The Seagull nebula is a complex of HII regions and reflection nebulae ionised by the CMa OB1 association in the direction of the constellation Canis Majoris.
Source: Dean Salman - http://www.sharplesscatalog.com
This sector contains the spectacular Seagull and Thor's Helmet nebulae as well as the beginning of a major fork in the Milky Way's spiral structure.
The most detailed study of the molecular clouds in this direction is the 2004 paper by Kim, Kawamura and colleagues using two 4-m radio telescopes. They monitor the radioactive variant of carbon monoxide (13CO - containing carbon-13) and find three major molecular complexes. The complex of 9 clouds surrounding (220.8°, -1.7°) is located at a distance of 1000 pc. Another 13 molecular clouds are associated with the CMa OB1 association at a distance of 1150 pc. Finally, a large single molecular cloud is associated with the nebula BFS64 at 3200 pc. 
The stars in this region are sometimes referred to as the CMa OB1 association and sometimes as the CMa R1 association. Van den Bergh originally defined CMa R1 as a "Major cluster of reflection nebulae located within the boundaries of the association CMa OB1" and gave the location as 222° 2] There seems to be little or no difference in practice between the two associations and many authorities use the terms interchangably to refer to the entire complex of gas and stars in this direction.
The Kim et.al. study identifies 13 clouds in the molecular gas complex associated with these stars, with a total of 42610 solar masses in radioactive carbon monoxide (13CO). More than 80 percent of this mass is concentrated in three clouds: [KKY2004] 3 (16000 solar), [KKY2004] 4 (12000 solar), and [KKY2004] 12 (7500 solar). 
Humphreys identifies 17 ionising stars in the CMa OB1 association, including 4 O-class stars. The O-class stars are:
- HD 54662 (O6.5 V)
- HD 53975 (O7.5 V)
- HD 55879 (O9.5 II-III)
- HD 54879 (O9.5 V)
She gives a distance of 1320 pc. 
The hottest of these O-class stars, the O6.5 V class HD 54662, is a runaway binary with a bow shock visible in infrared.  Its companion orbits with a period of 558 days and has a class of about O9 V.  It appears at the northern edge of the molecular cloud [KKY2004] 4.
HD 53975 appears to the east of the densest and most massive molecular cloud, [KKY2004] 3. It is also a binary, with a B3 V companion orbiting with a period of only 6 days  and according to Avedisva, may be one of the ionising stars for the HII region Sh 2-297. 
The giant HD 55879, which some sources identify as a B0 III star, is the sole ionising star in the cluster NGC 2353, which is north of the molecular cloud [KKY2004] 4. According to Kharchenko, NGC 2353 is 11.2 million years old and is located at a distance of 1119 parsecs.  NGC 2353 may be the oldest portion or nucleus of CMa OB1, from which star formation spread outward, although it is now at the edge of the association.  NGC 2353 is at the centre of the shell of hydrogen-alpha emission surrounding CMa OB1 and is nearby the nebula [GS55] 114, visible in hydrogen-alpha.
This sector is dominated by Sh 2-296, the huge shell of glowing ionised gas surrounding the young hot stars of the CMa OB1 association. This hydrogen-alpha image identifies the major nebulae in this direction.
Source: Galactic Plane Explorer hydrogen-alpha image
The Seagull nebula is made up of 3 HII regions ionised by members of the CMa OB1 association. By far the largest of these is Sh 2-296 (Gum 2, RCW 1, IC 2177), which forms the "wings" of the Seagull nebula. Fainter hydrogen-alpha emission shows that Sh 2-296 forms a shell that surrounds the CMa OB1 association. Avedisova lists 4 ionising stars, including the O6.5 V class HD 54662 mentioned above. 
The smaller Sh 2-292 (Gum 1, RCW 2, VdB 93, NGC 2327) is both a reflection nebula and HII region and forms the "head" of the Seagull. It is primarily ionised by the B0e III class giant HD 53367, which is a young 20 solar mass Be star with a 5 solar mass companion in an eccentric (highly elliptical) orbit.  Avedisova suggests that the B3 V class BD -10 1839 also helps to ionise this nebula.  SIMBAD associates this star with the reflection nebula VdB 91.
Sh 2-292 appears to be embedded in the 16000 solar mass molecular cloud [KKY2004] 3, described above, which is also associated with the Lynds dark nebula LDN 1657.  The nearby B3 V class HD 53691 appears to be embedded in the densest part of [KKY2004] 3.
