Kirtland’s Warbler Habitat Update
Philip W. Huber, Keith Kintigh, Stephen Sjogren
July 2013
Background The total acres of habitat dedicated to maintain the Kirtland’s warbler population above the minimum 1,000 pair recovery goal is based on data collected on how the birds are using their habitat. In 2001, a habitat analysis was completed based on a limited data set collected on the Huron-Manistee National Forests (HMNFs) (Bocetti, Probst, & Huber, 2001). That analysis included the following calculation:
Average Acres per Pair = 38 acres (based on HMNF data)
38 acres/pair x 1,000 pairs minimum = 38,000 acres
At that time, 38,000 acres was the amount of occupiable breeding habitat needed to be available at all times to sustain 1,000 pairs of Kirtland’s warblers. Managers also assumed that Kirtland’s warblers used a stand for an average of 10 years (duration of use).
Average Duration of Use = 10 years
Because jack pine is typically managed on a 50-year rotation, managers concluded that 190,000 acres of jack pine should be dedicated to Kirtland’s warbler management over the long term:
38,000 x 50-year rotation Total habitat dedicated to management = ------= 190,000 acres 10-year duration of use
Since 2001, additional acres have been dedicated to Kirtland’s warbler management on state and federal lands in the Lower and Upper Peninsulas of Michigan. Today, approximately 219,400 acres are dedicated to Kirtland’s warbler management, exceeding the minimum needed to manage for the 1,000 pair recovery goal. In addition, the managing agencies have agreed in a 2011 Memorandum of Understanding to continue to annually develop Kirtland’s warble breeding habitat, even if the species is removed from the federal list of endangered species.
Table 1. Annual Habitat Development Objectives and Manageable Habitat for the Kirtland’s Warbler by Agency/Forest.
Annual Habitat Dedicated Habitat Agency Development Objectives (Acres) (Acres) Michigan DNR 1,560 90,700 Lower Peninsula US Forest Service 1,600 88,300 Huron-Manistee National Forests US Forest Service 670 33,700 Hiawatha National Forest US Forest Service 0 0 Ottawa National Forest US Forest Service 0 0 Chequamegon-Nicolet NFs US Fish & Wildlife Service 0 6,700 Kirtland’s Warbler WMA Total: 3,830 219,400
Need for Change The 2001 Habitat Update recognized that “the recovery efforts for this species are based on an adaptive management approach, and, therefore, the estimate of the average density of male warblers per acre will certainly change with additional research and monitoring through time” (Bocetti, Probst, & Huber, 2001).
With the writing of a new Kirtland’s warbler conservation plan, managers understand that this information is in need of review and update. The most recent data is needed to complete this plan, as these metrics are the foundation for the agencies’ habitat management objectives.
Other reasons to update this analysis include:
• Singing males counted on the census are at an all-time high and much higher that what the 2001 formulas predict. • Analysis of census data shows that the average density of singing males (pairs) is higher than what was predicted (acres per singing male) for the core of the range in the northern Lower Peninsula. • Analysis of census data indicates that Kirtland’s warblers may use areas for shorter or longer periods (Duration of Use). • The agencies recognize the need to manage for a Kirtland’s warbler population above the minimum of 1,000 pairs at a maintenance level above 1,500 pairs (50% above the minimum). A maintenance level is needed to: o Ensure the long-term viability of Kirtland’s warbler population o Ensure that the Kirtland’s warbler continues to be considered for down-listing or delisting o Ensure that, once the Kirtland’s warbler’s status changes, it won’t be relisted o Establish a trigger point that would call managers to action to prevent the Kirtland’s warbler population from slipping below the minimum of 1,000 pairs • The agencies in the Lower Peninsula (LP) are broadening regeneration approaches away from traditional Kirtland’s warbler management on up to 25% of the acres managed for breeding habitat. The expected result is lower occupancy densities on these acres. • Managers need to consider the difference in habitat use between the northern Lower Peninsula core and peripheral habitats in the Upper Peninsula (UP), Wisconsin and Canada, and incorporate range expansion dynamics. Analysis In March 2013, the authors of this paper conducted an analysis of Kirtland’s warbler census data to update the acres/singing male and duration of use for their respective agencies. The results for each agency/forest are presented below in figures 1, 2 and 3 below.
Figure 1. Acres per Singing Male on the Hiawatha National Forest, 2003-2012.
