Laying sequence has important effects on eggshell color and embryonic development in birds. Some birds can allocate resources unevenly among the eggs within a clutch, prioritizing those at the beginning of the laying sequence, in order to maximize reproductive success. The changes in egg color according to laying sequence may be an adaptation to pressure from predators or brood parasites.
In this study, effects of laying sequence on egg color and embryonic heart rate in Russet Sparrows (Passer cinnamomeus) were investigated using artificial nest boxes. The eggs were divided into three groups: first to be laid, intermediate in the laying sequence, and last to be laid. We maintained the eggs in an incubator and measured embryonic heart rates.
Avian visual modeling showed that the background color brightness of the last eggs laid was significantly higher (whiter) than those of the other eggs. All eggs were about the same size and hatched around 13 days, indicating that laying sequence significantly affected embryonic development speed; the last eggs to be laid developed significantly faster than did the first in the clutch.
Our study quantified the effect of laying sequence on egg color variation and proved that laying sequence has an important effect on embryonic heart rate in Russet Sparrows.
Laying sequence in birds strongly affects the color, mass, metabolic rate, and embryonic development rate of the eggs (Ruxton et al. 2001; Moreno and Osorno 2003; Griffith and Gilby 2013). The pigments covering eggshells are deposited in the 4 h preceding egg laying by pigment glands (Burley and Vadehra 1989). During the laying period, the pigments in some birds gradually decrease as the number of eggs laid increases, which results in a shift in eggshell color from dark to light over the course of the laying sequence. This phenomenon, which is relatively common among the Passer genus (Yom-Tov 1980; Ruxton et al. 2001; Poláček et al. 2017), produces a clutch with a diversity of egg colors; the lightest egg is the last one laid (Lowther 1988; Poláček et al. 2017). Previous studies showed that variation in egg color reflected differences in egg quality; the last eggs to be laid with lighter coloration were the poorest quality (Moreno and Osorno 2003; Soler et al. 2005). Parent birds often distribute resources, such as antioxidants and male hormones, unevenly over the course of the laying sequence (Hall et al. 2010; Boncoraglio et al. 2011). Therefore, eggs laid at different points in the sequence receive different resources, which could influence their embryonic development and ultimate fate (Lipar and Ketterson 2000; Nicolai et al. 2004; Groothuis et al. 2005; Granroth-Wilding et al. 2014; Poláček et al. 2017; Cao et al. 2018). Some female use big size and other maternal resources by last laid eggs to compensate for the survival disadvantage experienced (Saino et al. 2010; Newbrey et al. 2015).
Magrath et al. (2009) showed that, given consistent environmental conditions, eggs laid earlier start to develop before the female bird has finished laying the entire clutch, so the first eggs laid hatch before the last eggs do. As a result, the clutch includes nestlings with an extreme imbalance in mass quality (Magrath et al. 2009). However, other research indicated that the eggs of Canada Geese (Branta canadensis) from the same clutch hatched at about the same time, with a maximum time interval of less than 24 h (Boonstra et al. 2010). Therefore, the embryonic development rate differed for eggs within the same clutch.
So far only a small number of studies have focused on the relationship between laying sequence and embryonic internal development rate in birds showing that laying sequence can affect embryonic development (Nicolai et al. 2004; Boonstra et al. 2010; Cao et al. 2018; Yang et al. 2018). Zebra Finch (Taeniopygia Guttata) clutches number can alter the developmental time of embryos, egg maternal resource allocation across the laying sequence can influence development (Griffith and Gilby 2013). In this study, using artificial nest boxes, we compared the effects of laying sequence on eggshell color variation and embryonic development speed of Russet Sparrows (Passer cinnamomeus), a passerine species.
WHAT ARE EGGSHELLS MADE FROM?
Calcium carbonate and certain proteins combine to form the eggshell, giving it strength.
For example, a typical laying hen’s shell forms in about 20 hours.
Certain structures in the oviducts of birds add pigmentation to the outside of the shell.
