Bird Egg Diversity Q3

ARTICLE 1

What is clutch size and why do we study it?

A clutch is the total eggs a bird lays per each nesting attempt. Some birds have more than one nesting attempt per year.  

Clutch sizes differ not only among major taxonomic groups of birds and among species, but even within an individual. For instance, albatrosses, shearwaters, tropicbirds, and frigatebirds characteristically lay only one egg per clutch. Loons, goatsuckers, most pigeons, and hummingbirds lay two eggs per clutch, and most shorebirds lay four eggs per clutch. With the exception of shorebirds, practically all species that normally lay more than two eggs per clutch show marked variation in clutch size. Many factors appear to influence the number of eggs in a clutch. They include:

  • Age of the female. Within populations, the age of a female bird is related to the size of her clutch.

  • Temperature and time of season. Cold weather may reduce the size of a clutch and clutches laid later in the breeding season may contain fewer eggs than clutches laid by the same individual earlier in the season.

  • Health of the female. If a female is unhealthy, her clutch size will probably be smaller than if she were in peak physical condition. A significant amount of energy is required to produce each egg.

  • Food availability. Abundant food supplies can mean more eggs per clutch.

  • High population density. Females lay fewer eggs per clutch when breeding in colonies or other high population areas.

  • Geographic location. On average, within a species, birds lay smaller clutches when breeding at lower latitudes or at higher altitudes.

The topic of clutch size variation has been a source of fascination for generations of bird biologists. We know there is variation in clutch size both within, and among, species. We want to know more about what factors might influence this variation in clutch size. For example, within a given species, does the number of eggs a female lays per clutch vary with latitude? With altitude? Why do birds that are colonial or nest at relatively high densities often lay fewer eggs than their solitary-nesting relatives? Why do small species tend to have larger clutches than large species? It is thought that birds lay about the number of eggs that will produce the maximum number of surviving offspring over the parents’ reproductive lifetime, but how do they regulate this? What ecological factors determine this number for different birds?

Increasing our understanding of the biological and ecological factors that affect clutch size allows scientists and wildlife managers to make better-informed decisions about bird conservation

ARTICLE 2

Abstract

Simple Summary

This study intended to understand the variation in egg and clutch mass among coexisting bird species at high altitudes. We investigated several life-history or ecological factors that could explain this variation. The results show that both egg and clutch mass were related to body mass across species. Contrary to the hypothesis of a trade-off between egg mass and clutch size, egg mass variation was not explained by clutch size when accounting for allometric effects. Clutch mass was found to be positively associated with parental care and negatively associated with predation rate. When considering clutch size and egg mass together, clutch size was significantly correlated with parental care, predation rate, and lifespan, while egg mass was only associated with development period. These results support the idea that reduced clutch size or mass is linked to a higher risk of predation, reduced parental care, and longer adult lifespan. Our findings suggest that clutch size has a greater impact on these factors compared to egg mass, possibly because smaller clutches represent a more significant reduction in energetic investment. This study has increased our understanding of how different factors influence the size of eggs and clutches in coexisting high-altitude bird species.

Abstract

The variation in egg and clutch mass in sympatric species at high altitudes is poorly understood, and the potential causes of variation are rarely investigated. This study aimed to describe the interspecific variation in avian egg and clutch mass among 22 sympatric bird species at an altitude of 3430 m. Our objective was to reduce potential confounding effects of biotic/abiotic factors and investigated hypotheses concerning allometry, clutch size, parental care, nest predation, and lifespan as possible correlates and explanations for the observed variation. Our findings indicated that both egg and clutch mass evolve with body mass across species. We found that egg mass variation was not explained by clutch size when controlling for allometric effects, which contrasts the “egg mass vs. clutch size trade-off” hypothesis. Additionally, we found that clutch mass was positively associated with parental care (reflected by development period) but negatively associated with predation rate. By substituting egg mass and clutch size into the models, we found that clutch size was significantly correlated with parental care, predation rate, and lifespan, while egg mass was only significantly associated with development period. Overall, these findings support life-history theories suggesting that reduced clutch size or mass is associated with a higher risk of predation, reduced parental care, but longer adult lifespan. Interestingly, our results indicate that clutch size has a greater influence on these factors compared to egg mass. This could be attributed to the fact that smaller clutch sizes result in a more notable decrease in energetic allocation, as they require a reduced effort in terms of offspring production, incubation, and feeding, as opposed to solely reducing egg size. These findings contribute to the growing evidence that life-history and ecological traits correlate with egg and clutch mass variation in sympatric species. However, further research is needed to explore the potential evolutionary causes underlying these patterns.

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Results

Our results reveal that egg mass and clutch mass are strongly correlated with body mass, consistent with previous studies across various taxa [2,3,13]. This relationship remains significant when other life-history and ecological traits were included (Table 2; Figure 1), indicating that egg and clutch mass evolve similarly with body mass and are associated with life-history and ecological correlates. Our findings also show that absolute egg mass decreases with increasing clutch size across coexisting species, as predicted by theoretical models [47] and observed in other studies [12]. However, this relationship disappears when controlling for body mass, suggesting that the trade-off between clutch size and egg mass may be more complex than previously thought [13,48]. Previous studies have proposed that the inverse relationship between number and mass is due to finite resource allocation [7]. However, our findings suggest that egg production represents a minor fraction of the energy investment made by altricial birds, as they primarily allocate a substantial amount of energy towards parental care [4]. Consequently, the lack of a significant relationship between clutch size and egg mass may be attributed to intricate selection pressures influencing these two interconnected life-history traits [2,49].

