Free Love? Mating Patterns in Bushtits

The first time I encountered a bushtit, or Psaltriparus minimus, I was holding it in my hand!  It was really cute: a rather round little bird, with a light gray body, darker gray wings, and a brownish-gray crown.  I was at the Starr Ranch Bird Observatory in California for a bird banding workshop.  Using mist nets, the ornithologists captured various birds in order to band them for future studies.  I was allowed to hold one of the bushtits as the ornithologist attached a metal band to its leg, and I amazed by how small it was—about the length of my middle finger!  I also noticed that there was a flock of bushtits in the trees nearby the mist net, and that they weren’t flying off even though there was a bunch of people nearby.  However, when I released the bushtit, it flew immediately to its flock, and all the bushtits left.  Apparently flock was waiting for its captive member!  I became interested in the bushtit’s population dynamics, and wondered if all flocks were this close to one another.

According to Stephen Ervin’s 1977 study, the answer is yes!  For three years, Ervin observed a population of bushtits nearby the campus of the University of California Santa Barbara, and came out with some cool observations.  The population consisted of thirteen flocks of varying sizes, from nine to thirty birds in a flock.  Despite frequent interaction between the different flocks, bushtits rarely switched flocks.  In fact, the interactions were usually quite agitated, with loud calling and fighting between the males.  Ervin recorded only two occasions in which birds permanently switched flocks: when a flock split, and when three birds switched flocks before the two flocks merged into one.  Ervin tried to artificially switch a bushtit to a different flock by capturing one and releasing it nearby another flock, but the displaced bushtit was chased away; later, the bushtit was found back with its original flock.  Overall, flocks tended to be constant in size and composition.  With this information about bushtit flocks, I can understand why the captured bushtit’s flock would be reluctant to leave it behind!

Ervin also observed that bushtits do not form flocks during the breeding season; after the breeding season is over, the flocks reform—almost always with the same members!  Not only that, but also the young bushtits that were born that year joined the flocks of their parents.  Ervin realized that the bushtit flocks were composed of family groups.  At the bottom of my post, there’s an interesting figure showing the relations between bushtits in one of the flocks if you want to check it out!

Bushtits usually rejoined the same flock because they tend to nest in their flock’s territory; in this study, 94 of the 101 birds did so.  The seven other birds—all female—that nested in another flock’s territory were incorporated into each territory’s respective flock.  Ervin suggested that gene flow in bushtit populations is possible because flocks accepted foreign birds right after breeding season; flock merging also facilitated gene flow.

In another 1977 paper relating to his Santa Barbara study, Ervin discussed more specifics about bushtit behavior during breeding season.  He noticed both nest appropriation—when one pair takes over a nest built by other birds—and mate replacement—switching mates during the breeding season.  However, because not all the birds were banded, Erving could not determine how often birds switched nests or mates.  He did notice that when a nest was appropriated, usually the builders of the nest were not dead; basically, the appropriators took advantage of another pair’s hard work!

Although Ervin did observe bushtits during breeding season, he had difficulty drawing solid conclusions.  I wondered if I could find more detailed information on this subject.  Luckily, Alice Baldwin Addicott, Sarah Sloane, and a group of scientists (referred to here as Bruce et al) studied bushtit behavior during breeding season.

Addicott studied a population of bushtits on the Stanford University campus.  She noticed that bushtits exhibit cooperative breeding; this means that bushtits helped care for young that were not theirs.  Pairs of bushtits did not act aggressively against individual bushtits that intruded on their nesting territory; instead, the birds foraged together, and later all contributed to building the nest, incubating the eggs, and feeding the young.  Based on Ervin’s study, the helper bushtits were probably from the same flock, and thus the same family group.

Like Ervin, Addicott also observed that bushtits change mates and nests during the middle of the breeding season.  Whenever a pair was disturbed, they would separate; if one flew off and the other remained with the nest, the one that stayed took a new mate.  If both deserted the nest, both took new mates.  The abandoned nests were taken over by other bushtit pairs.

I also read a paper by Sarah Sloane, who studied a population of bushtits in the Chiricahua Mountains in Arizona from 1986 to 1991 to quantify and explain the presence of the helper birds, or “supernumeraries,” as she called them.  She banded 607—wow!—bushtits for her study.  She determined that 39 percent of bushtit nest had supernumeraries, and that of the nests with supernumeraries 35 percent had two or more helpers.  One of the nests had six helpers at once!  Sloane also discovered that 89 percent of the helpers were adults, while only 11 percent were juvenile, and that 81 percent were male while 19 percent were female.  Sixteen percent of the adult supernumeraries were unmated males, and 84 percent were birds that had failed to produce a brood.  The failed breeders could be a lone male, a lone female, or even a pair that had their nest destroyed!  Among birds, it’s pretty rare to have female supernumeraries.  In general, most of the helpers were adult males that were failed breeders.  Sloane believed that most of the supernumeraries were males because the sex ratio in bushtits is skewed towards males: about 1.35 males to 1 female; she attributed this skewed ratio to the higher death rate for female bushtits, since they are the ones that disperse, as found by Ervin.

