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Keywords:

  • avian;
  • embryonic development;
  • Leucophaeus pipixcan;
  • life-history trade-offs;
  • parent–offspring conflict;
  • phenotypic plasticity;
  • photoperiodism;
  • seasonality

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

1. When predictable seasonal changes affect offspring fitness, we expect offspring to evolve phenotypes that minimize the costs of seasonal variation in timing of breeding. For species with parental care during embryonic development, offspring receive seasonal cues of the environment from parents that are biased by their parent's fitness (which is not equivalent to offspring fitness). Therefore, mechanisms enabling offspring to detect environmental cues independent of parents should be strongly favoured.

2. We experimentally evaluated the ability of avian embryos to integrate cues of season from photoperiod and maternal environments present in eggs to produce seasonal variation in phenotypes among Franklin's gull (Leucophaeus pipixcan) hatchlings. Eggs were collected early and late in the season and some were separated into their component parts and others were incubated under short (early season) and long (late season) photoperiods. After hatching, we measured the structural size of the chicks and the amount of yolk sac reserves.

3. We found that hatchling size, a phenotype linked with fitness, is sensitive to both egg contents provided by mothers and photoperiod, and development time decreases across the season. The effects of integrating cues of season from eggs and photoperiod on offspring phenotype are complex, and when cues of season from eggs are mismatched with cues of season from photoperiod, alternate phenotypes are created.

4. We also found that seasonal variation in egg size, yolk, albumen or shell content of the eggs do not account for the seasonal maternal egg effect on hatchling size. This seasonal maternal effect could be a result of other egg constituents or reflect heritable variation in timing of breeding that is linked with offspring size.

5. Changes in breeding phenology of adults could result in a mismatch between cues from parents and photoperiod cues of season. For example, if breeding seasons advanced such that late season birds initiate breeding at an early season photoperiod, offspring would then be integrating maternal cues of late season with photoperiod cues of early season and alter their phenotypes. We expect our results to initiate new studies on how vertebrate embryos integrate environmental cues with maternal effects and offspring responses to optimize the expression of offspring phenotype.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Animals exploiting seasonal environments take advantage of regular changes in resources, which are responsible for organizing biological processes and the annual cycle of organisms (Bradshaw & Holzapfel 2007). In temperate regions, seasonal increases in temperature, water and energy provide times for which fitness is maximized by reproduction and rearing young, whereas decreases in these resources result in times when fitness is maximized by migration or quiescent life-history phases. The ability to predict seasonal changes correctly and initiate appropriate behavioural, physiological and biochemical responses that maximize fitness is shaped by natural selection (Bradshaw & Holzapfel 2007; Lyon, Chaine & Winkler 2008). Photoperiod provides a consistent cue of season that animals use to predict optimal timing for annual events (Bradshaw & Holzapfel 2007; Lyon, Chaine & Winkler 2008), and the refinement of timing of breeding within a season is provided by additional biotic and abiotic cues (Wingfield & Kenagy 1991; Visser, Holleman & Caro 2009; Visser et al. 2011). The influence of photoperiod in orchestrating seasonal changes in adult phenotype has a long history of study, but the influence of photoperiod and season on juvenile phenotype has been little explored outside invertebrate systems (Dmitriew 2011).

A rich body of literature documents the influence of season and photoperiod on producing adaptive phenotypes in juvenile and larval invertebrates (Kingsolver & Huey 1998; Bradshaw & Holzapfel 2010; Sniegula & Johansson 2010; Beldade, Mateus & Keller 2011). However, the life histories of many of these invertebrates (i.e. mosquitos, pea aphids and butterflies) do not include parental care. In the absence of parental care, juveniles interact directly with their environments, which minimizes any conflict between offspring and parent because the offspring phenotype is unlikely to affect future reproductive success of parents. In contrast, when parents provide prenatal or natal care, embryos and juveniles experience environments as translated by or through parents and the offspring phenotype can directly affect future reproductive success of parents. For example in mammals, photoperiods experienced by females can be translated to their foetuses through changes in maternal hormones, resulting in long-term effects on offspring life-history traits (Horton 2005) and adult condition (French et al. 2009). Such maternal effects are also present in oviparous animals, although direct exchange of maternal resources with offspring is limited to a short period of time prior to oviposition (Schwabl 1996b; Groothuis & Schwabl 2008). Bird eggs contain maternally derived androgens (Reed & Vleck 2001; Groothuis et al. 2005; Boonstra, Clark & Reed 2009), melatonin (Bozenna et al. 2007), thyroid hormones (Wilson & McNabb 1997), antibodies (Grindstaff, Brodie & Ketterson 2003), carotenoids (Newbrey & Reed 2009), vitamins (Biard et al. 2009) and RNA transcripts (Knepper et al. 1999; Malewska & Olszanska 1999), all of which are biologically active, potentially affect embryonic growth and development and can vary seasonally (Watanabe et al. 2007; Hargitai et al. 2009). Although birds have played a critical role in evaluating both the impacts of maternal provisioning on offspring (Groothuis et al. 2005) and changes in adult phenology in response to changing seasons (Lack 1950; Perrins 1970), the ability of avian embryos to integrate both maternal and photoperiod cues of season has not been explored.

