Data from: Hatching failure in an aerial insectivore, the tree swallow (Tachycineta bicolor), is driven by embryonic mortality, not infertility
Data files
Jun 23, 2026 version files 81.02 KB
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Data_All_eggs.csv
59.25 KB
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Data_Embryo_staging.csv
6.46 KB
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Data_Fertility_results.csv
11.59 KB
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README.md
3.72 KB
Abstract
Egg development is a crucial stage in the life cycle of birds, and across wild species, up to 17% of eggs fail to hatch under normal conditions. Hatching failure is due to either infertility or embryonic mortality; however, methods to distinguish between these two causes have been underused, partly due to an assumption that eggs had to be refrigerated and tested relatively quickly. In 2022 and 2023, we monitored 1090 eggs laid by tree swallows (Tachycineta bicolor) from 191 nests at three study sites in British Columbia, Canada. Using fluorescence microscopy to identify embryonic and sperm nuclei, we conservatively estimated that 98.3% – 100% of eggs were fertile, and most cases of hatching failure were therefore due to embryonic mortality, not infertility. Embryonic mortality was most likely to occur during early incubation, before development is usually macroscopically visible. We also used fluorescence microscopy to examine 114 eggs laid by tree swallows kept in long-term frozen storage and found that not only could fertility be assessed, but the fertility rate of these eggs was similarly close to 100%. To our knowledge, this study is the first to quantify fertility using embryonic nuclei in tree swallows, and provides the basis for further investigation into factors contributing to hatching failure in this species. Our success using frozen eggs for fertility testing is especially relevant for conservation groups and captive breeding facilities that may already have unhatched eggs stored in freezers, or are limited by capacity or funding and need to place eggs in long-term storage until resources are available for testing.
Dataset DOI: 10.5061/dryad.w3r22817c
Description of the data and file structure
The methods described in the manuscript correspond to these datasets.
Files and variables
File: Data_All_eggs.csv
Description: A record of the fate of all eggs in the 2022-2023 dataset. All blank cells are non-applicable.
Variables
- Year: The year the egg was laid.
- Site: The site at which the egg was laid (agricultural, wetland, or mixed).
- Late_Nest: Was the egg laid exceptionally late in the season? (yes/no).
- Egg_Code: The unique identifier for the egg.
- Treatment: If the egg was included in a separate experiment unrelated to this study, it is entered as "control" or "manipulated". If the egg was not included in another experiment, it is entered as "none".
- Fate: The ultimate fate of the egg.
- Test_Type: Eggs that did not hatch and that were available for testing are entered with "fertility" for fertility testing or "staging" for embryonic staging. Eggs that went straight to staging had macroscopically visible embryos and therefore did not require fertility testing.
- Fertility: The fertilization status of the egg.
- Embryo_Stage: The general timeframe (early stages, middle stages, or late stages) that the embryo died, as described in the Methods section.
File: Data_Embryo_staging.csv
Description: Embryo staging results for the eggs from the 2022-2023 dataset. All blank cells are non-applicable.
Variables
- Site: The site at which the egg was laid (agricultural, wetland, or mixed).
- Year: The year the egg was laid.
- Egg Code: The unique identifier for the egg.
- Dissection notes: Qualitative notes about the development of the embryo. IP = internal pip; FIS = fecal in shell; GD = germinal disc; emb = embryo; PVL = perivitelline membrane.
- HH Stage: The Hamburger-Hamilton stage that the embryo died.
- General Stage: The corresponding general staging timeframe (early, middle, or late) from the Hamburger-Hamilton stage.
- Day of Inc: The day of incubation that the embryo died, based on the Hamburger-Hamilton stage.
- Justification: The key developmental features observed that aided in staging the embryo, based on Murray et al. (2013). Eggs that required fluorescence microscopy were automatically classified as early stage (exact HH stage unknown). EL = eyelid; GP = gape flange.
File: Data_Fertility_results.csv
Description: Results for the eggs that required fertility testing, including the 2016 dataset. All blank cells are non-applicable.
Variables
- Year: The year the egg was laid.
- Site: The site at which the egg was laid (agricultural, wetland, or mixed).
- Egg Code: The unique identifier for the egg.
- Germinal disc visible: A note if the germinal disc was visible upon initial dissection of the egg. y = yes; n = no; faint = the germinal disc was faintly visible; faint spot = a faint spot of development was seen in lieu of a germinal disc; red spot = a red spot was seen in lieu of a germinal disc; embryo = development had progressed past the germinal disc stage and an embryo was visible; raised dev = development was raised in the germinal disc, but an embryo could not be macroscopically seen yet.
- Contents disintegrated: A note if the perivitelline membrane had broken down and the yolk was disintegrated (yes/no).
- Conclusion: Results of fertility testing. See the Methods section for more details.
- Notes: Qualitative notes about the contents of the egg. GD = germinal disc; PVL = perivitelline membrane.