Hiawatha National Forest Acres Per Singing Male 800 758.6 Occupied Habitat Blocks 700 All Suitable Habitat 600 545.6 559.7 500
400 366.3 329.4 284.6 286.7 274.5 300 239.8 239.9 Acres Per SiningMaleAcres 200 150.5 105.8 93.8 89.2 90.8 104.6 104.5 64.6 82.1 100 43.5
0 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 Year
Figure 2. Acres per Singing Male on the Huron-Manistee National Forest, 2001-2012.
Huron-Manistee National Forests Acres Per Singing Male
35.0 31.3
28.0 30.0 26.7
25.0 20.8 19.0 18.4 19.3 18.6 20.0 16.7 16.7 15.2 14.0 15.0
10.0
Acres Per Singing Male Per Acres 5.0
0.0 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 Year
Figure 3. Acres per Singing Male on Michigan State Forest, 2001-2012.
Michigan State Forest Acres Per Singing Male 35.0
30.0
26.1 25.0 23.9 24.1 23.8 22.5 20.7 21.3 21.6 20.0 19.7 20.2 17.8 18.2 15.0
10.0 Acres Per Singing Male Per Acres 5.0
0.0 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 Year
These numbers were then used to calculate the pairs1 of Kirtland’s warbler that would result from management at current levels:
Average Acres Available = Acres / Year Goal x Average Duration of Use
Average Acres Available Pairs of Kirtland’s warblers = ------Acres / Pair
1 The general assumption is that one singing male represents one nesting pair of Kirtland’s warblers. The results are displayed in Table 2.
Table 2. 2013 Kirtland’s Warbler Habitat Model.
2013 Kirtland’s Warbler Habitat Model Average Average Duration Dedicated Rotation Acres/Year Average Acres Predicted Agency/Forest Acres/Pair of Use Habitat Age Goal Available Pairs USFS / Hiawatha NF 100 10 33,700 49 670 6,700 67 USFS / Huron- Manistee NFs 19 9 88,300 49 1,600 14,400 724 MDNR 22 10 90,700 49 1,560 15,600 709 USFWS 6,700 219,400 36,100 1,500
Discussion
General Overview The 2001 habitat analysis only included data from the Huron-Manistee National Forests. This analysis includes data from the Huron-Manistee NFs, Michigan Department of Natural Resources, and the Hiawatha National Forest in Michigan’s Upper Peninsula.
In the Lower Peninsula, the acres needed per pair of Kirtland’s warblers has declined by at least 40 percent since the 2001 analysis. In the Upper Peninsula, KW does not currently breed in the high density colonies found in the NLP core, resulting in more acres of habitat used per pair.
The average duration of use is the number of years a block of habitat is occupied by Kirtland’s warblers. The average for the Hiawatha and MDNR remained at 10 years, the same as what was estimated in 2001. The estimate for the Huron-Manistee NFs declined to nine years.
The predicted number of Kirtland’s warbler pairs in the model (Table 2) exceeds the goal of a minimum of 1,000 pairs by approximately 64 percent. Therefore, the agencies can continue to manage as planned and meet the desired population maintenance level (50% above the minimum or 1,500 pairs).
Hiawatha National Forest In 2012, the Upper Peninsula Kirtland’s Warbler census recorded 38 singing males, with 26 of those on the Hiawatha NF. Kirtland’s warblers have been increasing in the UP every year since 1992, have been documented to breed in the UP, and travel between the NLP and UP in a single breeding season (Probst et al 2003). Although data are limited in the UP, we were able to assess Kirtland’s warbler density outside of the NLP, to provide contrast within an area of expanding range.
The timing of Kirtland’s warbler patch colonization and abandonment is influenced by patch and large- scale regional factors. Isolated patches are generally colonized later and abandoned earlier. (Donner et al 2010). On the Hiawatha NF, 6,237 acres of Kirtland’s warbler breeding habitat are available, with about 2,716 acres occupied. Each singing male occupies approximately 104 acres in occupied habitat blocks (Figure 1). However, each singing male occupies approximately 240 acres in all available habitat. We used the higher density, occupied habitat block number to estimate pair production in the habitat model (Table 2). It is likely that the density of Kirtland’s warblers in the UP will change as more data becomes available, as populations rise, or with range shift. Although data were limited, we observed a trend towards higher numbers of Kirtland’s warblers in larger blocks and in wildfire burned blocks. In the habitat model, we used the established 10 year, age 6-16, occupancy duration.
