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Incubation Matters

by Caren B. Cooper and Tina Phillips
 

A new angle on why birds in warmer climes lay fewer eggs

If you peek into the nest of a bluebird, will you find three eggs, seven eggs, or something in between? A new study from the Cornell Lab of Ornithology suggests that the answer may depend in part on how warm the weather is when a bluebird lays its eggs. Using data from citizen-science participants, the study showed that laying a large number of eggs is apparently disadvantageous in warm weather. Eastern Bluebird eggs in large clutches have higher rates of hatching failure at warm southerly latitudes than at cooler northern latitudes. They also have higher rates of hatching failure late in the season, when temperatures are presumably warmer than in early spring.

The notion that birds typically lay fewer eggs in warmer climates isn’t new; for decades, researchers have documented how birds at more southerly latitudes have smaller clutches than those farther north. Previously, Lab researchers used data from citizen-science participants to show that Eastern Bluebirds generally lay more eggs from south to north and east to west, and that they lay larger clutches at the beginning of the season than near the end (BirdScope, Spring 2000).

The reason, though, was unclear. Most studies of clutch-size variation have focused on constraints during the nestling period, examining how factors such as day length, predators, and food availability might limit the number of young that parents raise at different latitudes. These studies largely ignored what goes on in the eggs before they hatch.

The new study is one of the first to show that egg-laying and incubation constraints help account for both seasonal and latitudinal trends in hatching failure and clutch size. The results suggest that Eastern Bluebirds lay fewer eggs in warmer climates, in part to avoid temperature-related hatching failures associated with incubating large numbers of eggs. These hatching failures are not caused by the overheating of eggs; it rarely gets hot enough in a nest box to kill the embryos. Rather, the danger appears to be developmental abnormalities that occur during prolonged exposure of some eggs to temperatures just below the warmth of normal incubation.

The study was based on more than 11,000 nesting records of Eastern Bluebirds from across their breeding range, submitted to the Lab by participants of The Birdhouse Network and the Cornell Nest Record Program. Researchers Caren Cooper, Wesley Hochachka, and André Dhondt at the Lab of Ornithology and Greg Butcher at Audubon tested the egg viability hypothesis, proposed by Scott Stoleson at the USDA Forest Service and Steven Beissinger at the University of California, Berkeley. This hypothesis suggests that in warm weather, eggs in large clutches are more likely to develop abnormally than eggs in small clutches because of prolonged exposure to suboptimal temperatures.

Most songbirds lay just one egg per day and do not begin sitting on their eggs until the last or second- to-last egg has been laid. By waiting to incubate until the clutch is complete, the birds ensure that the eggs will start developing, and later hatch, at about the same time.

Before a female begins incubation, eggs remain viable for extended periods at temperatures below 75˚F. When she begins sitting on the eggs, her body heat warms them to 96˚F–105˚F, the optimal range for embryonic development.

However, if the egg is exposed to warm weather for extended periods, embryonic development may begin even before the female starts incubating. At temperatures between 75˚F and 96˚F, embryos experience unsynchronized tissue growth, abnormal development, and mortality (Figure 1, below). In a large clutch, the first few eggs that a female lays may fail to hatch if they are exposed to these prolonged suboptimal temperatures before incubation begins.


The study’s results showed that, indeed, bluebird nests in more southerly climates were more likely to have at least one unhatched egg (Figure 2, below). Additionally, clutches laid late in the breeding season, when temperatures are presumably warmer, were 1.5 times more likely to contain an unhatched egg than clutches laid at the beginning of the breeding season. Hatching failure was high among larger clutches, late in the season, and under sunny, humid conditions with low precipitation. In warm weather, hatching failure may be an important constraint in limiting clutch size.



Birds attempting to nest in the South or late in the season seem to face an inescapable penalty if they lay a large clutch. If they wait to begin incubation until after they lay the last egg, their first eggs may be exposed to prolonged suboptimal temperatures, possibly leading to hatching failure. If they begin incubation as soon as they lay the first egg, they could ensure better success at hatching, but nestlings would hatch over a number of days. When that happens, the last young to hatch may die because the parents are more likely to feed its larger siblings.

