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Up close and personal, A window on genetic variability in the Wrentit

August 14, 2015

My home away from home this spring and summer was a thickly vegetated patch of land near Point Blue’s Palomarin Field Station (“Palo”), in the Point Reyes National Seashore. From mid-March to late July, I was a nest-searching intern, learning skills that I’ll need to become a wildlife biologist. I spent well over 500 hours on 12 hectares (30 acres) of coastal scrub and young fir forest. My goal – the same as two other interns working in study plots nearby – was to chart all the territories, mates, nests, and chicks of resident breeding birds.

Candace Stenzel
Candace Stenzel

As I got to know the White-crowned Sparrows, Wilson’s Warblers, and 5 other study species, I began asking questions about their habits. Wrentits, in particular, captured my curiosity. They are by far the most abundant species at Palo: I mapped 27 of their territories just on my study plot this year, meaning 27 different male-female pairs, most of them building nests, laying eggs, and trying to produce more Wrentits!

The Wrentit does not migrate but commonly lives its whole life in one small area. The young that survive to breeding age may establish territories very near where they were born. Pairs stay together and may remain mated for years. These habits set the context for my investigation of their family lines – a project that capped my Point Blue internship.

One particular bird became a focus of my questions. Because we band many resident birds as nestlings – giving them unique combinations of lightweight color-bands – we can follow them throughout their lives. The bird of interest had a blue band above two white ones on his left leg, and a silver (numbered) band on his right leg. So his shorthand name was “BWW/S.” We knew he was born in 2004 and now was a ripe 11 years old. (Wrentits live for 3 to 5 years on average, but the oldest one on record is still living, near Palo, at age 13.)

I first found BWW/S in my study area with a color-banded female. In late March I found their nest with 4 eggs in it. But by early April they had lost the nest to a predator – not an uncommon fate. Weeks went by with no sight of the original male, but a new male (also color-banded) was singing throughout the territory. In early May, I discovered a new nest, belonging to the original female and this new male. Though BWW/S had probably met his demise, I couldn’t stop wondering about him.

The 2015 nest-searching crew at Palomarin Field Station.
The 2015 nest-searching crew at Palomarin Field Station. Left to right: David Sherer, Candace Stenzel, Xeronimo Castaneda (supervisor), Kyle Marsh

Did a good long life mean that this individual Wrentit had produced plenty of descendants? And did they inherit some of his breeding traits, such as choosing very similar and nearby territories? Details like these would help us evaluate how closely related a localized group of birds might be. The variation in their gene pool is important to know, including for conservation. More genetic variability can give a wildlife population more resilience if the environment changes.

A curious scientist can seek answers to questions like mine at the Palomarin Field Station, where the database on resident nesting birds is exceptionally long-term and detailed.

I began mining this wealth of information to trace out the “family nesting lines” of BWW/S. I learned that, over time, this male Wrentit built 17 different nests that we know of. His mates at those nests laid 53 eggs, and 43 of those chicks survived long enough for us to color-band. Of those color-banded nestlings, just 5 later built nests of their own or produced offspring. This survival rate is not unusual for small birds that have to find food and avoid predators. While some of the banded nestlings fathered by BWW/S may have established territories off our study plots, we know of 5 that survived and bred.

This map shows all of BWW/S’s nest sites (red circles) and those of his successful offspring. The solid black shapes at right represent Point Blue’s Palomarin Field Station. The lower large rectangle was my study area. The symbols depict 5 different generations; colors distinguish 5 individual birds in the 2nd generation. The color blue shows that nesting birds in 3 generations all had a common ancestor in the 2nd generation. Next year’s nest-searching may find out if this Wrentit family line continues.
This map shows all of BWW/S’s nest sites (red circles) and those of his successful offspring. The solid black shapes at right represent Point Blue’s Palomarin Field Station. The lower large rectangle was my study area. The symbols depict 5 different generations; colors distinguish 5 individual birds in the 2nd generation. The color blue shows that nesting birds in 3 generations all had a common ancestor in the 2nd generation. Next year’s nest-searching may find out if this Wrentit family line continues.

I was able to chart the locations of nest territories belonging to BWW/S and all his known descendants (to date). My report on these findings rounded out my Point Blue internship at Palo.

This experience taught me a lot about field techniques and songbirds’ breeding behavior – and the value of long-term databases. I’ll use what I learned at Point Blue during my next adventure, pursuing a master’s degree in biology with a strong focus on conservation.

And I’ll probably think about BWW/S and his family line whenever I hear or see another Wrentit.

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