Finally, Sh 2-297 (Gum 3, LBN 1039, Ced 90, VdB 94) is a small nebular knot at the tip of the Seagull's right wing. It contains the infrared cluster [BDS2003] 96.  Avedisova lists 5 possible ionising stars, including the O7.5V class HD 53975 mentioned above. 
There are two small nebulae "flying" in front of the Seagull's right wing, also ionised by stars in or near CMa OB1. According to Avedisova, Sh 2-293 is ionised by the B2 III giant HD 52721 and is located at a distance of 740 ± 200 pc. Sh 2-295 is ionised by three B2 V stars at a distance of 920 ± 150 pc. 
Kharchenko identifies the star cluster [KPR2004b] 119, associated with the reflection nebula VdB 92, in the same direction as CMa OB1. Authorities differ a bit on its properties, howver. Kharchenko confirms it as a cluster containing the ionising stars DM -11 1761 (B2) and HD 53456 (B0V), with a distance of 1500 pc and an age of 38.9 million years, placing it a bit behind the CMa OB1 association.  Vazquez and colleagues, on the other hand, identify it as a "just a sparse group of young and midage stars" at a distance of 970 ± 70 pc, placing it a bit in front of the association.  Both studies place it fairly close to CMa OB1 and agree on an age of about 40 million years.
Nearby CMa OB1
The shell of hydrogen-alpha emission surrounding CMa OB1 is much larger than the traditional boundaries of CMa OB1. It also includes the HII regions [FT96] 222.9+1.0 and [FT96] 222.5+0.8, as well as the runaway O9 V star HD 57682.  Humphreys give a distance estimate of 1600 pc for HD 57682 , which is associated with the infrared source IRAS 07196-0852.  It has 20 solar masses and an age of about 4 million years. 
The Bok globule [CB88] 54 is a known star formation region  located to the southeast of CMa OB1. Launhardt and Henning give a distance of 1500 pc.  The Avedisova catalog associates it with the reflection nebula Bran4, the nebula LBN 1042, the Mira-class variable star V* Y CMa, and a second Bok globule, [CB88] 55. [Avedisova 2013]
This microwave image shows the three major complexes of molecular clouds in this direction as well as the ionising stars, clusters and associations.
Source: Galactic Plane Explorer microwave image
The origin of CMa OB1
The presence in CMa OB1 of two runaway O-stars (HD 54662 and HD 57682) as well as an expanding shell of gas suggests the possibility that the structure of the region may have been shaped by a supernova explosion approximately 1 million years ago. Comeron, Torra and Gomez conclude that both sequential star formation and a supernova explosion may have played a role in the history of the region, and that HD 57682 may have been a companion of the massive star that exploded in the supernova. 
CMa OB1 and spiral structure
The CMa OB1 association and its associated molecular clouds play an interesting role in the spiral structure of the Milky Way. The Orion spur, the local spiral structure, runs roughly parallel to the Perseus arm from its origin near the huge star formation region W51 , and then on through Cygnus X, the second quadrant, and until it reaches CMa OB1 and the Seagull nebula. Just east of this region, it branches, with the dominant portion crossing the Perseus arm and connecting to the Outer arm. A second branch continues on in the direction of the constellation Vela and ends in the massive molecular cloud complex called the Vela molecular ridge. 
The Seagull nebula thus lies at a celestial river fork or street intersection and must be influenced by stellar winds, supernovae and other events originating in both branches of the Orion spur. The faint but enormous ionised clouds visible in hydrogen-alpha to the east of the CMa OB1 association hint at the structure of this enormous celestial river reaching into the outer galaxy.
The (220.8°, -1.7°) region
The nebulae BFS61, BFS62, BFS63 and the complex of three nebulae (A,B and C) associated with [FT96] 220.9-2.5 are located in the direction of the molecular clouds surrounding (220.8°, -1.7°). Kim, Kawamura and colleagues conclude that these 9 molecular clouds lie at a distance of 1000 pc with a total of 7510 solar masses of radioactive carbon monoxide (13CO).  Fich and Blitz conclude that BFS62 and BFS63 lie at a distance of 930 ± 520 pc, and that BFS61 is at 670 ± 500 pc.  These distance estimates are all compatible with association with the molecular clouds.
The Avedisova catalog reveals more information about objects associated with these nebulae.