One consequence of seasonal environments is variation in reproductive value of offspring across the season. Offspring produced at the beginning of the season typically have a higher likelihood of survival than offspring produced late in the season (Moreno 1998; Drent 2006; Verhulst & Nilsson 2008). Parents are predicted to favour offspring with higher likelihood of survival, especially when allocation of resources to offspring with low likelihood of survival decreases future reproductive potential of parents [i.e. the reproductive constraint hypothesis (Winkler 1987; Stearns 1992; Verhulst & Nilsson 2008)]. The consequences of seasonal variation in reproduction have been considered mainly from the perspective of consequences for parental fitness and trade-offs between current and future offspring. In seasonal environments, parents must transition from breeding to preparation for migration and winter. Offspring and parental fitness, however, are not equivalent (Wolf & Wade 2001; Müller et al. 2007) and the consequences of season on current offspring fitness are large. Offspring are under similar seasonal time constraints as adults as well as additional constraints; that is, they must grow, develop and moult, as well as learn to forage and fly to be ready to migrate. When the costs of seasonal timing are high for offspring, we expect offspring to evolve mechanisms to detect timing of season, and mechanisms to mitigate negative fitness consequences of seasonal variation in timing of breeding.

Examining the effect of seasonality on offspring fitness is particularly relevant in birds, which have served as a critical model and indicator of shifts in timing of seasonal events (Lyon, Chaine & Winkler 2008). Consistent changes in temperatures across local and global scales are affecting annual cycles of plants and animals (Bradshaw & Holzapfel 2008); however, photoperiod is not labile. The extent to which a mismatch between photoperiodic cues of season and temperature driven changes in seasonality affects biological systems requires knowledge of the mechanisms by which organisms sense and respond to seasonal environments. Avian embryos are capable of sensing and responding to photoperiod as demonstrated in poultry (Siegel et al. 1969b; Shafey 2004a), but has only recently been explored as an adaptive mechanism affecting incubation period in wild populations (Cooper et al. 2011). Current hypotheses of photoperiod effects on avian embryonic development focus on the function of day length in setting the circadian rhythm in the final stages of incubation (Nichelmann, Hochel & Tzschentke 1999; Okabayashi et al. 2003), photo acceleration of development (reviewed in Cooper et al. 2011; and Reed & Clark 2011) and photoperiodic effects on post-hatching growth in artificially incubated poultry (Rozenboim et al. 2003).

Under the reproductive constraint hypothesis, animals that are long-lived, undergo long migrations and provide parental care are expected to invest more in their own survival at a cost to their current offspring when those offspring have a low probability of survival (Winkler 1987; Stearns 1992). Gulls and other seabirds have high adult survivorship, make long migrations, provide significant parental care to their chicks and exhibit seasonal declines in fecundity and reproductive success (Moreno 1998) as expected by the reproductive constraint hypothesis. These characteristics make them particularly good models to evaluate our hypothesis that offspring are able to alter development in response to seasonal cues obtained independent of parents, which has not been well documented in free-living populations.

In a wild population of Franklin's gull (Leucophaeus pipixcan), we observed systematic changes in hatchling size as the breeding season progressed (Fig. 1). In this population, late season hatchlings had shorter tarsi than early season hatchlings (Fig. 1a), but this pattern was not explained by smaller hatchling mass (Fig. 1b). These observations were unique because absolute mass at hatch did not change across the season; yet structural size at hatch (i.e. tarsus length) did change with season. Moreover, hatchling size and hatch date were related to observed differences in growth and survival (Berg 2009). Seasonal variation in structural size at hatch suggests that the way in which offspring grow and develop varies across the season, with consequences for fitness. We hypothesized that the observed seasonal variation in tarsus length at hatch resulted from seasonal effects of either photoperiod or maternal egg composition on embryonic development. We tested our hypotheses by experimentally controlling day length for artificially incubated Franklin's gull eggs collected both during the early and late stages of the nesting season. To understand how maternal investments vary with season, we characterized maternal investments in egg size, yolk, albumen and eggshell across the season. To evaluate whether or not embryos were using egg resources differently across the season, we evaluated the effects of treatments on hatchling composition (carcass mass and residual yolk-sac mass).

image

Figure 1. (a) Tarsus length at hatching (filled circles) declines significantly with hatch day (F1,45 = 17·36, P < 0·001, r2 = 0·28, n = 47; solid line) but (b) mass at hatching (filled circles) does not change with hatch day (F1,45 = 1·67, P = 0·203, r2 = 0·04, n = 47; solid line) in Franklin's gull chicks.

