Many seabird species with biparental care are known to retain the same partner across multiple breeding attempts. The prevailing explanation for this behavior is that reproductive success increases with mate familiarity due to improved coordination. This leads to selection for pairs to remain together, which gives rise to long-term pair bonds.
While this explanation is intuitive, almost nothing is known about the specific behaviors that benefit from retaining the same breeding partner. For example, do more familiar pairs simply begin breeding earlier by foregoing the customary courtship period? Or are more familiar pairs better able to coordinate parental duties than less established couples? These are some of the questions that I aim to address in my dissertation. To do so, I am studying the breeding behavior of the Cassin’ auklet (Ptychoramphus aleuticus), a small, planktivorous seabird.
A Cassin’s auklet on Southeast Farallon Island.
In particular, I am working with a well-studied population that breeds on Southeast Farallon Island, a rocky island off the coast from San Francisco. Point Blue Conservation Science (formerly Point Reyes Bird Observatory) has been closely monitoring this population continuously for nearly 50 years.
A view from the top of Southeast Farallon Island.
A central part of this monitoring effort is tracking the mating history of Cassin’s auklets throughout their reproductive careers. This is done by using artificial nest boxes, in which Cassin’s auklet breeding pairs will nest, lay a single egg, and, hopefully, ultimately raise a chick. As all birds that are handled are outfitted with a uniquely coded metal band, we can keep track of individual birds from year to year. This allows us to determine which birds are paired together and also to follow their reproductive success each year. Over time, this creates a picture of how mate familiarity impacts the likelihood that a pair will successfully hatch an egg and fledge a chick. Preliminary analyses suggest that pair experience does indeed correlate with reproductive success. For example, pairs that have been together for three years are approximately 15% more likely to successfully hatch their egg than pairs that are together for the first time.
But what are these more experiences pairs doing better? This is the question that I’ve been seeking to answer by studying the breeding behavior of Cassin’s auklets. During the last two breeding seasons, I used custom built radio-frequency identification (RFID) readers and passive integrated transponder (PIT) tags to monitor the nest attendance patterns of Cassin’s auklet breeding pairs. To do this, RFID readers are placed at nest boxes and breeding pairs are outfitted with a uniquely coded PIT tag on a leg band. Then, whenever a PIT tagged bird enters a nest with an RFID reader, the unique code of the PIT tag along with the time is recorded. Over the entire breeding season, this generates a continuous record of the comings and goings of breeding birds. Ultimately, this simple information can then be used to determine various measures of parental investment, such as incubation length and provisioning frequency, and also to investigate how pair members may be coordinate these behaviors.
Above: An RFID reader next to the nest box of a Cassin’s auklet.
Right: This Cassin’s auklet has been equipped with a PIT tag on its left leg. PIT tags are incredibly light, weighing less than 0.02% of a typical Cassin’s auklet.
I am currently in the process of analyzing the RFID detection data. So far, however, one thing is clear: Cassin’s auklet pairs are impressively coordinated. During incubation, partners will alternate between one day incubation stints and one day foraging trips. This lasts for about 36 days until the egg hatches and then chick provisioning begins, during which both adults will return each night to feed the chick.
Occasionally, however, this coordination does break down. During incubation, for example, one partner may not return to the nest for several days, which leads to a period of egg neglect during which neither partner is incubating. This is one indicator of reduced coordination that may occur when a pair is less experienced and therefore potentially less able to coordinate.
In the coming months, I will be exploring whether these periods of egg neglect occur more frequently for new pairs relative to their more experienced counterparts. Ultimately, the will work will provide one of the first tests of a possible behavioral explanation for why more experienced pairs of birds are more reproductively successful. This may have important implications for our understanding of the evolution of long-term pair bonds, the extreme form of social monogamy.