Aug 18 2015
Now that the breeding season is over and dry weather is suppressing native flowers, ruby-throated hummingbirds are swarming to backyard feeders in Pennsylvania. All of them are small and feisty, but did you know the males are even smaller and more belligerent than the females?
Ruby-throated hummingbirds are sexually dimorphic in size though they’re all so tiny that only a bander could know. At banding, birds are weighed and measured and so we’ve learned that male ruby-throats are about 87% the size of females in wing length and weight(*). Their size is related to their lifestyle.
Male hummingbirds are the original deadbeat dads. Ruby-throated males rush north in the spring to claim territories with lots of food which they vigorously defend with aerial displays, chasing, and bill-to-bill sword battles.
When a female shows up the male doesn’t welcome her at first (he acts annoyed) but he switches to intensive courtship displays when she perches. Good hovering technique really impresses her and to do it well he needs lots of energy, smaller wings, and a lighter body than hers — which he has.
As soon as he’s mated with one lady he looks for the next. He never helps with nesting and young and is so focused on attracting another female and warding off other males that he may forego feeding for much of the day. Banders have found that adult males lose weight in June and July, though they regain it in August.
By the end of the breeding season there are noticeably fewer adult males than females at bird banding stations. In a study done at Powdermill Nature Reserve, Bob Mulvihill and Bob Leberman found that the adult sex ratio is most skewed in the fall when there are 4.1 adult females for every 1 adult male.
Their paper(*), published in The Condor in 1992, describes why more adult males die in the summer than at other times of year:
“As a species, the Ruby-throated Hummingbird is near the extreme of small size that is physiologically possible for an endothermic vertebrate. It is conceivable that males approach a critical body mass during the summer. Below this critical mass they may have to abandon nocturnal homeothermy for hypothermic torpor, and may starve overnight or during periods of inclement weather.”
Male ruby-throated hummingbirds are so small and belligerent that it shortens their lives.
(photos taken at the Neighborhood Nestwatch bird banding at Marcy & Dan Cunkelman’s by Kate St. John, 18 July 2015. Bob Mulvihill is the bander holding the birds.)
(*) The paper by Robert S. Mulvihill and Robert C. Leberman is entitled A Possible Relationship Between Reversed Sexual Size Dimorphism and Reduced Male Survivorship in the Ruby-throated Hummingbird, published in The Condor 94: 480-489. It’s available as a PDF here at Sora. Their work is cited in the ruby-throated hummingbird account at Cornell’s Birds of North America.