Egg laying, egg temperature, attentiveness, and incubation in the Western Bluebird.
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).
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 (SuperCircuits Inc., Austin, TX, USA). Nest boxes used the previous year were fitted with dummy cameras well before the breeding season to minimize disturbance associated with installing video equipment. We exchanged dummy cameras for real cameras after the first egg was laid. Swapping nest-box lids did not seem to affect the birds' behavior as no nests were deserted. We recorded the audio and video signals continuously with a PVR-330 digital video recorder (Bolide Technology Group, San Dimas, CA, USA) after passing the video signal through a SuperCircuits time/date stamp; video records were analyzed using VirtualDub open-source video software.
Deducing Behavior From Egg Temperature.--WWW monitored bluebird incubation behavior simultaneously with nest-box video cameras and egg thermocouples in 2005-2007. Twenty-eight days of simultaneous video, [T.sub.egg], and Ta measurements at four nests demonstrated that attentiveness and incubation could reliably be deduced from changes in [T.sub.egg], provided [T.sub.a] was -2[degrees]C below [T.sub.egg] during incubation bouts. Little or no change in [T.sub.egg] occurred as a consequence of the bird's behavior if shade [T.sub.a] was close to or equal to [T.sub.egg]. A second measure of incubation, the number of minutes [T.sub.egg] exceeded 27[degrees]C, was used to indicate the total time eggs were above developmental zero, regardless of whether the elevated [T.sub.egg] was due to active or passive sources. The threshold of avian embryonic development is widely regarded to be 24-27[degrees]C (Webb 1987). Data Analysis.--We adopted fixed time periods for day and night. The daytime interval, 0400-1959 hrs PST, was 960 min in length and began, depending upon calendar date, an average of 24 min before the start of morning civil twilight (range = 13-45 min). The night-time interval, 2000-0359 hrs PST, was 480 min in length and began on average 5 min before the end of evening civil twilight (range = -26 to 13 min). Our daytime interval averaged 19 min longer than the period between morning and evening civil twilight. We chose this interval so the daytime period would include all oft'- and on-bouts during the active day. Thirty-seven percent of first off-bouts (35 of 95) occurred before sunrise (mean = 24 min, range = 1-67 min); timing of the last on-bout of the day was quite variable (range = 140 min before to 52 min after sunset).
We numbered days and nights relative to the day each clutch was completed to accommodate variation in clutch size. The final egg was laid on day 0, -1 denotes the day before clutch completion, and 1 denotes the day after clutch completion. We distinguished between attentiveness, in which an adult was in the nest box for any reason, and incubation, in which an adult raised [T.sub.egg] above physiological zero, 27[degrees]C.
We accommodated repeated measures with unbalanced sample sizes in our data using mixed models in SAS 9.1.3 PROC MIXED (Littell et al. 2006). All models included the number of days relative to laying completion as a repeated factor within the random effect of nest. Means and standard errors were estimated using intercept-only models. We tested the significance of fixed effects in models with random effects as structured above. Descriptive statistics and comparisons not involving repeated measures used parametric analyses.
Egg Laying.--Females that entered the nest box to lay an egg appeared to incubate eggs already in the nest causing [T.sub.egg] to increase abruptly (Fig. 1). We used the increase in [T.sub.egg] to calculate the duration of laying bouts in nests without video cameras. Females (n = 6) entered the nest box on egg-laying days between 0522 and 1037 hrs PST, laid an egg on average 11.3 [+ or -] 1.3 min later (random-effect model intercept [+ or -] SE: n = 3 females, 11 eggs), and spent on average 35.7 [+ or -] 4.2 min (random-effect model intercept [+ or -] SE: n = 6 females, 18 eggs) in the nest box during egg-laying (Table 1). Laying bouts began 113 [+ or -] 36 min (SE) earlier in the day, relative to sunrise, in 2003-2004 than in 2007 (type 3 test: n = 18, df = 4, F = 10.02, P = 0.034). Laying bouts in 2003-2004 began 98 [+ or -] 28 min after sunrise versus 211 [+ or -] 22 min after sunrise in 2007 (least-squares means [+ or -] SE) (Table 1).