The data revealed further complexities in incubation patterns that suggest the need for additional study. For example, the amount of time it took the eggs to hatch varied from 11 to 19 days, depending on the time of the season, the geographic location, and the number of eggs in the clutch. As the season progressed and ambient temperatures rose, the eggs hatched on average 1.5 days earlier, irrespective of latitude. Did the eggs hatch sooner because the females began incubating their clutches before all the eggs had been laid, perhaps to reduce the eggs’ exposure to the suboptimal temperatures that can cause hatching failure? Or did the hotter weather make it easier for the females to keep the eggs warm during incubation, speeding up the embryos’ development?

To help answer these questions, The Birdhouse Network is continuing to collect information on clutch size and will soon be analyzing data from dime-sized data loggers that record temperatures in the nest. Temperature fluctuations reveal details of the female’s incubation activity (BirdScope, Summer 2002). These data will help show whether the female waits to begin incubation until after she lays the last egg. They will also show whether females are better at keeping eggs in the optimal temperature range when the weather is warm. Researchers will even be able to investigate whether large clutches are advantageous in cooler weather because they retain heat better, allowing the female to incubate the clutch more efficiently.

The current study is one of the first to show how widespread temporal and geographic trends in clutch size can be explained by processes that occur during the incubation period. The implications extend to potential impacts of global warming on birds. If global temperatures rise high enough to influence egg viability, hatching failures may increase, and birds may be forced to lay smaller numbers of eggs.

These insights were made possible by the powerful data set collected by citizen-science participants over large geographic scales and long time periods. We thank all of the citizen-science participants who contributed their data to this study, and encourage further contributions through The Birdhouse Network.

Caren B. Cooper is a research associate in the Lab’s Bird Population Studies program. Tina Phillips is project leader of The Birdhouse Network.

If you peek into the nest of a bluebird, will you find three eggs, seven eggs, or something in between? A new study from the Cornell Lab of Ornithology suggests that the answer may depend in part on how warm the weather is when a bluebird lays its eggs. Using data from citizen-science participants, the study showed that laying a large number of eggs is apparently disadvantageous in warm weather. Eastern Bluebird eggs in large clutches have higher rates of hatching failure at warm southerly latitudes than at cooler northern latitudes. They also have higher rates of hatching failure late in the season, when temperatures are presumably warmer than in early spring.

The notion that birds typically lay fewer eggs in warmer climates isn’t new; for decades, researchers have documented how birds at more southerly latitudes have smaller clutches than those farther north. Previously, Lab researchers used data from citizen-science participants to show that Eastern Bluebirds generally lay more eggs from south to north and east to west, and that they lay larger clutches at the beginning of the season than near the end (BirdScope, Spring 2000).

The reason, though, was unclear. Most studies of clutch-size variation have focused on constraints during the nestling period, examining how factors such as day length, predators, and food availability might limit the number of young that parents raise at different latitudes. These studies largely ignored what goes on in the eggs before they hatch.

The new study is one of the first to show that egg-laying and incubation constraints help account for both seasonal and latitudinal trends in hatching failure and clutch size. The results suggest that Eastern Bluebirds lay fewer eggs in warmer climates, in part to avoid temperature-related hatching failures associated with incubating large numbers of eggs. These hatching failures are not caused by the overheating of eggs; it rarely gets hot enough in a nest box to kill the embryos. Rather, the danger appears to be developmental abnormalities that occur during prolonged exposure of some eggs to temperatures just below the warmth of normal incubation.

The study was based on more than 11,000 nesting records of Eastern Bluebirds from across their breeding range, submitted to the Lab by participants of The Birdhouse Network and the Cornell Nest Record Program. Researchers Caren Cooper, Wesley Hochachka, and André Dhondt at the Lab of Ornithology and Greg Butcher at Audubon tested the egg viability hypothesis, proposed by Scott Stoleson at the USDA Forest Service and Steven Beissinger at the University of California, Berkeley. This hypothesis suggests that in warm weather, eggs in large clutches are more likely to develop abnormally than eggs in small clutches because of prolonged exposure to suboptimal temperatures.

Most songbirds lay just one egg per day and do not begin sitting on their eggs until the last or second- to-last egg has been laid. By waiting to incubate until the clutch is complete, the birds ensure that the eggs will start developing, and later hatch, at about the same time.

Before a female begins incubation, eggs remain viable for extended periods at temperatures below 75˚F. When she begins sitting on the eggs, her body heat warms them to 96˚F–105˚F, the optimal range for embryonic development.