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Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Field Observations

Observational data on tarsus length of newly hatched chicks were collected as part of a previous study on chick survival (Berg 2009). Briefly, Berg (2009) visited nests to determine hatch dates and measure chicks at hatching in a Franklin's gull colony at J. Clark Salyer National Wildlife Refuge (NWR), which is located along the Souris river in north-central North Dakota, USA. Eggs in nests were marked with a unique code, and nests were checked daily for evidence of pipping (the onset of hatching) eggs near the estimated hatch dates. Within 12 h of hatching, hatchling mass (±0·5 g) and right tarsus (±0·1 mm) were measured and the hatchling was returned to the nest. Franklin's gulls hatch asynchronously (Burger & Gochfeld 1994), and the first hatchling in each nest was the only one measured from the nest.

Experimental Egg Incubation

To test our hypotheses about the factors affecting embryonic development, we collected freshly laid eggs between May 7 and June 10, 2009, at two field sites in north-central North Dakota (J. Clark Salyer NWR and Lake Alice NWR). We collected eggs from the beginning (May 7 at J. Clark Salyer and May 27 at Lake Alice) and at end of the nest initiation period (June 3 at J. Clark Salyer and June 10 at Lake Alice) and artificially incubated them in the laboratory. We determined whether eggs were freshly laid by (i) collecting eggs from nests only containing single eggs, (ii) checking eggs for incubation by touch (feeling eggs for warmth) and (iii) by floating eggs (Nol & Blokpoel 1983; Ackerman & Eagles-Smith 2010). We collected the first-laid egg in nests on a single day within the first week nesting was observed (i.e. early season) and on a single day approximately 3 weeks later, at which point approximately 80% of nests had been initiated (i.e. late season) in the 2009 breeding season at J. Clark Salyer NWR and Lake Alice NWR. We measured length (±0·1 mm) and breadth (±0·1 mm) and brought the eggs to our laboratory within 8 h of collection for either artificial incubation under experimental day length treatments, or dissection to determine egg composition. We randomly assigned each egg to a short (14 : 10 light : dark cycle) or long (18 : 6 light : dark cycle) day length treatment during incubation, or to a group that would be dissected for determination of egg composition (yolk, albumen and eggshell). Eggs in the incubation treatments were assigned to one of four incubators maintained at 37·5 °C and 65% relative humidity. Incubators had large, insulated plexi-glass windows and automatic egg turners (Hova-Bator 1586 Picture Window incubator).

Incubators were placed inside environmental chambers that maintained a constant temperature (24 °C), relative humidity (40%) and differed only by the assigned photoperiod. Both environmental chambers were equipped with full-spectrum light bulbs set to timers. One chamber maintained a short day length (14 : 10 light : dark cycle) that mimicked an early season photoperiod, and the other chamber maintained a long day length(18 : 6 light : dark cycle) that mimicked a late season photoperiod typical for latitudes where Franklin's gulls nest. In each chamber, a bank of four full-spectrum bulbs was placed approximately 25 cm above each incubator. Incubators were thus subjected to a photoperiod/chamber treatment; however, we refer to this as a photoperiod treatment henceforth. After 21 days of incubation, we removed eggs from the automatic turners, checked the eggs daily for evidence of hatching, collected freshly hatched (i.e. within 6 h of hatching) chicks and recorded time to hatching (in days). Time to hatching was measured as the number of days elapsed between the day eggs were placed in incubators to the day that they hatched. We measured mass (±0·01 g) and right tarsus (±0·1 mm) of hatchlings, and dissected a subset to determine mass (±0·01 g) of the residual yolk sac and yolk-free carcass after drying (at 40 °C) to a constant mass.

Egg Composition

Eggs assigned to dissection were weighed, gross components (i.e. shell, albumen and yolk) separated and weighed, then dried and reweighed. We measured mass (±0·001 g) of the whole egg on a digital balance, and carefully opened each egg with scissors to separate shell, albumen and yolk. We then dried (40 °C) the components to a constant (dry) mass (±0·001 g).

Statistical Analyses

We used nested general linear models to analyse hatchling characteristics and egg composition statistically. For tarsus size, hatchling mass, hatchling composition and incubation time, we included terms for season (early vs. late), photoperiod (14 h vs. 18 h day length), egg mass (estimated fresh egg mass based on 0·0011·length0·78 · breadth2·04 from Berg (2009)), collection location (J. Clark Salyer vs. Lake Alice) as a random effect, incubator nested within photoperiod treatment and an interaction between season and photoperiod. A post-hoc student t-test was used to compare least-square means among season × photoperiod groups to compare phenotypes created under matched cues of season to phenotypes created from mismatched cues. For models of egg composition, we included terms for season, collection location (henceforth location) and an interaction between season and location. For all models, Shapiro–Wilk tests indicated that residuals were normally distributed. Significance was based on α = 0·05.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

We collected 120 eggs and assigned them to incubators (30 per incubator, with 15 from early season nests and 15 from late season nests). A total of 79 hatched (21 from the early season short day length group, 21 from the early season long day length group, 24 from the late season short day length group and 13 from the late season, long day length group) and we had estimates of egg mass for 61 of these eggs (21 from the early season, short day length group, 13 from the early season long day length group, 15 from the late season short day length group and 12 from the late season, long day length group). Hatchling mass measurements were available for 59 of the 61 hatchlings for which egg mass estimates were available because hatching mass measurements for two of the 12 hatchlings from the late season long day length group were missing.