Currently, less than 5 percent of the Kirtland’s warbler population exists outside the NLP core. A goal of the Kirtland’s Warbler Recovery Team is to expand the range of Kirtland’s warblers as part of a long term strategy for recovery. Range expansion can provide protection for the species if adverse events occur in the core of the range in the Lower Peninsula. These events may include wildfire, insect or disease outbreaks, non-native species infestation or climate change.
Large blocks of unoccupied, but suitable, habitat are available to facilitate future colonization by Kirtland’s warblers. Ecosystem management is used to create these large blocks of jack pine habitat that are managed with barrens, fire and timber production to provide multiple-species benefits.
Huron-Manistee National Forests The number of singing males counted during the census on the Huron-Manistee National Forest has generally been increasing each year since 2003, even though the acres of occupied breeding habitat has remained relatively constant.
As indicated in Figure 2, the acres per singing male on the Huron-Manistee National Forests have been declining steadily since 2001 In other words, Kirtland’s warblers are occupying breeding habitat in higher densities. This improvement in breeding habitat use may be attributed to a number of factors. The overall population of Kirtland’s warblers has been increasing and more birds are available to occupy available breeding habitat. At the same time, managers have attempted to improve breeding habitat by increasing the size of treatment blocks, moving toward the maximum allowable block size of 550 acres (per the HMNF Plan). In addition, blocks are developed in close proximity to one another to increase the effective area of breeding habitat in any given location.
For the habitat model (Table 2), 19 acres/pair was selected. This is the average for the past 10 years of data. While the current trend is downward and the lowest figure is 14 acres/pair, this trend may not continue. USFS managers will continue to monitor the breeding habitat use and adjust this figure accordingly.
The average duration of use in the habitat model (Table 2) was calculated from 60 blocks of breeding habitat that occurred over five Kirtland’s Warbler Management Areas (KWMAs). Most KWMAs had a duration of use close to eight years. The McKinley KWMA had a duration of six years, while the Mack Lake Burn area had a duration of 11 years. This number will be adjusted as more stands move out of breeding habitat.
For the Huron-Manistee NFs, the predicted number of Kirtland’s warbler pairs in the model (Table 2) exceeds the Forest Plan goal of a minimum of 420 pairs by about 80 percent. This will allow the HMNFs to continue to manage under the current Forest Plan direction and meet the desired maintenance level for the HMNFs (630 pairs for the HMNFs).
Michigan Department of Natural Resources The number of singing males counted during the census on the NLP State Forest lands has gradually increased since 2003. Over the last couple of years, 1,100 to 1,200 singing males have been counted. In addition, the acres of occupied breeding habitat have increased by nearly 70% from about 14,000 acres in 2003 to about 24,000 acre in 2012.
As indicated in Figure 3, the acres per singing male on NLP State Forest lands has fluctuated since 2001, averaging about 22 acres/bird. Since 2007, bird densities on State Forest KWMAs have generally decreased with increasing occupied habitat. This may be a result of the rate of increase in habitat creation exceeding population growth, particularly in State KWMAs outside of the core breeding range of Ogemaw, Crawford, and Oscoda counties.
For the habitat model (Table 2), 22 acres/pair was selected for State KWMAs. This is the average for the past 10 years of data. The current trend has been upward with the highest value over 24 acres/pair, however, preliminary analysis indicate a decline to about 20 acres/pair in 2013.
The average duration of use in the habitat model (Table 2) was calculated as 10 years for the 13 State Forest KWMAs. Occupancy has only been tracked since 2000 on State Forest lands, limiting the size of the data set for analysis. Durations of use range from 1 year to 14 years and counting. Generally, the longest durations of use are in the larger blocks (>300 acres)and within the core range while the shortest are in smaller blocks (<200 acres)and outside the core range.
Conclusions Based on the revised acres per pair and duration of use, management at current levels should be sufficient to maintain the population at 1,500 pairs.
Works Cited Bocetti, C. I., Probst, J. R., & Huber, P. W. (2001). Kirtland's Warbler Habitat Update. Cadillac, MI: USDA Forest Service. Unpublished.
Donner, D. M., C. A. Ribic, and J. R. Probst. 2010. Patch dynamics and the timing of colonization – abandonment events by male Kirtland’s warblers in an early succession habitat. Biological Conservation. 143: 1159-1167.
Probst, J. R., D. M. Donner, C. I. Bocetti, and S. Sjogren. 2003. Population increase in Kirtland’s warbler and summer range expansion to Wisconsin and Michigan’s Upper Peninsula, USA. Oryx. 37: 365- 373.