It’s a sing-off! Myth-busting about birds and sex when it comes to defending the nest
- Written by Benjamin Freeman, Assistant Professor, School of Biological Sciences, Georgia Institute of Technology
Each spring, birds across America are in full voice. Cardinals chatter, sparrows sing and warblers warble. Birdsong lifts the human spirit – “‘Hope’ is the thing with feathers[1],” after all. Yet birds are not singing to soothe our nerves after a stressful day at the office. Instead, they sing to defend their territories and attract mates[2].
The traditional view of birdsong focuses on the male bird: He is like a gladiator who fiercely defends his territory against rivals to ensure sufficient space and resources to feed and raise his chicks.
A European robin defends its territory.Female birds, on the other hand, are often thought to be quiet spectators when it comes to territorial defense. This holds true for the red-winged blackbird[3] and many other North American birds.
But it is far from the complete picture[4].
Female rose-breasted grosbeaks[5] and many other birds sing and defend territories across the globe[6].
The growing recognition[9] that females often participate in territorial defense leads to a puzzle: If two is better than one, why do male-female pairs cooperate to defend territories in some species, while just the male defends home turf in other species?
To find out, we performed[10] over 3,000 playback experiments[11] across the Americas, playing recorded bird songs from the same species so the bird would think it was hearing an intruder.
We measured territory defense in 264 species. By studying many types of birds in many different environments, we were able to figure out some answers.
Simulating a bird intruder
Humans are well aware of their property lines and don’t take kindly to intruders. Imagine you are relaxing at home and you see your neighbors digging in your flower garden. You might rush out to tell them to stop; your prize dahlias aren’t for them to take.
For birds, these sorts of disputes happen all the time, with territory owners engaging in song battles with neighbors. The songbirds aren’t just defending their garden. They’re defending their food resources, nest locations and even their mates from rival birds, within territories that often span several acres in size[12].
To study how birds defend their territories[13], we pretended we were an intruding bird. But because we can’t sing like the average bird, we used technology.
One example of how birds responded to the study’s audio of their calls.We surreptitiously placed a speaker in a bird’s territory, hid in the bushes nearby, and then broadcast that bird species’ song. We then counted how many individuals came out from other parts of their territory to respond to the speaker. Some sang at the sound, clearly agitated. A few tried to attack the speaker itself.
At the end of a two-minute experiment, we would leave – and the rightful territory owner presumably felt proud that it had successfully repelled the invisible intruder. Then, we analyzed variables that could explain why some female birds participate[14] in territory defense while others stay out of the fray.
Birds that hang out together defend together
Some birds stick with their mate for life, while others pair up just for one short breeding season.
Studies have found that birds in long-term relationships cooperate in many daily tasks, whether it’s foraging for meals[15], gathering nest materials[16] or feeding the babies[17].
We found that this cooperation extends to guarding their home.
Females in species with long-term bonds that last for years, such as Carolina chickadees[20], often defended their territory.
However, among pine warblers[21] and other species that form temporary pairs only during the breeding season, males typically took responsibility for defending the territory.
Some families took it one step further by including the kids.
The brown-headed nuthatch[22] might look cute and sound like a squeaky toy[23], but these birds are no joke when they team up to defend their territory.
The nuthatches employ the previous seasons’ offspring as nannies – nest helpers that help take care of their babies. We often saw three or more adult nuthatches attacking the speaker to defend their territory when we conducted playback experiments on this species, meaning that the mated pair was joined by at least one helper. It seems to be a good strategy to get the whole family involved in territory defense too.
Brown-headed nuthatches, common in the southeastern U.S., often stick together as a family. Cornell Lab of Ornithology.There were some exceptions to these patterns. When we simulated invasions on the territory of the blue grosbeak, a species thought to be monogamous during breeding season[24], in multiple instances only the female bird defended the territory[25].
No time to relax in the tropics
Location also matters when it comes to bird defenses.
In the rainforests of Costa Rica and the mountains of Peru, we found that males and females cooperating to defend their territory together was generally the rule.
While humans living in places with cold winters associate tropical climates with vacations, birds living near the equator are not afforded the luxury of rest. Instead, they need to stay vigilant year-round to ward off any birds looking to usurp their resource-rich habitats. The need for year-round territorial defense may mean that teaming up is the best strategy to ward off competitors.
Lots of bird personalities
You might think it would get boring observing bird behavior day after day. And, indeed, we dealt with heat and humidity, hordes of biting insects, and early morning wake-ups.
But every experiment brought a peek into the personalities of these birds. There were the pugnacious tufted titmice[26], which seemed as if they were eagerly awaiting an opportunity to fight, given how quickly they came in to investigate the apparent intruder, and the nonchalant American robins[27], which took their sweet time in responding, only briefly peering at the speaker before returning to their daily routines.
Our adorable feathered friends are not afraid to get up close and personal with anything they deem a threat, either, including any gadgets. Many times we’d see small birds such as chipping sparrows[28] scrapping with a speaker twice its size. The birds focus on the song, and it can take birds a while before they realize the speaker is not, in fact, a rival bird.
Tenacious chipping sparrows spotted the audio speaker used in the experiment and tried to attack it.
Mdf/Wikimedia Commons, CC BY-SA[29][30]
Male birds sing and defend territories, but so do many female birds. We found that cooperative territorial defense is especially common in birds with long-term social bonds or that live close to the equator.
So, the next time you hear birds singing as you walk around your neighborhood, listen closely to what each voice is really saying – and who is doing the singing.
References
- ^ Hope’ is the thing with feathers (www.poetryfoundation.org)
- ^ sing to defend their territories and attract mates (doi.org)
- ^ true for the red-winged blackbird (macaulaylibrary.org)
- ^ far from the complete picture (doi.org)
- ^ rose-breasted grosbeaks (macaulaylibrary.org)
- ^ across the globe (doi.org)
- ^ Cephas/Wikimedia Commons (commons.wikimedia.org)
- ^ CC BY-SA (creativecommons.org)
- ^ growing recognition (femalebirdsong.org)
- ^ we performed (benjamingfreeman.com)
- ^ 3,000 playback experiments (doi.org)
- ^ within territories that often span several acres in size (home.dartmouth.edu)
- ^ how birds defend their territories (doi.org)
- ^ explain why some female birds participate (royalsocietypublishing.org)
- ^ foraging for meals (www.audubon.org)
- ^ gathering nest materials (www.audubon.org)
- ^ feeding the babies (www.audubon.org)
- ^ Paul Balfe/Wikimedia Commons (commons.wikimedia.org)
- ^ CC BY (creativecommons.org)
- ^ Carolina chickadees (macaulaylibrary.org)
- ^ pine warblers (macaulaylibrary.org)
- ^ brown-headed nuthatch (macaulaylibrary.org)
- ^ sound like a squeaky toy (macaulaylibrary.org)
- ^ thought to be monogamous during breeding season (www.allaboutbirds.org)
- ^ only the female bird defended the territory (doi.org)
- ^ tufted titmice (www.allaboutbirds.org)
- ^ American robins (macaulaylibrary.org)
- ^ chipping sparrows (macaulaylibrary.org)
- ^ Mdf/Wikimedia Commons (commons.wikimedia.org)
- ^ CC BY-SA (creativecommons.org)
Authors: Benjamin Freeman, Assistant Professor, School of Biological Sciences, Georgia Institute of Technology