Hatchling Size

The full model for tarsus length at hatching explained a significant amount of variation (n = 61, F7,53 = 5·96, P < 0·001, r2 = 0·44), with significant effects as a result of season (i.e. smaller tarsi in hatchlings from late season eggs; F1,53 = 4·44, P = 0·040, r2 = 0·05), photoperiod (i.e. smaller tarsi in hatchlings from eggs incubated with longer day lengths; F1,53 = 11·29, P = 0·002, r2 = 0·12), incubator nested within photoperiod treatment (F2,53 = 5·16, P = 0·009, r2 = 0·11) and egg mass (i.e. longer tarsi in hatchlings from larger eggs; F1,53 = 7·32, P = 0·029, r2 = 0·08) (Fig. 2), but no significant interaction between season and photoperiod (F1,53 = 1·30, P = 0·260, r2 = 0·01) or location (F1,53 = 1·45, P = 0·234, r2 = 0·02) (Table 1). When we included all 79 hatchlings (and therefore removed the egg mass term), the model explained a significant amount of variation in tarsus length (F6,72 = 4·14, P = 0·001, r2 = 0·26), and still had significant effects of season (i.e. smaller tarsi in hatchlings from late season eggs; F1,72 = 8·59, P = 0·005, r2 = 0·09), photoperiod (i.e. smaller tarsi in hatchlings from eggs incubated with longer day lengths; F1,72 = 12·24, P = 0·001, r2 = 0·13) and incubator nested within photoperiod treatment (F2,72 = 3·14, P = 0·049, r2 = 0·06) (Table 1). There were no differences in tarsus length between early season eggs receiving short or long day length photoperiod cues (t = 1·69, P = 0·097), but late season eggs receiving short day length photoperiods had longer tarsi at hatching than late season eggs receiving long day length photoperiod (t = 3·11, P = 0·003) (Fig. 2). In contrast to tarsus length, hatchling mass (full model F7,51 = 10·51, P < 0·001, r2 = 0·59) was affected by egg mass (i.e. larger hatchlings hatched from larger eggs; F1,51 = 42·63, P < 0·001, r2 = 0·34) and location (F1,52 = 11·24, P = 0·002, r2 = 0·09), but not by season (F1,51 = 0·06, P = 0·816, r2 < 0·01), photoperiod (F1,51 = 3·74, P = 0·059, r2 = 0·03), the interaction between photoperiod and season (F1,51 = 0·06, P = 0·813, r2 < 0·01) or incubator nested within photoperiod treatment (F2,51 = 2·43, P = 0·098, r2 = 0·04) (Table 1).

image

Figure 2. Tarsus length at hatching (least square means ±95% confidence interval) from eggs laid early and late in the season that were artificially incubated under experimental photoperiod treatments (14 : 10 vs. 18 : 6 light : dark 24-h cycle) varied significantly (F7,53 = 5·96, P < 0·001, r2 = 0·44), with effects as a result of season (F1,53 = 4·44, P = 0·040, r2 = 0·05), photoperiod (F1,53 = 11·29, P = 0·002, r2 = 0·12), incubator (F2,53 = 5·16, P = 0·009, r2 = 0·11) and egg mass (F1,53 = 7·32, P = 0·029, r2 = 0·08). Differences among treatments (a, b) and sample sizes (n) are indicated above each confidence interval.

