Point Blue Conservation Science: Palo Blog, May 2017
By Palomarin Field Station
This summary was compiled by Point Blue’s Palomarin nest searching interns Kelly Alm, Nicole Gaudenti, and Tyler Glaser with help from Hilary Allen, nest searching supervisor, and banding interns Meredith Heather and Emily Reich with help from Mark Dettling, banding supervisor.
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Observations from the Palo Grids:
May is typically a busy time on the Palomarin Field Station nest searching plots, or “grids”. Each spring/summer, three nest searching interns, or “gridders”, spend their days finding and monitoring nests and recording various information on all of the birds breeding on the grids. The data they collect provides us with valuable information on breeding bird densities, reproductive success, breeding phenology, and individual fitness and survival. All of the gridders’ careful observations from the field provide us with a clearer picture of the overall response of the breeding bird community to changes in environmental conditions.
By May, all of the Neotropical migrants (birds that breed here and winter in Mexico, Central America, or South America) have arrived, settled into territories, and begun breeding. Nesting of the resident species is already well underway. Nests in all stages of the nesting cycle are abundant on grid – some pairs still building, some incubating a clutch of eggs, and others busy feeding their growing nestlings. May also typically brings our first wave of young birds leaving their nests. With all of the breeding activity out on the grids, the gridders have hectic field days as they move from one pair of birds to the next, watching and listening for cues that will help them determine what stage of the nesting cycle a pair is in, and hopefully will lead them to a nest. Below are observations from this season’s gridders describing the process of searching for Wrentit nests (one of our 6 color banded focal species) during each of the nesting stages.
“Nest building is my favorite stage of nesting to observe, especially among Wrentits, for whom it is a joint effort. After reestablishing their territory for the breeding season, a pair prepares for what has yet to come. A flurry of duets, the affirmation of a pair’s bond, often accompanies building, making it seem more akin to a seasonal ritual than a biological duty. Over a week’s time they collect a myriad of disparate materials–repurposing cobwebs, bark, fine grass, animal hair, and lichen–to create a shelter for yet-to-be eggs and young, their impetus for joining forces each spring.
When following Wrentits who are building, gridders watch for the birds carrying nest material. Zooming in on a Wrentit with binoculars, only to see a wad of cobweb or tuft of grass in their bill, is always exciting. I especially enjoy watching Wrentits as they collect material. Whether one is tugging at needles on a Douglas-fir or carefully pulling fine strips of bark from a shrub stem, there is something captivating about watching another species collect material to use for creating.
A building pair of Wrentits travels to and from the nest site frequently, making it an opportune time to find their nest. They move swiftly when delivering material to a nest, so I try to follow close behind, a challenging feat in dense scrub and Douglas-fir forest. If I lose sight of the bird, I briefly search nearby before waiting in the area where I last saw them, hopeful they will cross my path again. If this fails, I return to the spot I observed them collecting material. Wrentits commonly return to the same source for material, even with closer sources available.
As I note a bird’s movements, I create a mental map of points where I saw the bird, what direction the bird was traveling, and whether he or she was carrying material (going to the nest) or not (coming from a nest). Pairs can be very sneaky while building, entering and exiting a large patch of dense scrub from various points. However, with enough time and patience, one can begin to narrow down the location of the nest.
Wrentits in Palo’s coastal scrub typically nest about waist high in some combination of blackberry, coyote bush, poison oak and Douglas-fir, which offer varying degrees of protection from ground predators and concealment from above. As such, even once one knows a nest is in a particular patch of scrub, it can be a tricky to find. In many cases, I have searched every inch of a patch, peering inside it from various angles. Over the season, gridders develop a search image of a nest, an eye for a subtle, small brown mass, buried behind dozens of intersecting woody stems and vines.
Nothing quite compares to the magic of parting coyote brush stems or blackberry leaves, to see a nest, perfectly tucked away. Sometimes a nest is still a work in progress, bark strips woven into a loose, disheveled nest cup, waiting to be tidied up and refined with cobweb. A completed nest is a site to behold—a meticulously woven bundle of dried vegetation lined with fine grass and animal hair and rimmed with lichen, waiting for the arrival of blue eggs.” — Kelly Alm
“A unique characteristic of Wrentits is that both the male and female incubate their clutch of eggs. Because of this, the primary cue for finding a Wrentit nest during incubation is the moment when the pair switches off who is on incubation duty. After determining that the pair must be incubating (usually by noticing that they are never together because one of them has to be on the nest at any given time) I need to choose where to focus my efforts. A good place to start is an area where the pair seems to be spending a lot of time, or a particular spot where a male seems to keep singing. Wrentits typically spend ~30 minutes incubating and the males often sing upon leaving the nest when they are relieved by their female, so noting exactly where the male sings after 30 minutes of silence is often a superb place to start looking for a nest.
It helps to create a little story in my head. If I hear a male sing, I may imagine that he just came off of a nest and will be busy foraging throughout his territory while his mate sits patiently on their eggs. I note the time at which he sang, and try to follow him. If I manage to flawlessly follow him, he may lead me right to his nest, but often that is difficult to manage in the dense scrub. If the male I’m following is still singing after 45 minutes, it is relatively safe to say that he does not have a nest with eggs. If he consistently sings for a while and then goes undetected for 30 minutes, there is a possibility that he was on a nest for those silent 30 minutes.