Furthermore, our results provided evidence for the coevolution of egg and clutch mass alongside other life-history strategies, as demonstrated by the evolution of larger eggs in species characterized by longer developmental periods, as documented in other studies [4,26,50]. Larger eggs and clutches typically require more energy for incubation and development, resulting in longer development times. Additionally, species with larger eggs tend to have slower paces of life, reduced development rates, and longer lifespans [11,50,51]. Therefore, increasing development period may be a general requirement for larger egg and clutch mass within and across altricial bird species.

Our study revealed a significant association between increased nest predation and reduced reproductive investment, as indicated by both clutch size and clutch mass, which aligns with previous predictions [19,22]. However, the impact of nest predation on egg mass was less pronounced (Table 2). It is widely observed that larger clutch size is associated with reduced nest predation [4,52]. In our study area, most bird species only produce a single brood per year [5,29,33,53], and our previous study revealed a clear connection between reduced nest predation risk and large clutch size [26,34]. However, some studies suggest that birds reduce egg mass as a response to heightened nest predation risk [14]. The missing association between nest predation and egg mass raises questions on the significance of nest predation in shaping egg mass variation within coexisting habitat. Testing the direct effects of nest predation favoring reduced egg mass is challenging due to potential conflicting direct and indirect impacts. Nest predation may directly promote reduced investment in the current clutch through decreased egg mass [23], but indirectly favor larger eggs by directly favoring smaller clutch sizes (see Section 4) [22,25] that correlated with larger eggs. Consequently, the indirect effect of nest predation promoting smaller clutches (associated with larger eggs) opposes direct effects promoting smaller eggs, thereby increasing the complexity to detect the effects of nest predation. Further studies involving diverse bird species with varying nest predation rates are required to elucidate the potential contribution of nest predation to the variation in egg mass.

Life-history theory suggests species with longer lifespans or higher adult survival rates are expected to exhibit reduced reproductive effort [4,19,24]. Our results tend to support this prediction in that lifespan was negatively associated with reproductive effort reflected by clutch size, although negative relationship between lifespan and clutch mass was not significant. Additionally, our study did not identify a positive correlation between egg mass and lifespan, which does not to support the notion that species with longer lifespan exhibit a greater investment in individual progeny through the production of larger eggs. This is probably due to the limited sample size of species. Additional studies across more sympatric species are needed to investigate the relationship between lifespan and egg or clutch mass. Maximum longevity is another issue that has been argued to be problematic in this study, as it represents the longest-lived known individual and is therefore highly sensitive to sample size [11,54,55]. Averaged lifespan or life expectancy is a more appropriate measure in future analyses.


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6. Conclusions

This study investigated the variation in egg and clutch mass among coexisting bird species at high altitude, and explored several life-history/ecological factors that may explain this variation. We found that both egg and clutch mass were correlated with body mass across species. Contrary to the hypothesis of a trade-off between egg mass and clutch size, we found that egg mass variation was not influenced by clutch size when accounting for allometric effects. Furthermore, clutch mass was positively associated with parental care and negatively associated with predation rate. We observed that clutch size was significantly correlated with parental care, predation rate, and lifespan, whereas egg mass was only associated with development period. These results support the notion that reduced clutch size or mass is linked to a higher risk of predation, reduced parental care, and longer adult lifespan. Importantly, our findings suggest that clutch size has a greater impact on these factors compared to egg mass, possibly due to the significant reduction in energetic investment associated with smaller clutches. Overall, this study enhances our understanding of the factors influencing the size of eggs and clutches in coexisting high-altitude bird species. Further study is needed to explore the evolutionary causes underlying these patterns.

Q: What is a clutch and how does clutch size vary among birds? What are the pros and cons of large v small clutch sizes?

A clutch is the total number of eggs a bird lays in each nesting attempt. Clutch sizes vary significantly among different bird species, taxonomic groups, and even within individual birds. For example:

  • Albatrosses, shearwaters, tropicbirds, and frigatebirds usually lay one egg per clutch.

  • Loons, goatsuckers, most pigeons, and hummingbirds typically have clutches of two eggs.

  • Most shorebirds tend to lay about four eggs per clutch.

Variations in clutch size can arise from several factors:

  • Age of the female: Older females often produce larger clutches.

  • Temperature and time of season: Cold weather can reduce clutch size, while earlier clutches in the breeding season may be larger.

  • Health of the female: Healthier females tend to have larger clutches due to better energy availability.

  • Food availability: Access to abundant food can allow for larger clutches.

  • Population density: Higher densities often lead to smaller clutch sizes.

  • Geographic location: Birds at lower latitudes or higher altitudes usually have smaller clutches.

The advantages of larger clutches include the potential for more offspring, which can increase the chances of survival for at least some of those offspring. However, larger clutches may also lead to increased competition among the chicks for resources.
Conversely, smaller clutches can reduce competition but may also result in fewer surviving offspring overall. Additionally, smaller clutches can allow for more parental care per offspring, increasing their chances of survival. Overall, the choice of clutch size reflects a balance between reproductive effort and survival strategies in varying ecological contexts.