The average time of egg laying was 222 [+ or -] 27 min after sunrise (random-effect model intercept [+ or -] SE: n = 3 nests, 11 eggs). Subsequent eggs within clutches were not laid significantly later than the egg directly preceding them, relative to sunrise (random-effect model intercept [+ or -] SE: 36.0 [+ or -] 20.7 min, df = 2, t = 1.74, P = 0.225; n = 3 nests, 6 eggs). First-brood clutch sizes at the Mendocino County field site averaged 5.1 [+ or -] 0.82 SD (range = 2-7: n = 175) from 2003 to 2007 (JMW, unpubl, data). Clutch sizes in our study sample were four to six eggs.
[T.sub.egg] During High Ambient Temperatures.--We recorded [T.sub.egg] in one nest from the first egg laid until the clutch hatched, 18 days later. [T.sub.egg] increased abruptly at 0649 hrs PST on the second day, when the female entered the nest box to lay, and decreased abruptly when she left the box 15 min later (Fig. 1A). The female did not return to the nest after laying that day, but [T.sub.egg] reached a maximum of 40.2[degrees]C when shade [T.sub.a] beneath the nest box reached 40.1[degrees]C at 1420 hrs PST (Fig. 1A). [T.sub.egg] averaged 31.5[degrees]C and exceeded 27[degrees]C for 709 min on that day. [T.sub.egg] again spiked in the morning on the third and fourth days as the female entered the nest to lay, and decreased when she left the nest (Fig. I B, C). High daytime [T.sub.a] elevated [T.sub.egg] above 27[degrees]C for over 540 min on day 3. All viable eggs in this nest hatched.
Daytime Attentiveness and Incubation.--Mean daytime [T.sub.egg] and the number of minutes that [T.sub.egg] exceeded 27[degrees]C appeared to increase across the egg-laying period for the six nests with multiple days of data (Fig. 2). These patterns were confounded by high [T.sub.a] at nest # 11 and were not significant (n = 12 eggs, 6 nests; type 3 tests: mean [T.sub.egg], df = 5. F = 2.20, P = 0.198; [T.sub.egg] > 27 C. df = 5. F = 2.61, P = 0.167). All six nests reached maximum values of mean [T.sub.egg] and minutes [T.sub.egg] > 27[degrees]C on the day after clutch completion.
The amount of time that [T.sub.egg] exceeded 27[degrees]C (incubation time) was significantly correlated with daytime attentiveness and mean [T.sub.a] in the sample of nests with full-day records, and where [T.sub.a] was not too high to estimate attentiveness using [T.sub.egg] (type 3 tests: n = 12 eggs, 6 nests; df 4, F = 69.37, P = 0.001: and df = 4, F = 66.27, P = 0.001; respectively). Incubation time increased for each additional minute of attentiveness by 0.68 [+ or -] 0.1 min SE (df = 4, t = 8.33, P = 0.001), when controlling for [T.sub.a]. Every degree of increase in mean [T.sub.a] increased incubation time by 42 [+ or -] 5.2 min SE (df = 4, t 8.14, P = 0.001), when controlling for attentiveness. This relationship was found for an ambient mean temperature range of 22.3-32.7[degrees]C.
Night-time Attentiveness and Incubation.--Western Bluebirds commonly began night-time attentiveness during the laying period. Videos of females roosting early in the laying period revealed the eggs were not in contact with the brood patch at night. Females often appeared to be standing above the eggs, and eggs were at times visible beneath them. One of three females videotaped starting with the day of clutch initiation roosted in the nest box the night after laying her first egg, another began roosting with the pre-penultimate egg (third of 5), and another started with the penultimate egg (fifth of 6).
Lower [T.sub.a] at night made [T.sub.egg] a reliable indicator of female attentiveness versus incubation. Five of seven bluebirds roosted in the nest box on the prepenultimate night (i.e., night -2). Lack of intimate physical contact with the eggs resulted in egg temperatures that exceeded Ta but were lower than those typical of steady incubation for three of these nests (Fig. 3A).
Night-time mean [T.sub.egg] and amount of time that [T.sub.egg] exceeded 27[degrees]C (incubation time) significantly increased during the laying period (type 3 tests: n = 22 eggs, 7 nests; df = 14, F = 11.18, P = 0.005; and df = 14, F = 15.24, P = 0.002; respectively), but night-time attentiveness did not (type 3 test: n = 22 eggs, 7 nests; df = 14, F 3.18, P = 0.096). The amount of time that [T.sub.egg] exceeded 27[degrees]C on nights with 100% attentiveness (480 min) ranged from 41 to 480 min and averaged 354 [+ or -] 43 min (random-effect model intercept [+ or -] SE: n = 6 nests, 16 nights).
