Renesting attempts reveal latitudinal trends in multiple broods
Bluebirds in the South tend to lay fewer eggs per nest than they do in the North, but a longer
breeding season allows them to nest more often.
How many eggs does a bluebird lay each year? On the surface, this might seem like an easy question. After all, participants in The Birdhouse Network send data to the Cornell Lab of Ornithology each year on the number of bluebird eggs that they count in their nest boxes. These data show that the answer depends on location—Eastern Bluebirds at northerly latitudes lay larger clutches than do bluebirds farther south. However, this doesn’t necessarily mean that bluebirds in the North lay more eggs overall, because the total number of broods that bluebirds can raise in a summer might also vary regionally. We have to know not only how many eggs birds put in a basket, but also how many baskets (broods) birds produce.
To find out how many times bluebirds nest, the most accurate approach would be to mark individual birds with colored leg bands and monitor their breeding activities throughout the season. Conducting this sort of work across the continent would be a daunting task and has never been done. Instead, our research team at the Lab of Ornithology analyzed data on repeated use of nest boxes from more than 7,000 Eastern Bluebird nesting attempts reported by The Birdhouse Network’s participants from 1998 to 2002. The results, to be published in The Journal of Avian Biology, show that the number of nest attempts per box was 26 percent higher in the South than in the North.
The number of nesting attempts per box by Eastern Bluebirds in the southern portion of their range was 26 percent higher than in the northern portion. Results are based on data submitted to The Birdhouse Network, 1998–2002. Nest attempts were counted as the number of nests with at least one egg and do not necessarily indicate a higher success rate.
Figure 1. Eastern Bluebird nest-box reoccupancy The number of nesting
attempts per box by Eastern Bluebirds in the southern portion of their range was 26 percent higher
than in the northern portion. Results are based on data submitted to The Birdhouse Network,
1998–2002. Nest attempts were counted as the number of nests with at least one egg and do not
necessarily indicate a higher success rate.
The higher rate of nest-box reuse farther south indicates a higher instance of multiple brooding and was apparently not the result of different females using the same box in the same year. Our analyses indicated that observer bias and movement of female bluebirds from one box to another were unlikely explanations for the striking pattern that bluebirds farther south produce more broods.
To our knowledge, this study is only the second one to show that the number of nesting attempts increases from north to south within a single species. Season length can account for the observed trend. For each incremental change in latitude of one degree to the south, the breeding season is longer by 2.3 days. The most northerly locations (48˚ latitude, North Dakota through Quebec, Canada) had breeding seasons of only 108 days, compared with 154 days in the extreme South (28˚ latitude, Texas through Florida). Thus, bluebirds in southerly climes have more time to produce extra broods. The number of nest attempts per box was 1.8 per season on average in the South, compared with 1.3 attempts in the North. Southern bluebirds were also more successful at fledging young from more nests: the number of nests that produced at least one fledgling was 19 percent higher in the South.
The evidence also suggests that bluebirds in the North may try to compensate for the shorter breeding period by initiating their second brood more quickly than their counterparts in the South. After fledging one brood, birds in the North began another clutch in the same box an average of 17 days later, whereas southern birds waited 26 days. Shorter time intervals between broods may increase the likelihood that a second brood can be raised, but at a cost to fledglings. By starting a new nest sooner, northern bluebird parents may very well be sacrificing time that could have been used to raise and feed fledglings from earlier nests.
The results of the study have implications far beyond bluebird natural history, because for half a century ornithologists have sought to explain variation in the number of eggs laid in an individual nest. Ecologists often assume that the number of eggs laid this year will affect the number that a bird can produce the following year, and that birds manage reproductive trade-offs from one year to the next. Our results, along with those from several other studies, indicate that nesting birds are making similar “calculations” within a single season. For birds capable of producing multiple nests in a season, the clutch size of a single nest may not be as significant as the total number of nestlings that can be produced. For total annual fecundity to be equal among bluebirds, birds in the North would have to lay much larger clutches than those in the South. However, Eastern Bluebirds show only a small amount of clutch size variation (less than half an egg) across latitudes (BirdScope, Spring 2000), which is not enough to compensate for observed differences in multiple brooding. This is not to say that southern birds have better reproductive success, only that they make more baskets full of eggs. But let’s not count those chicks before they hatch—recall that birds in the South have higher hatching failure rates (BirdScope, Summer 2003) and therefore endure their own trade-offs.
Caren Cooper, Wesley Hochachka, and André Dhondt are researchers in the Lab’s Bird Population Studies. Tina Phillips is project leader of The Birdhouse Network.