Besides knowing how often supernumeraries occurred and their gender and age, Sloane also wanted to know if the supernumeraries mated with one of the pair.  The supernumeraries usually began helping out during the feeding stage—too late to contribute genetically—but when the helpers were present during the mating stage, courtship displays were seen between all members of opposite-sex.  Sloane thought that the relationships between bushtits could be called “polygynous” (when a male mates with more than one female), “polyandrous” (when a female mates with more than one male), or “polygynandrous” (when both the male and female mate with multiple others).  Since there was no mate guarding behavior seen, the bushtits seemed to follow a “free love” approach to mating!

Sloane teamed up with three other scientists (the Bruce et al group referred to earlier) to learn if bushtits really are free lovers or are actually monogamous.  To do so, they used DNA probes to compare the DNA of the original pair, the supernumeraries, and the chicks.  They found that although there was an extra bird, even if it was a male, all the chicks came from the same pair—no cheating allowed!  However, if the female had a second brood, she usually mated with the helper; a fancy word for this type of reproductive behavior is “serial monogamy.”  What I found most interesting was that the helpers were unrelated to the pair that was mating; Ervin observed that flocks consist of multiple family groups, so it is possible that the supernumeraries could always be from a different family group within the same flock.  Perhaps only nonrelatives are present as supernumeraries because mating is a possibility, and the bushtits are avoiding inbreeding.  It would be interesting to see if related bushtits ever mate.  Alas, these data do not exist!

If you’d like to know more information about this cute bird, take a look at its Cornell Lab of Ornithology page here: http://www.allaboutbirds.org/guide/Bushtit/id.  You can also read the papers I referenced, listed below (hopefully you have a subscription to JSTOR!):

Addicott, A.B. 1938. Behavior of the Bush-Tit in the Breeding Season. The Condor. 40:49-63.

This paper covers Addicott’s observations of bushtits during the breeding season.  She covers such topics as nest territory, construction of the nest, and the behavior of mates.  (Link: http://www.jstor.org/stable/1363843)

Bruce, J.P., Quinn, J.S., Sloane, S.A., and White, B.N. 1996. DNA Fingerprinting Reveals Monogamy in the Bushtit, a Cooperatively Breeding Species. The Auk. 133:511-516.

Bruce et al did a DNA analysis on bushtit mating pairs, the supernumeraries, and the babies to learn if bushtits are monogamous or not.  (Link: http://www.jstor.org/stable/4088921)

Ervin, Stephen. 1977. Flock Size, Composition, and Behavior in a Population of Bushtits. Bird-Banding. 48:97-109.

This paper discussed a three-year study of a population of bushtits that looked into the dynamics within and between flocks of bushtits.  (Link: http://www.jstor.org/stable/20699069)

Ervin, Stephen. 1977. Nest Appropriation and Mate Replacement in the Bushtit. The Auk. 94:598-599.  (Link: http://www.jstor.org/stable/4085236)

This article summarized some of Ervin’s observations of bushtit behavior during breeding season.

Sloane, S.A. 1996. Incidence and Origins of Supernumeraries at Bushtit (Psaltriparus minimus) Nests. The Auk. 113:757-770.

In this paper, Sloane describes her detailed study of a bushtit population to quantify the supernumeraries present at bushtit nests and to also propose reasoning for their presence.  (Link: http://www.jstor.org/stable/4088855)

Here’s a picture of a bushtit.  It blends in really well with its habitat, as you can see!

This is a typical bushtit nest, known as a “pendant nest” because of its shape.

This is a typical bushtit nest, known as a “pendant nest” because of its shape.

This figure is from Ervin’s 1977 on flock size, composition, and behavior.  As you can see here, a flock consists of multiple family groups.

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4 Responses to Free Love? Mating Patterns in Bushtits

  1. Sonja says:

    Hey, cool post! I’ve always been really fascinated by bushtits and have been observing a breeding pair closely and beginning to develop some questions about their behavior. The studies you cite here will be very useful as I compile information and learn more about these incredible little birds!

    Thanks!

  2. Pam says:

    Great post! A small bushtit flock routinely visits my yard. Last year, a female (I presumed by the bird’s light-colored eyes) persistently ‘attacked’ her image in our window, even after we installed an exterior screen. I volunteered to write an article about bushtits for our Audubon chapter. Your post provides a wealth of information and makes my job MUCH easier. I’ll cite your excellent account and encourage subscriptions to your blog.

    • Sarah Sloane says:

      I’m fascinated that it was the female attacking her image! My research has suggested that females are more aggressive than males and I’ve suggested that eye color serves a communicative function in female bushtits. Can you contact me about this and can I cite your observation in my paper? Thanks, Sarah Sloane (sloane@maine.edu) P.S. My BNA account has more information not published anywhere else.

  3. Pingback: Bushtits Getting What They Want …

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