However, if the egg is exposed to warm weather for extended periods, embryonic development may begin even before the female starts incubating. At temperatures between 75˚F and 96˚F, embryos experience unsynchronized tissue growth, abnormal development, and mortality (Figure 1). In a large clutch, the first few eggs that a female lays may fail to hatch if they are exposed to these prolonged suboptimal temperatures before incubation begins.

The study’s results showed that, indeed, bluebird nests in more southerly climates were more likely to have at least one unhatched egg (Figure 2). Additionally, clutches laid late in the breeding season, when temperatures are presumably warmer, were 1.5 times more likely to contain an unhatched egg than clutches laid at the beginning of the breeding season. Hatching failure was high among larger clutches, late in the season, and under sunny, humid conditions with low precipitation. In warm weather, hatching failure may be an important constraint in limiting clutch size.

Birds attempting to nest in the South or late in the season seem to face an inescapable penalty if they lay a large clutch. If they wait to begin incubation until after they lay the last egg, their first eggs may be exposed to prolonged suboptimal temperatures, possibly leading to hatching failure. If they begin incubation as soon as they lay the first egg, they could ensure better success at hatching, but nestlings would hatch over a number of days. When that happens, the last young to hatch may die because the parents are more likely to feed its larger siblings.

The data revealed further complexities in incubation patterns that suggest the need for additional study. For example, the amount of time it took the eggs to hatch varied from 11 to 19 days, depending on the time of the season, the geographic location, and the number of eggs in the clutch. As the season progressed and ambient temperatures rose, the eggs hatched on average 1.5 days earlier, irrespective of latitude. Did the eggs hatch sooner because the females began incubating their clutches before all the eggs had been laid, perhaps to reduce the eggs’ exposure to the suboptimal temperatures that can cause hatching failure? Or did the hotter weather make it easier for the females to keep the eggs warm during incubation, speeding up the embryos’ development?

To help answer these questions, The Birdhouse Network is continuing to collect information on clutch size and will soon be analyzing data from dime-sized data loggers that record temperatures in the nest. Temperature fluctuations reveal details of the female’s incubation activity (BirdScope, Summer 2002). These data will help show whether the female waits to begin incubation until after she lays the last egg. They will also show whether females are better at keeping eggs in the optimal temperature range when the weather is warm. Researchers will even be able to investigate whether large clutches are advantageous in cooler weather because they retain heat better, allowing the female to incubate the clutch more efficiently.

The current study is one of the first to show how widespread temporal and geographic trends in clutch size can be explained by processes that occur during the incubation period. The implications extend to potential impacts of global warming on birds. If global temperatures rise high enough to influence egg viability, hatching failures may increase, and birds may be forced to lay smaller numbers of eggs.

These insights were made possible by the powerful data set collected by citizen-science participants over large geographic scales and long time periods. We thank all of the citizen-science participants who contributed their data to this study, and encourage further contributions through The Birdhouse Network.

Caren B. Cooper is a research associate in the Lab’s Bird Population Studies program. Tina Phillips is project leader of The Birdhouse Network.

If you peek into the nest of a bluebird, will you find three eggs, seven eggs, or something in between? A new study from the Cornell Lab of Ornithology suggests that the answer may depend in part on how warm the weather is when a bluebird lays its eggs. Using data from citizen-science participants, the study showed that laying a large number of eggs is apparently disadvantageous in warm weather. Eastern Bluebird eggs in large clutches have higher rates of hatching failure at warm southerly latitudes than at cooler northern latitudes. They also have higher rates of hatching failure late in the season, when temperatures are presumably warmer than in early spring.

The notion that birds typically lay fewer eggs in warmer climates isn’t new; for decades, researchers have documented how birds at more southerly latitudes have smaller clutches than those farther north. Previously, Lab researchers used data from citizen-science participants to show that Eastern Bluebirds generally lay more eggs from south to north and east to west, and that they lay larger clutches at the beginning of the season than near the end (BirdScope, Spring 2000).

The reason, though, was unclear. Most studies of clutch-size variation have focused on constraints during the nestling period, examining how factors such as day length, predators, and food availability might limit the number of young that parents raise at different latitudes. These studies largely ignored what goes on in the eggs before they hatch.