Kim, Kawamura and colleagues find that the nebula BFS64 is associated with a molecular cloud containing 23 thousand solar masses of radioactive carbon monoxide (13CO ) at a distance of 3200 pc.  Fich and Blitz give a compatible distance estimate of 3510 ± 980 pc. 
BFS64 may be associated with the infrared star clusters [IBP2002] CC04 and [IBP2002] CC05 , as well as the infrared complex [FT96] 221.9-2.0  and the reflection nebulae DG 114 and VdB 87.  The reflection nebula VdB 87c contains the B1 V class ionising star BD -08 1665, according to SIMBAD. The Avedisova catalog includes BFS64 and the young stellar object CPM 33 (WB89 935) in the same star formation region. [Avedisova 2002]
There are a number of distant objects visible through the hydrogen-alpha emission surrounding the CMa OB1 association.
The spectacular Thor's Helmet nebula is formed from gas and dust released by the massive hot Wolf-Rayet star WR 7.
Source: Canada-France-Hawaii Telescope / Coelum / J.-C. Cuillandre & G. Anselmi
The spectacular Thor's Helmet (NGC 2359, Sh 2-298, RCW 5) is the combination of a reddened arc of emission and a wind blown bubble surrounding the WN4 class Wolf-Rayet star WR 7 (HD 56925). Avedisova gives a distance of 5000 ± 1500 pc.  A 2001 study gives the same estimate of 5000 pc. 
Although the Thor's Helmet bubble is clearly formed from dust and gas released by the Wolf-Rayet star, there are several possible explanations for the reddened arc. Some of these are discussed in a 1995 paper by Chen, Wang and Qu. 
Sh 2-294 (RCW 3, LBN 1032) looks like a celestial octopus. According to Avedisova, it is ionised by a B0.5 V class star at a distance of 4300 ± 1000 pc.  BFS give a compatible estimate of 4600 pc.  It contains the infrared cluster [BDS2003] 95.  A 2008 study finds a double star cluster within this nebula, confirms the B0.5 V class ionising star, and suggests a distance of 3200 pc.  This is taken from a Russeil estimate of 3240 ± 560 pc. 
The B-class emission star BD -13 1825 (WB89 949), located to the east of Sh 2-297, is part of a star formation region according to the Avedisova catalog [Avedisova 2009] and is located at a distance of 8710 pc according to Wouterloot and Brand. 
The suspected star forming regions WB89 985 and WB89 986 are visible in the direction of what seems to be an indentation in the northeast region of the hydrogen-alpha emission surrounding CMa OB1. They are estimated to lie at distances of 5290 and 7930 pc respectively. 
The obscure nebula Sh 2-291 is visible in the southwest corner of this sector. Wouterloot and Brand give a distance of 7950 pc for Sh 2-291, based on an identification with the infrared source IRAS 06529-0755.  Very little else appears in the scientific literature on this nebula.
The Southern Filament of the Orion molecular clouds is visible in the south of this sector. These molecular clouds will be discussed in detail in a future commentary on our local system of stars, the Gould Belt.
The RCW catalog identifies a large faint nebula, RCW 4, at the northern edge of the hydrogen-alpha emission surrounding the CMa OB1 association. However, no distance estimates are available and according to SIMBAD, no references in the scientific literature have ever been made to this nebula beyond the RCW catalog itself.
Nebulae in this sector
1. ^ Kim, Bong Gyu, Kawamura, Akiko, Yonekura, Yoshinori, & Fukui, Yasuo 2004, Publications of the Astronomical Society of Japan, <SUP>13</SUP>CO (J = 1 -- 0) Survey of Molecular Clouds toward the Monoceros and Canis Major Region
3. ^ Humphreys, R. M. 1978, Astrophysical Journal Supplement Series, Studies of luminous stars in nearby galaxies. I. Supergiants and O stars in the Milky Way. and the unpublished catalogs available here.
