I’m not sure what to think about our cardinals. We saw the egg cache go from two to three—than back to two in the same day. No kidding, the nest gained a third egg in the morning and lost it in the afternoon.
Three eggs… 
And then there were two…
I looked all over the ground and couldn’t find it. Before that, I was hunting around the web trying to learn more about cardinals and discovered that robins and cardinals will sometimes lay their eggs in the same nest. It’s not always clear why this happens, maybe competition or mutualism. Maybe they’re just swingers.
There was an article published about nest-sharing between cardinals and robins several years ago, published in The Wilson Journal of Ornithology. The authors observed cardinals and robins sharing a nest with mixed eggs in Polk County, Iowa of all places. Iowa is a happening place. Both species incubated the eggs; however, only the robins fledged.
“Govoni, P. W., et al. (2009). Nest Sharing Between an American Robin and a Northern Cardinal, BIOONE.
Mixed-clutch nest sharing was observed between an American Robin (Turdus migratorius) and a Northern Cardinal (Cardinalis cardinalis) in Saylor Township, Polk County, Iowa in May 2007. The nest contained three American Robin eggs and two Northern Cardinal eggs, but only American Robin young were fledged successfully. This was not a case of brood parasitism, as both females were observed alternating incubation of the nest. Competition for desirable nest sites might be a possible cause for this type of interspecific behavior.”
Others speculate that robins will eat cardinal young. I’m not so sure about that. Based on what little I found on the web about it, it’s controversial whether robins actually raid cardinal nests to eat the eggs. They rarely will eat shrews and small snakes. Like me, they hate coconut. They eat a lot of chokecherries, often after they’ve fermented into wine, on which they get pretty drunk and could play pranks on cardinals (“Hey, let’s go cardinal-tipping and steal some eggs!”).They sure like worms and follow my wife around as she waters the lawn, snacking on them as they emerge from their flooded tunnels, gasping and frantically hunting for their flood insurance policies. They also ham it up for the camera.
My wife has spied a robin or two flying around the back yard. It raises questions about competition because robins nested and raised a brood last year in the same tree and in the exact same spot where the cardinals are settled this year.
It’s hardly prime real estate in my opinion. We’re always out in the back yard, making noise and flinging water and grass clippings. And we’re continually nosing around the nests, which makes the adult birds pretty nervous and fussy, putting up Do Not Trespass signs and privacy fences.
If robins ate the third egg, they had excellent table manners. There’s no trace of shell or yolk anywhere. I wondered if the cardinal had carried off one of the eggs out of impatience with our continual spying on their nest. But how? The eggs look too big for a bird’s beak. Can they carry it in their feet? Or do they own luggage?
And where would they take it? I supposed it’s possible they could be taking it to another nest they previously built—but it would be occupied by a previous brood. Cardinals nest more than once a season; the male feeds the young while the female builds another one, according to Birds of Iowa Field Guide by Stan Tekiela (2000).
I have no idea what’s going on with these birds. I’ll keep you posted as the situation develops.
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