[FIGURE 1 OMITTED]
Information on both onset of night-time attentiveness and incubation for the five nests from 2003-2004 revealed two bluebirds started attentiveness on the same night as incubation, two began attentiveness at least one night before incubation, and one commenced attentiveness at least two nights before incubation. Four of the birds engaged in steady night-time incubation on the penultimate night and two began incubation one night earlier, as illustrated by an abrupt increase in mean night-time [T.sub.egg] and in number of minutes [T.sub.egg] exceeded 27[degrees]C (Fig. 3). Two birds skipped a night of incubation after commencing steady night-time incubation (Fig. 3).
Regular incubation patterns in passerines commonly develop gradually during the laying period (Haftorn 1978, 1979, 1981; Zerba and Morton 1983; Morton and Pereyra 1985; Anderson 1997). Western Bluebirds had a gradual increase in daytime incubation during egg laying that reached a maximum the day after clutch completion (Fig. 2). Incubation bouts occurring before establishment of a steady incubation rhythm are present in most species that have been continuously monitored during egg laying (Norton 1972, Holcomb 1974, Ashkenazie and Safriel 1979, Afton 1980, Zerba and Morton 1983, Morton and Pereyra 1985, Bortolotti and Wiebe 1993, Poussart et al. 2000). Most Western Bluebirds in our sample fit the "slow rise" pattern of incubation onset characterized by Grenier and Beissinger (1999), but the non-monotonically rising incubation at two nests fits the "irregular" pattern first described by Wiebe et al. (1998) and correlated with poorer body condition. Partial incubation results from proximate factors such as an increase in prolactin, a hormone correlated with incubation's onset (Sockman et al. 2000), and is likely an adaptive behavior serving to maintain the viability of earlier-laid eggs (Arnold et al. 1987). More research is needed to explore whether partial incubation is the result of constmints on adult physiology, an adaptive trait subject to behavioral modification, or a combination of both (Sockman et al. 2006).
Roosting in the nest before the onset of consistent nocturnal incubation may be a common trait in temperate cavity-nesting passerines. Western Bluebird females often roosted in the nest box at night after laying the pre-penultimate egg, resulting in mean [T.sub.egg] rising above [T.sub.a] but remaining below that typical of later incubation (Fig. 3). This behavior is similar to roosting accompanied by a gradual increase in nocturnal incubation during the egg-laying period in Goldcrest (Regulus regulus), Willow Tit (Poecile montana), and Great Tit (Parus major) (Haftorn 1978, 1979, 1981). Great Tits and Blue Tits (Cyanistes caeruleus) roost in the nest cup during the early laying period without raising egg temperature above physiological zero, suggesting that roosting in the nest lessens the adult's thermoregulatory costs (Pendlebury and Bryant 2005). Nocturnal nest attendance before the penultimate egg and before onset of full daytime incubation has been noted in species as varied as Wood Ducks (Aix sponsa), American Tree Sparrows (Spizella arborea), Green-rumped Parrotlets (Forpus passerinus), American Kestrels (Falco sparverius), Baird's Sandpipers (Calidris bairdii), Semipalmated Sandpipers (C. pusilla), and Cedar Waxwings (Bombycilla cedrorum) (Putnam 1949, Weeden 1966, Norton 1972, Bortolotti and Wiebe 1993, Wilson and Verbeek 1995, Grenier and Beissinger 1999).
Western Bluebirds demonstrated an abrupt increase in night-time incubation during the egg-laying period when measured by the amount of time [T.sub.egg] exceeded 27[degrees]C or by mean [T.sub.egg] with two of six females beginning steady nocturnal incubation on the pre-penultimate egg (Fig. 3). The onset of steady night-time incubation before the penultimate egg has been documented in cavity-nesters that hatch asynchronously: Goldcrest (2 of 2 nests), Great Tit (7 of 15 nests), and House Sparrow (Passer domesticus) (3 of 5 nests) (Haftorn 1978, 1979; Anderson 1997). Western Bluebirds are reported to hatch synchronously (Guinan et al. 2000), but the hatching spread at the Hopland study site from 2003 to 2007 was 0.76 [+ or -] 0.73 days (n = 175 first nesting attempts) (JMW, unpubl, data). Other passerine species that vary in onset of full incubation also span the hatching synchrony/asynchrony spectrum with some clutches hatching within 24 hrs and others in over 24 hrs (Haftorn 1979, Nilsson 1993, Clotfelter and Yasukawa 1999). The relationship between onset of partial and steady (full) incubation in passerines and the resulting extent of hatching asynchrony merits more consideration.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
The phenomenon of skipping a night of incubation after onset of steady nocturnal incubation has seldom been reported for this or other passerine species (Wang and Beissinger 2009). Extended periods of neglect after onset of full incubation, and of eggs cooling at night while the adult is present, have been reported for Dusky Flycatcher (Empidonax oberholseri) during inclement weather (Morton and Pereyra 1985). The weather during nights of skipped incubation in our study was not noticeably different from other nights. The successful hatching of most eggs in these nests suggests that Western Bluebird eggs are resistant to episodes of nocturnal egg neglect.