Since eggs are typically laid blunt-end first, this side typically experiences greater pressure during travel, leading to stronger colors.
Darker eggshells are typically found in birds that lay their eggs in open nests and in colder climates.
The possible drivers of egg colours are varied and contradictory.
For instance, darker pigments tend to absorb more heat than lighter ones, which may indicate that darker eggs are preferred in colder climates.
However, darker eggs provide a stronger defense against UV radiation, which is generally stronger in warmer climates.
Additionally, they have more potent anti-microbial qualities that are more advantageous in warmer climates, which together imply that the reverse may be true.
Similarly, in warmer climates where predators are more common, one might anticipate that the obvious nature of lighter eggs would be a disadvantage.
To settle the matter, biologist Phillip Wisocki of the C. W. Long Island University’s Post Campus and associates measured the color and brightness of 643 different bird species’ eggs that were kept in collections held by different natural history museums.
After mapping these colorations onto each known geographic breeding area for the species, the authors discovered a pattern.
According to the researchers’ findings in their publication, birds that reside in colder climates, especially those whose nests are exposed to incident solar radiation, lay darker eggs.
On the other hand, eggs that were laid in enclosed nests, cavities, or warmer climates generally had lighter colors.
The group then exposed the different colored and brightly colored eggs from chicken, duck, and quail to direct sunlight.
They discovered that darker-pigmented eggs could sustain their incubation temperatures for longer than their lighter-colored counterparts.
Researchers measured the color and brightness of eggs from 643 different bird species that have been preserved and are housed in a variety of natural history museums. The authors then plotted these colorings onto the known geographic breeding areas of each species, as shown in the picture.
The researchers noted in their paper that darker eggs are laid by birds in colder environments, especially those whose nests are exposed to incident solar radiation (stock )
According to the researchers, this data implies that pigmentation of eggs may be crucial for thermoregulation in cold climates.
However, they noted that in warmer climates, a variety of opposing selective pressures further affect eggshell colors.
The full findings of the study were published in the journal Nature Ecology & Evolution.
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Study site and species
At an elevation of roughly 1500 meters, this study was carried out in the subtropical evergreen broadleaf forest of Kuankuoshui National Nature Reserve (28°10ʹN, 107°10ʹE) in southwest China (see Yang et al. 2010 for more details) from April to August 2013.
Russet Sparrows are members of the Passeridae family and are primarily found in the Himalayan region and southern China (Summers-Smith 1988). They live here year-round and construct their nests in chimney tubes, stone wall cavities, and eaves (Yang et al. 2012b; Huo et al. 2018). Since the entrances to sparrows’ natural nests are typically too small and deep for easy access, artificial nest boxes were provided to draw them in (see below).
The nest boxes measured 35 cm in depth and had a 4 cm diameter entrance hole (Fig. 1). We kept an eye on each box until the first Russet Sparrow egg was seen in the nest. After that, we checked the nests once a day to make sure that the incubation period began with the first egg and continued until the clutch was finished. To identify the order in which eggs should be laid, the tip of each newly found egg in the nest was numbered using a light blue marker. Once a full clutch of eggs had been laid, they were all transferred to an incubator together and replaced with model eggs. We took this action because Russet Sparrows appear to not exhibit any egg discrimination, begin their incubation after the clutch is complete, and accept any model eggs in their nests (Huo et al. 2018). The nestlings were raised by their parents in their original nests after they hatched.
The nest box used and a male Russet Sparrow. Photo by Bruce Lyon.