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Table 1. General linear modelling results for hatchling size, composition and time to hatch. Sample size (n), F statistic value (F), P value (P) and coefficient of determination (R2) for the full general linear models (with × indicating interaction and [] indicating a nested effect) as well as respective individual model terms (shown in italics). Power (Power) is reported only for individual categorical variables. The number of levels for each categorical variable is also listed in the sample size column
ModelnFPR2Power
Tarsus = Season + Photo + Season × Photo + Inc[Photo] + Loc + Emass615·96<0·0010·44 
Season24·440·0400·050·54
Photo211·290·0020·120·91
Season × Photo41·300·2600·010·20
Inc [Photo]45·160·0090·110·81
Loc21·450·2340·020·22
Emass 7·320·0290·08 
Tarsus = Season + Photo + Season × Photo + Inc[Photo] + Loc794·140·0010·26 
Season28·590·0050·090·82
Photo212·240·0010·130·93
Season × Photo40·680·4110·010·13
Inc [Photo]43·140·0490·060·59
Loc20·560·4560·010·11
Chick mass = Season + Photo + Season × Photo + Inc[Photo] + Loc + Emass5910·51<0·0010·59 
Season20·060·8160·000·06
Photo23·740·0590·030·48
Season × Photo40·060·8130·000·06
Inc[Photo]42·430·0980·040·47
Loc211·240·0020·090·91
Emass 42·63<0·0010·34 
Residual YS = Season + Photo + Season × Photo + Inc[Photo] + Loc + Emass245·000·0040·69 
Season213·320·0020·260·93
Photo219·190·0010·380·98
Season × Photo46·530·0210·130·67
Inc[Photo]43·830·0440·150·61
Loc2<0·010·9550·000·05
Emass <0·010·9950·00 
Residual YS = Season + Photo + Season × Photo + Inc[Photo] + Loc402·120·0770·28 
Season24·450·0430·100·54
Photo25·150·0300·110·60
Season × Photo40·760·3880·020·14
Inc[Photo]40·030·9670·000·05
Loc2<0·010·9620·000·05
Yolk free carcass = Season + Photo + Season × Photo + Inc[Photo] + Loc + Emass232·700·0510·56 
Season22·940·1070·090·36
Photo23·050·1010·090·37
Season × Photo40·460·5060·010·10
Inc[Photo]41·160·3390·070·22
Loc21·000·3320·030·16
Emass 6·490·0220·19 
Time to hatch = Season + Photo + Season × Photo + Inc[Photo] + Loc + Emass422·740·0230·36 
Season24·840·0350·090·57
Photo23·590·0670·070·45
Season × Photo40·620·4380·010·12
Inc[Photo]41·480·2430·060·29
Loc22·190·1480·040·30
Emass 1·130·2950·02 
Time to hatch = Season + Photo + Season × Photo + Inc[Photo] + Loc492·450·0400·26 
Season26·880·0120·120·73
Photo20·420·5200·010·10
Season × Photo43·280·0770·060·42
Inc[Photo]40·530·5920·020·13
Loc21·190·2810·020·19

Hatchling Composition

We determined body composition (i.e. dry residual yolk sac mass and dry yolk-free carcass mass) for a subset of 40 hatchlings (nine from the early season, short day length group, nine from the early season, long day length group, 15 from the late season, short day length group and seven from the late season, long day length group), and 24 had estimates of egg mass (nine from the early season, short day length group, two from the early season, long day length group, seven from the late season, short day length group and six from the late season, long day length group). Yolk-free carcass mass was not available for one of the nine early season, short day length hatchlings that had estimates of egg mass. The full model explained a significant amount of the variation in dry yolk sac mass (F7,16 = 5·00, P = 0·004, r2 = 0·69; Fig. 3) with a significant negative effect of season (i.e. smaller residual yolk sac in hatchlings from late season eggs; F1,16 = 13·32, P = 0·002, r2 = 0·26), positive effect of photoperiod (i.e. greater residual yolk sac in hatchlings from eggs incubated under longer day lengths; F1,16 = 19·19, P = 0·001, r2 = 0·38), an interaction between season and photoperiod (F1,16 = 6·53, P = 0·021, r2 = 0·13), and an effect of incubator nested within photoperiod treatment (F2,16 = 3·83, P = 0·044, r2 = 0·15) (Table 1). Effects of location (F1,16 < 0·01, P = 0·955, r2 < 0·01) and egg mass(F1,16 < 0·01, P = 0·995, r2 < 0·01) were not significant (Table 1). When all dry yolk sac masses were included (and thus the egg mass term removed), the full model did not explain a significant amount of variation (F6,33 = 2·12, P = 0·077, r2 = 0·28) (Table 1), but there were still significant negative effects of season (F1,33 = 4·45, P = 0·043, r2 = 0·10) and positive effects of photoperiod (F1,33 = 5·15, P = 0·030, r2 = 0·11). The full model for yolk-free dry carcass mass at hatching did not explain a significant amount of variation (F7,15 = 2·70, P = 0·051, r2 = 0·56), but was positively affected by egg mass (i.e. greater yolk-free carcass mass of hatchlings from larger eggs; F1,15 = 6·49, P = 0·022, r2 = 0·19) (Table 1) and power was limited (Table 1). Early season eggs receiving a long day length photoperiod cue hatched with more residual yolk sac than early season eggs receiving short day length photoperiod cues (t = 4·14, P = 0·001) (Fig. 3). In contrast, there were no differences in yolk sac reserves between late season eggs receiving short or long day length photoperiods (t = 1·71, P = 0·108).

image

Figure 3. Residual yolk sac mass at hatching (least square means ±95% confidence interval) from eggs laid early and late in the season that were artificially incubated under experimental photoperiod treatments (14 : 10 vs. 18 : 6 light : dark 24-h cycle) varied significantly (F7,16 = 5·00, P = 0·004, r2 = 0·69) with effects of season (F1,16 = 13·32, P = 0·002, r2 = 0·26), photoperiod (F1,16 = 19·19, P = 0·001, r2 = 0·38) and incubator (F1,16 = 3·83, P = 0·044, r2 = 0·15). Differences among treatments (a, b) and sample sizes (n) are indicated above each confidence interval.