4. ^ Noriega-Crespo, Alberto, van Buren, Dave, & Dgani, Ruth 1997, Astronomical Journal, Bow Shocks Around Runaway Stars.III.The High Resolution Maps
5. ^ Boyajian, T. S., Gies, D. R., Dunn, J. P., Farrington, C. D., Grundstrom, E. D., Huang, W., McSwain, M. V., Williams, S. J., Wingert, D. W., Fullerton, A. W., & Bolton, C. T. 2007, Astrophysical Journal, The Long-Period, Massive Binaries HD 37366 and HD 54662: Potential Targets for Long-Baseline Optical Interferometry
6. ^ Gies, D. R., Fullerton, A. W., Bolton, C. T., Bagnuolo, W. G., Jr., Hahula, M. E., & Wiemker, R. 1994, Astrophysical Journal, HD 53975: an O-type spectroscopic binary with a large mass ratio
7. ^ Avedisova, V. S. & Kondratenko, G. I. 1984, Nauchnye Informatsii, Exciting stars and the distances of the diffuse nebulae
8. ^ Kharchenko, N. V., Piskunov, A. E., Röser, S., Schilbach, E., & Scholz, R.-D. 2005, Astronomy and Astrophysics, Astrophysical parameters of Galactic open clusters
9. ^ Clariá, J. J. 1974, Astronomy and Astrophysics, A study of the stellar association Canis Major OB 1.
10. ^ Pogodin, M. A., Malanushenko, V. P., Kozlova, O. V., Tarasova, T. N., & Franco, G. A. P. 2006, Astronomy and Astrophysics, The Herbig B0e star HD 53367: circumstellar activity and evidence of binarity
11. ^ Bica, E., Dutra, C. M., Soares, J., & Barbuy, B. 2003, Astronomy and Astrophysics, New infrared star clusters in the Northern and Equatorial Milky Way with 2MASS
12. ^ Vázquez, Ruben A., May, Jorge, Carraro, Giovanni, Bronfman, Leonardo, Moitinho, André, & Baume, Gustavo 2008, Astrophysical Journal, Spiral Structure in the Outer Galactic Disk. I. The Third Galactic Quadrant
13. ^ Comeron, F., Torra, J., & Gomez, A. E. 1998, Astronomy and Astrophysics, Kinematic signatures of violent formation of galactic OB associations from HIPPARCOS measurements
14. ^ de Wit, W. J., Testi, L., Palla, F., Vanzi, L., & Zinnecker, H. 2004, Astronomy and Astrophysics, The origin of massive O-type field stars. I. A search for clusters
15. ^ Shevchenko, V. S., Ezhkova, O. V., Ibrahimov, M. A., van den Ancker, M. E., & Tjin A Djie, H. R. E. 1999, Monthly Notices of the Royal Astronomical Society, The stellar composition of the star formation region CMa R1 - I. Results from new photometric and spectroscopic classifications
17. ^ Launhardt, R. & Henning, T. 1997, Astronomy and Astrophysics, Millimetre dust emission from northern BOK globules.
18. ^ Avedisova, V. S. 1996, Astronomy Letters, The spiral structure of the inner Galaxy from a catalog of star-forming regions
20. ^ Teixeira, P. S., Fernandes, S. R., Alves, J. F., Correia, J. C., Santos, F. D., Lada, E. A., & Lada, C. J. 2004, Astronomy and Astrophysics, Near infrared imaging of <ASTROBJ>NGC 2316</ASTROBJ>
21. ^ Ivanov, V. D., Borissova, J., Pessev, P., Ivanov, G. R., & Kurtev, R. 2002, Astronomy and Astrophysics, Discovery of new Milky Way star clusters candidates in the 2MASS point source catalog
22. ^ Fich, Michel & Terebey, Susan 1996, Astrophysical Journal, IRAS Observations of the Outer Galaxy. I. Discrete Sources and Large-Scale (Diffuse) Emission
23. ^ Cappa, C. E., Rubio, M., & Goss, W. M. 2001, Astronomical Journal, CO Observations of NGC 2359: The Molecular Clouds Revisited
24. ^ Chen, Yang, Wang, Zhen-Ru, & Qu, Qin-Yue 1995, Astrophysical Journal, On the wind-powered nebula NGC 2359 and the WN star HD 56925
25. ^ Blitz, L., Fich, M., & Stark, A. A. 1982, Astrophysical Journal Supplement Series, Catalog of CO radial velocities toward galactic H II regions
26. ^ Yun, J. L., Djupvik, A. A., Delgado, A. J., & Alfaro, E. J. 2008, Astronomy and Astrophysics, A young double stellar cluster in an HII region, emerging from its parent molecular cloud
28. ^ Wouterloot, J. G. A. & Brand, J. 1989, Astronomy and Astrophysics Supplement Series, IRAS sources beyond the solar circle. I - CO observations