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Egg laying, egg temperature, attentiveness, and incubation in the Western Bluebird.

Article Excerpt
Timing and duration of egg-laying bouts have been mostly studied in open-cup or groundnesting species that are amenable to direct observation (Wiebe and Martin 1995, Oppenheimer et al. 1996, McMaster et al. 1999). Egglaying times are commonly estimated by visiting a nest before and after a new egg appears, and deducing when during that interval the egg was laid (Weatherhead et al. 1991, Oppenheimer et al. 1996, McMaster et al. 2004). Indirect methods also have been used in incubation studies with nest attendance (presence at the nest) often implicitly equated with parental incubation, and attentiveness often equated with incubation attentiveness. Observations of nocturnal behavior during the laying period are rarely able to separate incubation sensu stricto from nest attendance without egg incubation (Haftorn 1978, 1979, 1981). Morton and Pereyra (1985) recommended distinguishing attentiveness from incubation based on measured egg temperature. Egg temperature may, however, be raised well beyond the threshold of embryonic development in the absence of parental attendance during periods of high ambient temperature or under direct solar radiation (Grant 1982, Zerba and Morton 1983, Ward 1990), which potentially compromises the usefulness of egg temperature.

Distinguishing nest attendance from active incubation and parental incubation from periods of passive incubation due to high ambient temperature is possible with continuous in-nest-box video-recording and simultaneous egg temperature measurements. We used this approach to document: (1) exact laying times and laying-bout durations, (2) patterns of nest attendance, and (3) development of incubation during the laying period for Western Bluebirds (Sialia mexicana), which are socially monogamous, single-gender intermittent incubators that produce up to two successful clutches per season (Guinan et al. 2000).

METHODS

Study Sites.--Our study sites were the Hopland Research and Extension Center in Mendocino County, California (39[degrees] 00' N, 123[degrees] 04' W) in 2004 and Gates Canyon, Solano County, California (38[degrees] 22' N, 122[degrees] 02' W) from 2003 to 2007. Both sites consist of pasture land and mixed oak woodland (mainly Quercus douglasii and Q. lobata) with elevations ranging from 150 to 915 m at Hopland and 213 to 609 m at Gates Canyon. We distributed >100 top- or front-opening wood nest boxes affixed to wood or metal fence posts, metal poles, or trees at both sites. Nest-box entry holes were 3.8 cm in diameter, and entrance heights ranged from ~1.2 to 2.0 m above ground. Individual boxes were 1 to 10 years old when the study began.

Egg Temperature.--We recorded egg temperature ([T.sub.egg]) from the egg-laying period to after clutch completion in seven bluebird nests during the 2003 and 2004 breeding seasons. Measurements started with the first (n = 1), second (n = 1), third (n = 4),or fifth (n = 1) egg laid and typically ended after the clutch hatched. One egg for each monitored clutch (usually from an unmonitored clutch) was sacrificed by inserting a 40-gauge copper-constantan thermocouple (Omega Engineering Inc., Stamford, CT, USA) into its center and spot-gluing it in place with 5-min epoxy. The thermocouple leads were threaded through the bottom of the nest cup and connected to either a Campbell Scientific CR21X data logger (Campbell Scientific Inc., Logan, UT, USA) or a Veriteq Spectrum SP-1700 N data logger (Veriteq Instruments Inc., Richmond, BC, Canada). The original clutch size was maintained by removing one egg from each clutch to which an egg with a thermocouple was added.

We measured shade air temperature ([T.sub.a]) beneath nest boxes with 20-gauge Cu-Cn thermocouples connected to data loggers. The CR21X data loggers measured temperatures every 1 or 6 sec and averaged temperatures at 60-sec intervals. The Spectrum SP-1700 N data loggers sampled temperatures at 1-min intervals. Measured temperatures were accurate to 0.3[degrees]C for CR21X data loggers (Campbell Scientific 1984) and to 0.4[degrees]C for Spectrum SP-1700 N data loggers.

Video Observation.--We monitored the behavior of incubating birds during the 2005-2007 breeding seasons by fitting the underside of nest-box lids with a miniature infrared CCD video camera and miniature microphone...

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