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Egg Composition

We determined gross egg components (i.e. egg mass, dry yolk mass, dry albumen mass and dry eggshell mass) for 45 freshly laid Franklin's gull eggs (18 early season and six late season eggs from J. Clark Salyer NWR, and 14 early season and seven late season eggs from Lake Alice NWR). There were no differences in egg mass (F3,41 = 1·27, P = 0·300, r2 = 0·09), dry yolk mass (F3,41 = 1·83, P = 0·156, r2 = 0·12), dry albumen mass (F3,41 = 1·10, P = 0·359, r2 = 0·07) or dry eggshell mass (F3,41 = 1·19, P = 0·324, r2 = 0·08) associated with season or location (Table 2).

Table 2. General linear modelling results for egg composition. Sample size (n), F statistic value (F), P value (P) and coefficient of determination (R2) for the full general linear models (with × indicating interaction) as well as respective individual model terms (shown in italics). Power (Power) is reported only for individual categorical variables. The number of levels for each categorical variable is also listed in the sample size column
ModelnFPR2Power
Egg mass = Season + Loc + Season × Loc451·270·3000·09 
Season23·050·0880·070·40
Loc20·430·5140·010·10
Season × Loc40·360·5500·010·09
Yolk mass = Season + Loc + Season × Loc451·830·1560·12 
Season20·180·675<0·010·07
Loc20·900·3480·020·15
Season × Loc42·200·1450·050·31
Albumen mass = Season + Loc + Season × Loc451·100·3590·07 
Season23·150·0830·070·41
Loc20·040·852<0·010·05
Season × Loc4<0·010·959<0·010·06
Shell mass = Season + Loc + Season × Loc451·190·3240·08 
Season23·270·0780·070·42
Loc20·130·725<0·010·06
Season × Loc40·140·709<0·010·07

Incubation Period

We were able to determine time (to the nearest day) to hatching for 49 of the hatchlings (16 from the early season short day length group, 17 from the early season long day length group, nine from the late season short day length group and seven from the late season long day length group). Egg mass estimates were available for 42 of these hatchlings (16 from the early season short day length group, 13 from the early season long day length group, six from the late season short day length group and seven from the late season long day length group). Only negative season effects (i.e. shorter time to hatching for late season eggs; F1,34 = 4·84, P = 0·035, r2 = 0·09) were evident in the model for time to hatching (F7,34 = 2·74, P = 0·023, r2 = 0·36) (Table 1). Negative season effects (F1,42 = 6·88, P = 0·012, r2 = 0·12) were still evident in a reduced model (F6,42 = 2·45, P = 0·040, r2 = 0·26; Fig. 4) without the egg mass term, but no other effects were detected (Table 1). Early season eggs receiving a long day length photoperiod cue shortened time to hatch relative to early season eggs receiving short day length photoperiod (t = −2·40, P = 0·022), whereas late season eggs receiving short or long day length photoperiods hatched in a similar amount of time (t = 0·72, P = 0·474) (Fig. 4).

image

Figure 4. Time to hatch (least square means ±95% confidence interval) from eggs laid early and late in the season that were artificially incubated under experimental photoperiod treatments (14 : 10 vs. 18 : 6 light : dark 24-h cycle) varied significantly (F7,34 = 2·74, P = 0·023, r2 = 0·36) with effects of season (F1,34 = 4·84, P = 0·035, r2 = 0·09). Differences among treatments (a, b) and sample sizes (n) are indicated above each confidence interval.

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

We identified effects of both season and photoperiod on offspring phenotype. Late season eggs produced hatchlings with shorter tarsi than early season eggs (Fig. 2). Similarly, long day lengths during incubation resulted in shorter tarsus length at hatching relative to hatchlings exposed to short day lengths during incubation (Fig. 2). The main effects of season and photoperiod on tarsus length mimic the seasonal pattern identified in the field (Fig. 1). The effects of season and photoperiod on yolk sac reserves, which are a metric of the amount of energy and resources used during development, are complex. Early season eggs produce hatchlings with more yolk sack reserves remaining after development than hatchlings from late season eggs, but hatchlings from eggs exposed to short day lengths have less yolk sac reserves than hatchlings from eggs exposed to long day lengths during incubation. These differences may be a result of effects of increased photoperiod on decreasing incubation period (Cooper et al. 2011). Collectively, our results indicate that avian embryos are integrating maternal investments and photoperiod cues of season to adjust their development and that the phenotypes that arise result from complex interactions between egg environments and photoperiod.

The effect of mothers (early vs. late season eggs) on tarsus length and the amount of yolk sac reserves suggests that egg components vary across the season and affect offspring development (Figs 2 and 3). Female investment in egg mass and yolk, albumen and eggshell did not change seasonally and, thus, are unlikely the mechanism responsible for the maternal effect identified in our experiment. This suggests that maternal investment in egg constituents (e.g. hormones, carotenoids and antibodies) resulting in epigenetic regulation of gene expression(Natt et al. 2009) provide more likely mechanisms responsible for the maternal effect observed in the experiment. Alternatively, the seasonal egg effects could reflect heritable variation for growth, if growth is related to heritable variation in timing of reproduction like that found in great tits (Parus major) (Visser et al. 2011). This particular hypothesis warrants further evaluation and careful study to separate genetic effects from maternal effects derived from seasonal variation in phenotype (Kruuk, Merila & Sheldon 2001). Chick growth and survival decline with hatch date in this population (Berg 2009), thus a seasonal change in the quality of maternal investments is consistent with the reproductive constraint hypothesis and trade-offs between current and future offspring.

