We all know now that bushtits are one of the most social species of birds. They like to do everything in groups whether it is to forage for food or to conserve their energy at night by ‘huddling’ to abnormally cold conditions (Chaplin, 1982). Not only that but bushtits usually have helpers at nests, this is called cooperative breeding. Helpers are individuals that help raise offspring that are not their own (Sloane, 1996). There have been plenty of research on these topics that I would like to explore for you today.

Have you ever seen a group of tiny birds huddling? Well that’s what (Smith, 1972) observed in a hawthorn tree of a flock of 29 bushtits. She had found their roosting location and began to observe them regularly from January until spring when the flock broke up. Normally if multiple bushtits landed on the same branch, there would be an even spacing of about 2 inches from each other. But (Smith, 1972) noticed that on colder nights that were at least 10°F below normal, they were roosting in close contact with each other. (Chaplin, 1982) also noticed that this was used as their first line of defence against cold when she was studying their daily use of energy. Communal roosting among bushtit is normal during abnormal cold weathers and it helps to reduce both their nocturnal maintenance cost and their diurnal inactive cost in energy (Chaplin, 1982).

(Chaplin, 1982) measured the bushtits energy expenditures and energy intake of single and group individuals. She did this by measuring basal metabolism and cloacal body temperature at night over a 10-30° temperature range, amount of food eaten and feces produced per day, amount of activity during the light period and body weight changes over a 24h period. She found that a single individual must consume about 80% of its body weight daily to maintain itself at 20°C and probably close to or greater than 100% of its body weight at lower temperatures. But by being in a flock, not only can they conserve energy at night or during cold temperatures but can also reduce daytime expenditures and to obtain maximum energy possible (Chaplin, 1982).

Since bushtits are always around other birds, are they polygamous or monogamous? Well, with the development of DNA probes that detect high levels of genetic variation (Bruce et al, 1996) we can see if offspring have any genetic contributions from a second male. Bushtits are cooperative breeders, they usually have another bird other than the original pair at the nest to help care for the offspring, called helpers (Sloane, 1996). The helper is usually an unmated male or female who have failed in finding a mate and reproducing their own offspring. Sometimes the helper can get lucky and find an opportunity for genetic contributions through extrapair fertilizations or intraspecific brood parasitism (Bruce et al, 1996). Extrapair fertilization is when the helper (usually male) copulates with the female and passes his genes onto some of her eggs. And Intraspecific brood parasitism is when females lay her eggs into the nest of another pair of bushtits. Is this true? Does the helper get a chance to reproduce? Well helpers usually join either before or during egg laying and some were observed doing courtship behaviours such as fluttering chases and vocalization around the nest (Sloane, 1996).

According to this DNA probe results showed that the bushtits were sexually monogamous and so extrapair fertilization probably did not occur (Sloane, 1996). The DNA probe showed banding patterns of offspring and compared with those of its parents. They were likely related if the banding size and intensities were similar (Bruce et al, 1996). Which was found to be the case. So, does the helper get any reproductive success or is it purely altruistic? (Ervin, 1977) found that in 16 pairings between bushtits, 11 of them switched mate from one season to the next. The mates were different from the previous season even though the former mates were still alive. Therefore, helpers do have a chance at reproducing with a female second brood while adopting her first offspring (Bruce et al, 1996).

Bushtit can stick around the same flock for 1-4 consecutive flocking season (Ervin, 1977). They rely on each other for survival whether it is to endure the cold nights or to help raise offspring. And this is what makes them so successful. Besides who wouldn’t want to see 10 fluffy bushtits huddling together on a branch? So, make sure to keep an eye out for fluttering and chipping bushtits anywhere from shrubs to oak trees. They may even be in your backyard or your closest park!

Literature Cited

Bruce JP, Quinn JS, Sloane SA, White BN. 1996. DNA Fingerprinting Reveals Monogamy in the Bushtit, a Cooperatively Breeding Species. The Auk. 113(2): 511-516. [Link]

Chaplin SB. 1982. The Energetic Significance of Huddling Behavior in Common Bushtit (Psaltriparus minimus). The Auk. 99(3): 424-430. [Link]

Ervin S. 1977. FLOCK SIZE, COMPOSITION, AND BEHAVIOR IN A POPULATION OF BUSHTITS (Psaltriparus Minimus). JSTOR. 48(2): 97-109. [Link]

Sloane SA. 1996. Incidence and Origins of Supernumeraries at Bushtit (Psaltriparus minimus) Nests. The Auk. 113(4): 757-770. [Link]

Smith SM. 1972. Roosting Aggregations of Bushtits in Response to Cold Temperatures. The Condor. 74(4): 478-479. [Link]