Twin Cities bird lovers watched in alarm this week as robins gathered by the dozens and sometimes hundreds in trees and shrubs, puffing up their breasts and charging at each other, seemingly frantic for food in an unnaturally snowy April.

Was it hunger? The cold?

Actually, it’s simpler than that.

It’s sex.

So says Craig Mandel, a birder who belongs to the Minnesota River Audubon Chapter and who leads nature walks in the Minnesota Valley National Wildlife Refuge.

Most of the robins we’re seeing are males, Mandel said. Filled with testosterone and ready for spring, they’re all jammed up in the Twin Cities because they halted their migration in the face of bad weather and snow.

“Usually we don’t see such big flocks in spring, but the large amount of snow has bunched them up more than usual,” he said. “The males come back first, which is why you see all these birds with gorgeous orange breasts, all puffed out, kind of displaying around other males.”

Jim Williams, a lifelong birder who writes the Star Tribune’s “Wingnut” birding blog, said there are probably hundreds of thousands of robins in the metro area right now.

“They migrate based on the length of day; they don’t have weather forecasts,” he said. “When the storms cleared to the south of us, they came up at night … showed up and said, ‘Oh God.’ ”

Mandel counted 94 robins in his Minnetonka back yard on Thursday, and estimated that 90 percent of them were male. Williams, who was out looking for birds on Friday morning, said robins have stripped crabapple trees in the area and had begun eating sumac berries.

Robins eat worms and insects and fruit, and are probably getting along just fine with dried fruit they find on trees and shrubs, Williams said. They are well adapted to cold, with some overwintering here.

To feed or not to feed?

While bird watchers on social media are talking about buying dried fruit or mealworms for robins, Williams prefers to let nature take its course.

“This weather could take some of these birds, but they’re the weaker ones,” he said. “That’s the way it is, and that’s just improving the gene pool.”

Migrating birds often ride storm fronts, Mandel said. While that makes the trip less tiring, they can get stuck when they hit bad weather.

He’s not worried about robins. Mobs of juncos and sparrows have also been spotted, and scattering bird seed on the ground will help them. Mandel is more concerned about Eastern phoebes, kinglets and yellow-rumped warblers, which eat mostly insects and usually don’t visit bird feeders.

Sometimes early arrivers turn back in the face of extreme weather, Mandel said. Others die. Birders tend to be realists where nature is concerned, but sometimes even they go to extreme measures to aid feathered friends.

Purple martins, which nab their meals in midair by catching flying insects, are arriving at a colony in southern Minnesota. Mandel said posts on birding websites indicate people are trying to feed them with crickets and scrambled eggs.

To get the birds to eat, the food has to be thrown in the air. Some birds are taking it.