Most passerines are early-morning layers, but Western Bluebirds at our study sites laid well after sunrise. Western Bluebirds laid later in the day than European tits: Great Tit, Marsh Tit (Poecile palustris), Willow Tit, and Coal Tit (Periparusater) laid on average 1-43 min after sunrise (n = 22 nests, 62 eggs; Haftorn 1996). Timing of laying in Western Bluebirds is consistent with passerines of larger body size having later laying times (McMaster et al. 2004). To our knowledge, information on the timing of egg laying in other muscicapids is unavailable.
Maximum daytime ambient temperatures during our study commonly exceeded 30[degrees]C elevating [T.sub.egg] above physiological zero in the absence of female incubation. Elevated [T.sub.a] had a much greater effect on incubation time (defined as the time [T.sub.egg] > 27[degrees]C) than female attentiveness. Warm ambient temperatures may affect hatching success and hatching asynchrony by accelerating development of earlier-laid eggs. The trend for tropical species to hatch asynchronously (e.g., Grenier and Beissinger 1999) may reflect this constmint (Stoleson and Beissinger 1999). The apparent hardiness of eggs in our study to high temperature suggests altricial eggs of intermittently incubating passerines may illustrate adaptation to natural fluctuations in egg temperature. The effects of warm temperatures during the laying period on egg and nestling survival would benefit from systematic investigation, whether incubation is attributable to adults beginning incubation earlier during warm weather or whether incubation is passively derived from high ambient temperatures.
Support was provided to W. W. Weathers and J. M. Wang by the Hopland Research and Extension Center. S. R. Beissinger and the following members of the Beissinger Laboratory provided helpful feedback during manuscript preparation: Sophie Veran, O. M. Richmond, M. W. Tingley, and B. R. Risk. S. P. Kulbi provided assistance with video and data analysis. Research support was provided to W. W. Weathers by the California Agricultural Experiment Station. Eggs were sacrificed and birds videotaped under authority of USDI, Fish and Wildlife Service Permit # MB087256-0, California Department of Fish and Game Scientific Collecting Permits #s SC-007208 and 801026-02, and University of California Animal Use and Care Protocol #s 10863 and 07-12777 issued to W. W. Weathers, U.S. Fish and Wildlife Service permit # MB 101830-0 and IACUC protocol # R233 issued to S. R. Beissinger with J. M. Wang as an authorized subpermittee, and California student scientific collecting permits # 803052-03 and # 803012-03 issued to J. M. Wang.
Received 8 September 2008. Accepted 10 February 2009.
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JENNIFER M. WANG (l) AND WESLEY W. WEATHERS (2,3)
(1) Department of Environmental Science, Policy and Management, University of California, Berkeley, CA 94720, USA.
(2) Department of Animal Science, University of California, Davis, CA 95616, USA.
(3) Corresponding author; e-mail: firstname.lastname@example.org
TABLE 1. Time egg-laying bouts began, their duration, and minutes until eggs were laid. Time since Bout duration Min until Nest # n sunrise (min) (min) egg laid 98 (a) 3 308 (272-349) 53 (21-79) 12 (9-18) 102 (a) 3 212 (179-274) 42 (25-69) 10 (8-11) 67 (a) 5 152 (84-278) 31 (23-45) Il (8-21) 11 (b) 3 123 (212-126) 25 (15-31) 23 (c) 3 69 (36-130) 28 (15-34) 224 (c) 1 108 45 (a) Denotes 2007 nets. (b) 2003 nets. (c) 2004 nests. Values are mean (range), n = number of eggs laid.
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|Author:||Wang, Jennifer M.; Weathers, Wesley W.|
|Publication:||The Wilson Journal of Ornithology|
|Date:||Sep 1, 2009|
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