Making sense of the patterns that we observe helps lead us to a nest, but there are always the sneaky individuals who lead us astray with inconsistent cues. Some pairs incubate for longer than the typical 30 minutes. Individuals also differ in how vocal they are. I have been able to find a nest based on how aggressively a female was ‘churring’ (a rattling call that Wrentit’s make, often as an alarm), but there are also many individuals that seem to not be perturbed by my presence at all and therefore give me no vocal cues as to how close I am to their nest. Some males will actually sing from a nest, which is really helpful for us gridders, but makes me wonder how safe that must be for their precious nest contents. Once, while monitoring a Wrentit nest in a tree, the male sang not only during his whole route to the nest, but also for the first while that he sat on the nest. His female, who had just come off the nest and was still nearby, gave a brief angry chur as if to say ‘would you be quiet!’, and then her mate immediately lowered his volume and stopped singing.” — Nicole Gaudenti
“The nestling stage of the reproductive process is by far the riskiest phase of the young birds’ development. After the eggs have hatched, parents have a more difficult time keeping the location of the nest concealed due to nestling noises, scent, etc., and also because they begin to make more frequent trips to and from the nest to feed their young. Due to these factors, a large number of nests are depredated at this particular stage of the reproductive process.
Fortunately for us gridders, these factors also makes it a whole lot easier to locate nests during this stage! The most common way of identifying whether or not a nest is at this particular stage of the process is by observing food being carried towards the nest in either of the parents’ bills. At least in this particular month of the breeding season (May being the very peak of reproductive activity for Wrentits), it’s more than likely that if ‘food-carrying’ is observed, there is also an associated nest somewhere in the area (unless of course they’re feeding their fledglings already, which is also a possibility, but not quite as likely due to the wet weather and late start for most Wrentits this breeding season).
After observing a ‘food-carry’ from one of the parents, I usually find it best to stay put for a little while, as a means of isolating potential locations the nest could be, and also giving myself a chance to observe the parents’ behavior. Typically, if a ‘food-carry’ is observed and you happen to be anywhere near their nest, the parents will falter in their movement directly towards the nest location. Often they appear noticeably uncertain in how to reach their young without revealing the nest’s exact whereabouts to the watchful gridder sitting in their path. Once this kind of behavior is noted from either or both of the parents, I’ll give the pair a little space, and wait to see where they deliver the food!” — Tyler Glaser
During the thick of the breeding season, gridders encounter pairs of birds at every stage of nesting, from building a nest to incubating and feeding nestlings. Each stage is a privilege to witness, offering an intimate window into a particular bird’s life.
Let’s Do the Nest Numbers:
Since the beginning of the Palomarin nest searching internship in late March until May 31st, there were a total of 81 nests found for 61 pairs of 9 different species across the three nest searching plots or “grids”. The species for which we found the most nests was the Wrentit (60 nests found) followed by Wilson’s Warbler (8 nests found), and Spotted Towhee (5 nests found).
We banded a total of 74 nestlings from 24 nests of 5 of our study species (Wrentits, Song Sparrows, Nuttall’s White-crowned Sparrows, Spotted Towhees, and Wilson’s Warblers).
Of the nests found, 20 nests had successfully fledged young by the beginning of June. Plenty of noisy young birds and busy parents!
Quick Banding Highlight:
On May 2 at Redwood Creek (Golden Gate National Recreation Area) we caught an American Crow. This is the first time the Palomarin project has captured a crow in its 51 year history!
Let’s Do the Banding Numbers:
In 29 days (3,091.98 net hours) of mist-netting at Palomarin in May, we captured 56 new birds and recaptured 87 previously banded birds. A total of 143 birds of 19 species were caught. Approximately 5 birds were caught per banding day.
At our other West Marin banding sites, we captured 157 new birds and recaptured 149 previously banded birds. A total of 306 birds of 30 species were caught over 12 banding days in May (656.5 net hours), an average of approximately 26 birds per day.
The highest capture rates at Palomarin and our other West Marin banding sites were on May 22nd at Palomarin with 9 birds and May 22nd at Pine Gulch (Bolinas Lagoon Open Space Preserve) with 44 birds.
At Palomarin the highest numbers were captured for the following species: Wilson’s Warbler (37), Allen’s Hummingbird (17), Pacific Wren (15), Swainson’s Thrush (13), and Oregon Junco (11).
Across all off-sites, the highest numbers of captures by species were: Swainson’s Thrush (69), Song Sparrow (59), Wilson’s Warbler (56), Allen’s Hummingbird (18), Bewick’s Wren (14).
About these Summaries:
In an effort to share our science with the public, Point Blue interns and staff at our Palomarin Field Station (Palomarin or “Palo”) in Point Reyes National Seashore near Bolinas, CA produce these monthly bird-banding summaries. Our science interns create these summaries as part of their science outreach training.
Our Palomarin Field Station is open to the public. Consider visiting us! Learn how by visiting our mist-netting demonstrations web page.