Identifying minimum patch sizes for breeding success in birds

A new study illustrates the importance of understanding the minimum patch size that an at-risk bird needs to reproduce. Jerrod Butcher and fellow researchers from Texas A&M University looked at the effect of habitat patch size on two songbirds in north-central Texas.

For the federally endangered golden-cheeked warbler, they found that birds did not successfully breed in patches smaller than 15-20 hectares indicating that a threshold exists at that size below which the birds do not reproduce. However, the study also found that the warblers established territories in patch sizes as small as 2.9 hectares (the smallest studied) and paired in patches as small as 4.1 hectares.

From a conservation perspective, this study shows that research looking at just the presence-absence of the warblers (or even bird pairings) would substantially underestimate the minimum habitat patch size needed to sustain a viable population.

The researchers also looked at white-eyed vireos, a habitat generalist, and as predicted found no evidence of a minimum patch size threshold for breeding success - the vireos successfully reproduced in patches as small as 4.1 hectares.

One of the benefits of identifying patch size thresholds is that by looking at how different environmental variables change when you cross the threshold, you can get clues on what factors are influencing the bird behavior. In this vein, the researchers looked at two variables along the patch size gradient: 1) brown-headed cowbird parasitism; and 2) abundance of arthropods - the food source for the birds.

However, the researchers found no relationship between either variable and patch size, which means they likely are not explanations for the threshold. So more research is needed to figure out why golden-cheeked warblers do not breed in patches smaller than 15-20 hectares.

This study also raises the issue of whether small patches act as ecological traps for the golden-cheeked warblers resulting in population sinks that further threaten the species. This could occur if small patches are attractive to the warblers leading to numerous small territories where the birds are not able to reproduce. However, the study authors warn against jumping to that conclusion. They write,

"Patches of habitat less than 20 hectares in area may not be large enough to sustain a viable population of golden-cheeked warbler in the long-term; however, such patches may benefit populations if the patches can sustain breeding pairs in the short term."

Nevertheless, they advise that managers across the distribution range of golden-cheeked warbler be cautious about decreasing vegetation patches below 20 hectares. Similar research on minimum patch-size thresholds for breeding success could be important for the conservation of other at-risk birds.

--Reviewed by Rob Goldstein--Conservation Maven

Butcher, J., Morrison, M., Ransom, D., Slack, R., & Wilkins, R. (2010). Evidence of a Minimum Patch Size Threshold of Reproductive Success in an Endangered Songbird Journal of Wildlife Management, 74 (1), 133-139 DOI: 10.2193/2008-533

State endangered-species lists are failing to protect species that need help the most

A new analysis of birds on state endangered species lists suggests that some species receive insufficient protection while others are receiving protection unnecessarily.

This may be attributable to the fact that state endangered species lists are composed by policy process rather than by scientific process.

Birder's World has a good article highlighting the new analysis.

You can also find the analysis, by Jeff Wells of the Boreal Songbird Initiative, at PLoS ONE.

In Ohio, the following species are listed as endangered (e) or threatened (t) though their listing could be considered questionable based on the new analysis. Many of the species on these lists are based on local rarity and rarity is prioritized, however rarity may be detrimental to conservation efforts.

-Yellow-bellied Sapsucker(e): Primarily migratory(2) through Ohio with a resident population estimate in the state of about 400 birds, while the global population estimate is 9,000,000 individuals (1).

-Loggerhead Shrike (e): The global population for this species is estimated at 4,200,000 individuals while Ohio's estimated population is roughly 300 individuals(1). Loggerhead Shrikes invaded Ohio in the mid-1800's as deciduous forests were replaced by agricultural fields and then declined by the 1930's (2).

-Lark Sparrow (e): a species that spread into Ohio in the 19th century as land use became primarily agricultural. Their occurrence in the state was sparse and varied from place-to-place until in the 1960's they were restricted to the Oak Openings preserve near Toledo, Ohio (2). Peterjohn suggests that at most Ohio hosted about 12 breeding pairs in the Oak Openings region (2), while the global population estimate is 9,900,000 individuals (1).

-Dark-eyed Junco (t): The global population estimate for this species is 260,000,000 whereas(1) the Ohio breeding population is roughly 30-50 individuals(2).

Some species should probably be given greater priority given that Ohio hosts a relatively large portion of the global population of these species.

-Cerulean Warbler is considered a species of concern in Ohio, however the global population for this species is estimated at 560,000 individuals (1). Ohio hosts an esitimated 70,000 breeding Cerulean Warblers, or ~13% fo the total population(1). Ohio's role in the management of this species is potentially great and proper habitat management may bolster global population numbers.

-Henslow's Sparrow, an Ohio species of concern, hosts an estimated population of 6,000 individuals or ~7.5% of the global population of 80,000.

While some birders and state naturalists may be inclined to disagree with this new analysis, because it may impact their state lists, the proposal of species listing and management based on a more scientific process should result in more efficient and cost-effective management practices.


1 Partner in Flight Landbird Population Estimates Database, Version 2004

2 Peterjohn, B. G. 2001. The Birds Of Ohio. The Wooster Book Company. Wooster, Ohio.

Arctic Terns Longest Migrants....Still

By HENRY FOUNTAIN
NY TIMES

To reach elite status in many airline frequent-flier programs, you have to log at least 50,000 miles in the air in a year.

Somewhere in Greenland there’s an Arctic tern that could qualify.

Arctic terns have a reputation as long-distance travelers, migrating to the Southern Ocean from breeding grounds in the Arctic. Researchers have suggested the round-trip distance might be as much as 25,000 miles.

But those were only estimates. While location-tracking tags have been used with large birds like albatrosses, Arctic terns, at less than four ounces, were too small to carry them.

Now Carsten Egevang of the Greenland Institute of Natural Resources and colleagues have devised a miniature data logger that, at 1/20th of an ounce, is light enough. It records light intensity, using the timing of sunrise, sunset and length of twilight to determine latitude and longitude.

In The Proceedings of the National Academy of Sciences, the researchers report on the journeys of 11 terns fitted with the devices. The birds, which began their trips in Greenland or Iceland in August, took two routes south, some hugging the African coast and others crossing from West Africa to Brazil to follow the South American coast. They stopped for about three weeks in the mid-Atlantic east of Newfoundland, a rich feeding zone.

Once they reached the Southern Ocean, they spent four months flying primarily east and west, again in areas that are rich in food. They returned in May and June having traveled, on average, about 44,000 miles. One tern totaled 50,700 miles, which is the longest animal migration ever recorded electronically.