it as often as not.