We divided the sparrow eggs into three groups. The first group of eggs to lay was the first egg; the middle group of eggs to lay was the middle group (two or three eggs); and the last group of eggs to lay was the last egg (one egg). We use an average for the middle eggs to analyze the data because the middle group has two or three eggs. A caliper (model number 505-681, Mitutoyo, Japan), accuracy ± 0, range 0-150 mm 02 mm) was utilized to gauge the eggs’ length and width. An electronic balance (EHA501, Guangdong Xiangshan Weighing Apparatus Group Co. , Ltd. ; range 0–100 g; accuracy 0. 01 g) was used to weigh the eggs. Hoyt (1979) provided the formula used to determine the eggs’ volume, which is egg volume = 0. 51 × egg length × square egg width. Each group’s eggs were weighed every day until the nestlings hatched, starting on the first day of incubation. Using a USB4000 VIS–NIR spectrometer (Ocean Optics, Inc.), egg color was measured. , Dunedin, Florida, USA). Each egg had three randomly selected locations where the background and spot colors were measured. A thorough explanation of the procedure is provided in Yang et al. (2009, 2010). For analysis, the reflectance spectrum’s values in the visible (400–700 nm) and ultra-violet (300–400 nm) sections were used. The Mini EX digital incubator (Brinsea Co.) was used to hatch the eggs. Ltd. , UK; also see Yang et al. 2018).
A digital egg monitor MK1 type heart rate meter (Buddy Digital Egg Monitor, Avitronics Inc.) was used to measure the heart rates of embryos. , Cornwall, England). The eggs were placed horizontally in the heart rate meter for the measurement as soon as they were taken out of the incubator to prevent errors in heart rate measurement caused by temperature variations (also see Zhao et al. 2017; Cao et al. 2018; Yang et al. 2018). Once the heart rate stabilized, the heart rate (beats per minute) was measured. Beginning on the first day of incubation, heart rate readings were taken every other day until the nestlings hatched. The period of time between the first heart rate appearance, which happened 2-4 days after the start of incubation, and hatching was called the embryonic development time (Cao et al. 2018).
The incubator hatching temperature was set at 7. 5 ± 0. 5%20%C2%B0C; the relative humidity was %2055%20%C2%B1%205%; the egg turning interval was r/45%20min; and the cooling time was 2%20 hours per day. Utilizing the one-sample Kolmogorov-Smimov (K–S) test, the data’s normality distribution was examined. The means were compared using one-way analysis of variance (ANOVA), provided the data met the requirements for a normal distribution. Welch’s ANOVA was utilized if the data were not normally distributed. Differences at p < 0. 05 were considered statistically significant and at p < 0. 01 were highly statistically significant. All tests were two-tailed, and data were presented as mean ± SD unless otherwise indicated. Statistical analyses were performed in IBM SPSS 20. 0 for Windows (IBM Inc. , USA).
Each Russet Sparrow clutch contained 3–5 eggs (n = 69). Every day, usually in the early morning, one egg was laid. Since pale colors reflect more ambient light than dark colors, whitening in the final eggs was generally visible to the unaided eye in most nests (78 5%, n = 69) (Fig. 2). The final group’s eggs’ background color reflectance was noticeably higher than that of the other two, indicating that whitening had taken place (Fig. 3). The three groups’ spots all had the same spot color reflectance, which was dark brown (Fig. 3). The final eggs had a considerably higher brightness reflectance of the background color than the other groups (F = 10). 817, p = 0. 001). The chromaticity of the background color of the other lights, with the exception of the green light, varied significantly between the groups. The eggs that were last had the highest chromaticity of blue and ultraviolet light (F = 3). 531, p = 0. 041; F = 3. 986, p = 0. 028) and the least chromatic red and yellow light (F = 3). 890, p = 0. 030; F = 3. 655, p = 0. 037). But there was no variation in the background color hue between the three groups (F = 1). 518, p = 0. 234) (Table 1). There was no discernible difference in the brightness, hue, or chromaticity of the spots on the eggs across the three groups (Table 2).
Whitening of the final eggs laid in clutches of Russet Sparrows Three clutches of Russet Sparrow eggs, designated as a, b, and c, are arranged from left to right in accordance with the order in which they lay. Photo by Juan Huo.
Background reflectance of Russet Sparrow eggs. The terms G1, G2, and G3 denote the initial group, the final group, and the final eggs in each clutch, respectively. G1 stands for the first group, the first eggs to be laid. Back means eggshell background color, spot means eggshell spot color.
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