The new study is one of the first to show that egg-laying and incubation constraints help account for both seasonal and latitudinal trends in hatching failure and clutch size. The results suggest that Eastern Bluebirds lay fewer eggs in warmer climates, in part to avoid temperature-related hatching failures associated with incubating large numbers of eggs. These hatching failures are not caused by the overheating of eggs; it rarely gets hot enough in a nest box to kill the embryos. Rather, the danger appears to be developmental abnormalities that occur during prolonged exposure of some eggs to temperatures just below the warmth of normal incubation.

The study was based on more than 11,000 nesting records of Eastern Bluebirds from across their breeding range, submitted to the Lab by participants of The Birdhouse Network and the Cornell Nest Record Program. Researchers Caren Cooper, Wesley Hochachka, and André Dhondt at the Lab of Ornithology and Greg Butcher at Audubon tested the egg viability hypothesis, proposed by Scott Stoleson at the USDA Forest Service and Steven Beissinger at the University of California, Berkeley. This hypothesis suggests that in warm weather, eggs in large clutches are more likely to develop abnormally than eggs in small clutches because of prolonged exposure to suboptimal temperatures.

Most songbirds lay just one egg per day and do not begin sitting on their eggs until the last or second- to-last egg has been laid. By waiting to incubate until the clutch is complete, the birds ensure that the eggs will start developing, and later hatch, at about the same time.

Before a female begins incubation, eggs remain viable for extended periods at temperatures below 75˚F. When she begins sitting on the eggs, her body heat warms them to 96˚F–105˚F, the optimal range for embryonic development.

However, if the egg is exposed to warm weather for extended periods, embryonic development may begin even before the female starts incubating. At temperatures between 75˚F and 96˚F, embryos experience unsynchronized tissue growth, abnormal development, and mortality (Figure 1). In a large clutch, the first few eggs that a female lays may fail to hatch if they are exposed to these prolonged suboptimal temperatures before incubation begins.

The study’s results showed that, indeed, bluebird nests in more southerly climates were more likely to have at least one unhatched egg (Figure 2). Additionally, clutches laid late in the breeding season, when temperatures are presumably warmer, were 1.5 times more likely to contain an unhatched egg than clutches laid at the beginning of the breeding season. Hatching failure was high among larger clutches, late in the season, and under sunny, humid conditions with low precipitation. In warm weather, hatching failure may be an important constraint in limiting clutch size.

Birds attempting to nest in the South or late in the season seem to face an inescapable penalty if they lay a large clutch. If they wait to begin incubation until after they lay the last egg, their first eggs may be exposed to prolonged suboptimal temperatures, possibly leading to hatching failure. If they begin incubation as soon as they lay the first egg, they could ensure better success at hatching, but nestlings would hatch over a number of days. When that happens, the last young to hatch may die because the parents are more likely to feed its larger siblings.

The data revealed further complexities in incubation patterns that suggest the need for additional study. For example, the amount of time it took the eggs to hatch varied from 11 to 19 days, depending on the time of the season, the geographic location, and the number of eggs in the clutch. As the season progressed and ambient temperatures rose, the eggs hatched on average 1.5 days earlier, irrespective of latitude. Did the eggs hatch sooner because the females began incubating their clutches before all the eggs had been laid, perhaps to reduce the eggs’ exposure to the suboptimal temperatures that can cause hatching failure? Or did the hotter weather make it easier for the females to keep the eggs warm during incubation, speeding up the embryos’ development?

To help answer these questions, The Birdhouse Network is continuing to collect information on clutch size and will soon be analyzing data from dime-sized data loggers that record temperatures in the nest. Temperature fluctuations reveal details of the female’s incubation activity (BirdScope, Summer 2002). These data will help show whether the female waits to begin incubation until after she lays the last egg. They will also show whether females are better at keeping eggs in the optimal temperature range when the weather is warm. Researchers will even be able to investigate whether large clutches are advantageous in cooler weather because they retain heat better, allowing the female to incubate the clutch more efficiently.

The current study is one of the first to show how widespread temporal and geographic trends in clutch size can be explained by processes that occur during the incubation period. The implications extend to potential impacts of global warming on birds. If global temperatures rise high enough to influence egg viability, hatching failures may increase, and birds may be forced to lay smaller numbers of eggs.

These insights were made possible by the powerful data set collected by citizen-science participants over large geographic scales and long time periods. We thank all of the citizen-science participants who contributed their data to this study, and encourage further contributions through The Birdhouse Network.

This article originally appeared in the Summer 2003 issue of BirdScope.

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