The deposition of steroid hormones and carotenoids in avian eggs can vary across the breeding season (Rubolini et al. 2006; Saino et al. 2008; Hargitai et al. 2009; Schoech et al. 2009), which provides a potential mechanism for seasonal maternal effects. For example testosterone levels in egg yolks are reported to decline with season in several species of birds (Schwabl 1996a; Pilz et al. 2003; Gil et al. 2006; Tobler, Granbom & Sandell 2007), although see Müller et al. (2004) and Tomita et al. (2011). Moreover, female canaries (Serinus canaria) exposed to long day lengths produce eggs with lower levels of yolk testosterone than females exposed to short day lengths (Schwabl 1996a). Yolk androgens affect growth in bird embryos (Schwabl 1996b; Lipar, Ketterson & Nolan 1999; Lipar & Ketterson 2000), which is one approach for females to manipulate embryonic phenotype across the season. Likewise, carotenoids and antioxidant activity also have been shown to decline across the breeding season (Rubolini et al. 2006; Saino et al. 2008). Carotenoids are biologically active compounds that can mitigate oxidative stress and enhance immune function in developing embryos (Saino et al. 2003; Rubolini et al. 2006). Changing patterns of maternal investments in yolk have been interpreted as a decline in female quality across the season, with changes in investments as adaptive strategies for females to allocate resources between current and future offspring, or the changes are interpreted as a response to deteriorating environmental quality across the season (Nilsson 1999).

In addition to responding to investments made at the time eggs are produced (seasonal egg effect), Franklin's gull embryos are responding directly to cues about the time of season through photoperiod. Embryos exposed to long day lengths hatch with shorter tarsi than embryos exposed to short day lengths, and the effect on tarsus seems to be driven by late season eggs exposed to long photoperiods (Fig. 2). These responses by embryos may be adaptive in producing phenotypes that are best matched to early vs. late season growth and survival, which needs to be evaluated in field experiments. When parents vary care for offspring across the season based on the value of current offspring compared to future offspring (Winkler 1987), current offspring receive a signal of season biased by parental fitness. Because of this high level of control, parents can influence offspring phenotypes in a way that benefits their own fitness. However, offspring fitness is not equivalent to parental fitness (Wolf & Wade 2001; Müller et al. 2007). Thus, when the cost of seasonality is largely incurred by the offspring, we expect offspring to respond to cues of season independent of cues provided by parents, and selection to favour offspring phenotypes that mitigate these costs (Reed & Clark 2011). Therefore, cues of season that are derived independent of parental investment provide offspring with an unbiased signal by which they can adjust their phenotype. Our data indicate that avian embryos do have the ability to sense external environments and adjust their development independent of maternal effects in eggs.

Patterns of embryonic growth are altered when there is a mismatch between maternal cues of season and photoperiod cues of season. For example short day lengths appear to mitigate seasonal egg effects; when late season eggs are exposed to short days during development, tarsi lengths are similar to that of hatchlings from early season eggs (Fig. 2). In this case, embryos in late season eggs appear to be sensing photoperiod directly and altering their phenotype. The main effects of season and photoperiod on yolk sac reserves are driven by the early season eggs exposed to mismatched day lengths having more yolk sac reserves than hatchlings in all other treatments (Fig. 3). The amount of yolk in eggs prior to incubation does not change seasonally, suggesting that the amount of energy used during development is reduced for early season eggs exposed to long day lengths. This is likely because of decreased amount of time to hatch for the early season hatchlings receiving mismatched cues of day length relative to the early season eggs receiving matched cues of day length (Fig. 4). Cooper et al. (2011) also detected a direct effect of increased photoperiod on shortening development time in house sparrows (Passer domesticus), but could not test for seasonal egg effects. The pattern we found among early season eggs (Fig. 4) was consistent with that of Cooper et al. (2011). In contrast, the time to hatch in late season eggs does not appear to be sensitive to photoperiod (i.e. there was no change in time to hatch in late season eggs exposed to short days). We hypothesize that the seasonal egg effect driving time to hatch in late season eggs overrides any effect of photoperiod, but the same is not true for early season eggs. Our data do indicate that maternal cues (egg constituents) and photoperiod affect the efficiency and rate of avian embryonic development and that the embryo itself is integrating these cues in complex ways to produce phenotypic characteristics related to fitness (Kruuk, Merila & Sheldon 2001).

Photoperiods provide consistent cues of seasonal variation in environments, but they also vary consistently across latitude. Cooper et al. (2011) interpret changes in developmental time as an adaptive response to shorter breeding seasons in northern latitudes and partially responsible for shorter incubation periods that occur at higher latitudes in many species (Martin et al. 2007; Robinson et al. 2008). Shorter development time late in the season (or at higher latitudes) could be beneficial for late hatching offspring that are more time constrained than offspring hatching earlier in the season (or at lower latitudes). For example common murre (Uria aalge) chicks hatching from late season eggs have the capacity to grow faster and fledge sooner than chicks from early season eggs (Benowitz-Fredericks & Kitaysky 2005). The photoperiod effect found in house sparrow embryos (Cooper et al. 2011) and our findings in Franklin's gull hatchlings suggest that a single mechanism may be responsible for plasticity in offspring phenotypes that occur across seasons as well as latitudes. Seasonal variation in offspring phenotype often reflects phenotypic plasticity [e.g. older, more experienced adults nesting early and producing higher quality chicks than later nesting, inexperienced adults (Daunt et al. 2007; Nisbet & Dann 2009)]. Variation in body size among populations that vary across latitudes has been interpreted as evolutionary adaptation; however, recent studies suggest that phenotypic plasticity is a mechanism contributing to latitudinal variation in phenotypes (Gienapp et al. 2008; Husby, Hille & Visser 2011). The influence of photoperiod on offspring development and growth provides a mechanism by which phenotypic plasticity might be generated along both seasonal and latitudinal gradients, especially for species breeding across a wide range of latitudes (such as Franklin's gull).

In order for embryos to respond to a cue of season, they must possess the appropriate sensory capacity (Reed & Clark 2011). In birds, the central sensory system that detects and organizes the response to day length involves cells in the retina, pineal gland and the suprachiasmic nuclei in the hypothalamus (Dawson et al. 2001). The eye is one of the first sensory organs to develop (Hamburger & Hamilton 1951), and in birds, the retina can produce melatonin, a hormone produced in the absence of light that affects growth and immune function (Dawson et al. 2001; Paulose et al. 2009). In the case of Franklin's gull and other free-living populations (Cooper et al. 2011), we do not know the specific mechanism responsible for embryonic responses to photoperiod. However, consistent with our experimental evidence in the gull, agricultural researchers have shown that poultry embryos also respond to the length of light exposure during incubation (Siegel et al. 1969a; Shafey 2004b) and have identified that chicken embryos produce melatonin early in development (Paulose et al. 2009). Furthermore, when chicken embryos are exposed to increasing levels of light during incubation, they increase rates of feeding and growth after hatching (Archer, Shivaprasad & Mench 2009). Our hypothesis for the adaptive significance for an embryo in the wild that hatches late in the season, when day lengths are longer, is a change in growth, which allows them to leave the nest at an earlier age and prepare for migration in a shorter period of time.

The findings from this study enhance our understanding of the ecological and evolutionary context of shifts in seasonal timing of breeding events. Temperature affects nest initiation in birds (Visser, Holleman & Caro 2009), and temperate species are observed to initiate breeding earlier in conjunction with global changes in climate (Winkel & Hudde 1997; Dunn 2004; Potti 2009). Research on seasonality in avian reproduction must consider the direct impact of photoperiod on embryonic development and not only the effects of date mediated through parental care or provisioning. Our results, in conjunction with those of Cooper et al. (2011), suggest that differences in photoperiod resulting from earlier nesting or from shifts in habitat across latitudes can alter avian embryonic development and size at hatching, which is an unexplored consequence of climate change. Little is known about the impacts of maternal effects under climate change. However, our knowledge of the fitness consequences of maternal effects present after hatching suggests the impacts to be maladaptive (Coppack, Pulido & Berthold 2001; Visser 2008). If the timing of nesting shifts outside of the range experienced in the species' evolutionary history in response to environmental changes (e.g. warming spring temperatures), the interaction between maternal and photoperiodic effects on offspring development may compromise resilience to change.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

We thank B. Clark, J. Froehlich, D, Larsen, P. Martin, J. Peterson, S. Weissenfluh and E. Berg for assistance in the field and laboratory. Early drafts of the manuscript were generously reviewed by B. Lyon, J. Bowsher and the Gillam, Clark, Reed Lab group. Three anonymous reviewers and the Associate Editor provided helpful comments for revising an earlier draft of the manuscript. The U.S. Fish & Wildlife Service provided access to the site and logistical support, and we especially thank C, Dixon, G. Erickson, S. Fellows, T. Grant, T. Gutzke and B. Vose for their cooperation. Funding was provided by the National Science Foundation (IOS-0445848 to W.L.R. and M.E.C.), North Dakota Game & Fish Department (to M.E.C. and W.L.R.) and the U.S. Fish & Wildlife Service (M.E.C.). Fieldwork was conducted under permits from North Dakota Game & Fish Department, U.S. Fish & Wildlife Service and North Dakota State University Institutional Animal Care and Use Committee.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Supporting Information

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information
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